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UNIVERSEUNIVERSE

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Universe

Contents PICTURE ON PAGE 1Image of a planetary nebula.Planetary nebulae are amongthe most photogenic objectsin astronomy.

What Is theUniverse?Page 6

What Is in theUniverse?Page 18

The SolarSystemPage 38

The Earthand the MoonPage 66

Observingthe UniversePage 80

There was a time when people believedthat the stars were bonfires lit byother tribes in the sky, that the

universe was a flat plate resting on the shellof a giant turtle, and that the Earth,according to the Greek astronomer Ptolemy,was at the center of the universe. From themost remote of times, people have beencurious about what lies hidden beyond thecelestial sphere. This curiosity has led themto build telescopes that show with clarityotherwise blurry and distant objects. In thisbook you will find the history of the cosmosillustrated with spectacular images thatshow in detail how the cosmos was formed,the nature of the many points of light thatadorn the night sky, and what lies ahead.You will also discover how the suns thatinhabit space live and die, what dark matterand black holes are, and what our place is inthis vastness. Certainly, the opportunity to

compare the destiny of other worlds similarto ours will help us understand that for thetime being there is no better place than theEarth to live. At least for now.

In the Milky Way—according tomathematical and physicalcalculations—there are more than 100

billion stars, and such a multitude leads tothe question: Is it possible that our Sun isthe only star that possesses an inhabitedplanet? Astronomers are more convincedthan ever of the possibility of life in otherworlds. We just need to find them. Readingthis book will let you become betteracquainted with our neighbors in the solarsystem—the other planets—and the mostimportant characteristics that distinguishthem. All this information that explores themysteries of space is accompanied byrecent images captured by the newesttelescopes. They reveal many details aboutthe planets and their satellites, such as thevolcanoes and craters found on the surfaceof some of them. You will also learn moreabout the asteroids and comets that orbitthe Sun and about Pluto, a dwarf planet,which is to be visited by a space probe for

the first time. Less than a decade ago,astronomers began observing frozen worlds,much smaller than a planet, in a region ofthe solar system called the Kuiper belt. Weinvite you to explore all of this. The imagesand illustrations that accompany the textwill prove very helpful in studying andunderstanding the structure of all the visibleand invisible objects (such as dark matter)that form part of the universe. There arestellar maps showing the constellations, thegroups of stars that since ancient times haveserved as a guide for navigation and for thedevelopment of calendars. There is also areview through history: from Ptolemy, whothought the planets orbited around theEarth, and Copernicus, who put the Sun inthe center, and Galileo, the first to aim atelescope skyward, up to the most recentastronomical theories, such as those ofStephen Hawking, the genius of space andtime who continues to amaze with hisdiscoveries about the greatest mysteries ofthe cosmos. You will find these and manymore topics no matter where you look in thisfantastic book that puts the universe and itssecrets in your hands.

The Secrets ofthe Universe

CONE NEBULAThis nebula got its namefrom its cone shape, asshown in the image.

What Is the Universe?

The universe is everything thatexists, from the smallestparticles to the largest ones,together with all matter andenergy. The universe includes

visible and invisible things, such as darkmatter, the great, secret component ofthe cosmos. The search for dark matteris currently one of the most importanttasks of cosmology. Dark matter may

literally determine the density of all ofspace, as well as decide the destiny ofthe universe. Did you know that, secondby second, the universe grows andgrows? The question that astronomers

are asking—the question that concernsthem the most—is how much longer theuniverse can continue to expand like aballoon before turning into somethingcold and dark.

X-RAY OF THE COSMOS 8-9

THE INSTANT OF CREATION 10-13

EVERYTHING COMES TO AN END 14-15

THE FORCES OF THE UNIVERSE 16-17

DARK MATTEREvidence exists that dark matter, though invisibleto telescopes, betrays itself by the gravitationalpull it exerts over other heavenly bodies.

UNIVERSE 98 WHAT IS THE UNIVERSE?

The universe, marvelous in its majesty, is an ensemble of a hundredbillion galaxies. Each of these galaxies (which tend to be found inlarge groups) has billions of stars. These galactic concentrations

surround empty spaces, called cosmic voids. The immensity of thecosmos can be better grasped by realizing that the size of our fragileplanet Earth, or even that of the Milky Way, is insignificantcompared to the size of the remainder of the cosmos.

Originating nearly 14 billion years agoin an immense explosion, the universe

today is too large to be able to conceive. Theinnumerable stars and galaxies that populate itpromise to continue expanding for a long time.Though it might sound strange today, for manyyears, astronomers thought that the Milky Way,where the Earth is located, constituted the entireuniverse. Only recently—in the 20th century—was outerspace recognized as not only much vaster than previouslythought but also as being in a state of ongoing expansion.

The Universe

NEAR STARS Found closerthan 20 light-years from theSun, they make up our solarneighborhood.

2.

NEIGHBORS Within a spaceof one million light-years,we find the Milky Way andits closest galaxies.

3.NEAREST GALAXIES. At a scaleof one hundred million light-years,the galactic clusters nearest tothe Milky Way can be seen.

5.

FILAMENTS. From five billionlight-years away, the immensity ofthe cosmos is evident in itsgalactic filaments, each one hometo millions and millions of galaxies.

7.SUPERCLUSTERS. Within adistance of a billion light-years,groups of millions of galaxies,called superclusters, can be seen.

6.

LOCAL GROUP. Tenmillion light-years awayis Andromeda, theclosest to the Earth.

4.

EARTH Originated, togetherwith the solar system, whenthe universe was already 9.1billion years old. It is the onlyknown planet that is home to life.

1.

EARTH

Neptune

G51-15

Ross128

Lalande21185

Wolf359

Luyten’sStar

Procyon

Uranus

Saturn

Jupiter

Pluto

SUNAlphaCentauri

Sirius

270°

90°

180°

0.5

0°

180°

0°

12.5

EpsilonEridani

L372-58

L726-8

L725-32

EpsilonIndi

Lacaille9352Ceti

7.5

2.5

Struve2398

Ross248

Ross154

Groombridge34

61 Cygni

Bernard’s Star

L789-6

L789-6

0°

SextansDwarf

UrsaMinor Dwarf

Leo A

Leo I Leo II

Andromeda I

Sextans BSextans A

AntilaDwarf

NGC3109

DracoDwarf

SagittariusDwarf

TucanaDwarf

PhoenixDwarf

CetusDwarf Sagittarius

IrregularDwarf

AquariusDwarf

LGS 3

PegasusDwarf

IC1613

WLM

CanisMajor

SmallMagellanicCloud

LargeMagellanicCloud

CarinaDwarf

MILKY WAY

MILKY WAY

NGC6822

Triangle

Andromeda

M32

M110

NGC185

NGC147

IC 10

0.120.25

0.37

1.22.5

3.7

0°

180°

0°

180°

NGC7582

NGC6744

CapricornusSupercluster

Pavo-IndusSupercluster

SculptorSupercluster

SculptorVoid

Pisces-CetusSuperclusters

Pisces-PerseusSupercluster

ComaSupercluster

CentaurusSupercluster

HerculesSupercluster

ShapleySupercluster

BoötesVoid

LeoSupercluster

Ursa Major Supercluster

BoötesSupercluster

Corona Borealis Supercluster

Hydra

HorologiumSuperclusters Columba

Supercluster

NGC1023

NGC2997

NGC5128

NGC5033

NGC4697

12.5

25

37.5

50

1,000

750

250

Dorado

Sculptor

Maffei

M81

M101

Leo I

Canis

Ursa Major Group

VirgoGroup

Leo III Group

Virgo III Group

FornaxCluster

EridanusCluster

LOCALGROUP

VIRGO

SextansSupercluster

X-Ray of the Cosmos

100 billionThe total number of galaxies that exist,indicating that the universe is both largerand older than was previously thought


Region 1 Region 3

Region 2

Region 4

Region 5

Galaxy 1 Galaxy 2

Galaxy 5

Galaxy 4Galaxy 3

The Instant of CreationI

t is impossible to know precisely how, out of nothing, the universe began to exist. According to the bigbang theory—the theory most widely accepted in the scientific community—in the beginning, thereappeared an infinitely small and dense burning ball that gave rise to space, matter, and energy. This

happened 13.7 billion years ago. The great, unanswered question is what caused a small dot of light—filledwith concentrated energy from which matter and antimatter were created—to arise from nothingness. Invery little time, the young universe began to expand and cool. Several billion years later, it acquired theform we know today.

10-43sec 10-12

sec 3 min

Scientists theorize that, fromnothing, something infinitelysmall, dense, and hot appeared.All that exists today was

compressed into a ball smaller thanthe nucleus of an atom.

TIME

TEMPERATURE

Cosmic Inflation TheoryAlthough big bang theorists understood the universe as originatingin an extremely small, hot, and condensed ball, they could not

understand the reason for its staggering growth. In 1981, physicist AlanGuth proposed a solution to the problem with his inflationary theory. In anextremely short period of time (less than a thousandth of a second), theuniverse grew more than a trillion trillion trillion times. Near the end of thisperiod of expansion, the temperature approached absolute zero.

HOW IT DIDNOT GROWHad the universe notundergone inflation,it would be acollection of differentregions, each with itsown particular typesof galaxies and eachclearlydistinguishable fromthe others.

HOW IT GREWCosmic inflation wasan expansion of theentire universe. TheEarth's galacticneighborhood appearsfairly uniform.Everywhere you look,the types of galaxiesand the backgroundtemperature areessentially the same.

FROM PARTICLES TO MATTERThe quarks, among the oldest particles,interact with each other by forcestransmitted through gluons. Later protonsand neutrons will join to form nuclei.

PhotonMassless elementalluminous particle

GluonResponsible forthe interactionsbetween quarks

QuarkLight, elementalparticle

GravitonIt is believed totransmit gravitation.

0

Gravity

SUPERFORCE

Strong nuclear

Weak nuclear

ElectromagnetismEXPANSION

10-38sec

1032 ° F (and C) 1029 ° F (and C) 1015 ° F (and C) 2x109 ° F (1x109 ° C)-

1At the closest moment tozero time, which physics hasbeen able to reach, thetemperature is extremely

high. Before the universe's inflation,a superforce governed everything.

2The universe is unstable. Only10-38 seconds after the big bang, the universe increases in size more than a trillion trillion

trillion times. The expansion of the universe and the division of its forces begin.

3 The universe experiences agigantic cooldown. Gravityhas already becomedistinguishable, and the

electromagnetic force and the strongand weak nuclear interactions appear.

4

5 sec

9x109 ° F (5x109 ° C)

The electrons and theirantiparticles,positrons, annihilateeach other until the

positrons disappear. Theremaining electrons form atoms.

6

10-4sec

1012 ° F (and C)

Protons and neutronsappear, formed by threequarks apiece. Becauseall light is trapped within

the web of particles, the universeis still dark.

5The nuclei of thelightest elements,hydrogen andhelium, form.

Protons and neutrons unite toform the nuclei of atoms.

7

WMAP (WILKINSON MICROWAVE ANISOTROPY PROBE)NASA's WMAP project maps the background radiation of the universe. In theimage, hotter (red-yellow) regions and colder (blue-green) regions can beobserved. WMAP makes it possible to determine the amount of dark matter.

THE SEPARATION OF FORCESBefore the universe expanded, during a period ofradiation, only one unified force governed allphysical interactions. The first distinguishableforce was gravity, followed by electromagnetismand nuclear interactions. Upon the division of theuniverse's forces, matter was created.Energetic Radiation

The burning ball that gave rise to the universe remained asource of permanent radiation. Subatomic particles and

antiparticles annihilated each other. The ball's high densityspontaneously produced matter and destroyed it. Had this stateof affairs continued, the universe would never have undergone thegrowth that scientists believe followed cosmic inflation.

1 sec

1 A gluon interactswith a quark. 2 Quarks join by means

of gluons to formprotons and neutrons.

3 Protons andneutrons unite tocreate nuclei.

ElectronNegatively chargedelemental particle

ELEMENTARY PARTICLESIn its beginnings, the universe was a soup of particles that interacted with each otherbecause of high levels of radiation. Later, as the universe expanded, quarks formed thenuclei of the elements and then joined with electrons to form atoms.

The neutrinos separate from the initial particle soup through the disintegrationof neutrons. Though having extremely little mass, the neutrinos mightnevertheless form the greatest part of the universe's dark matter.

Proton

Neutron

Quark

Gluon


380,000 500 millionTIME(in years)

TEMPERATURE 4,900° F (2,700° C) -405° F (-243° C)380,000 years after the bigbang, atoms form. Electronsorbit the nuclei, attracted bythe protons. The universe

becomes transparent. Photons travelthrough space.

8Galaxies acquire their definitiveshape: islands of millions andmillions of stars and masses ofgases and dust. The stars explode

as supernovas and disperse heavierelements, such as carbon.

9

FIRST ATOMSHydrogen and helium were the first elements to be formed at the atomic level. They are the maincomponents of stars and planets. They are by far the most abundant elements in the universe.

The vast span of time related to the history ofthe universe can be readily understood if it isscaled to correspond to a single year—a yearthat spans the beginning of the universe, the

appearance of humans on the Earth, and thevoyage of Columbus to America. On January 1of this imaginary year—at midnight—the bigbang takes place. Homo sapiens appears at

11:56 P.M. on December 31, and Columbus setssail on the last second of the last day of theyear. One second on this timescale is equivalentto 500 true years.

1 HydrogenAn electron is attracted byand orbits the nucleus, whichhas a proton and a neutron.

Proton

Neutron

Electron

NUCLEUS 2

NUCLEUS 1

2 HeliumSince the nucleushas two protons,two electrons areattracted to it.

3 CarbonWith time, heavier and more complex elementswere formed. Carbon, the key to human life, has sixprotons in its nucleus and six electrons orbiting it.

Quasar

Starcluster

Nebula

Ellipticalgalaxy

Irregulargalaxy

Star

Spiralgalaxy

Barredspiral

galaxy

Galaxycluster

COLUMBUS'SARRIVAL

takes place onthe last second

of December 31.

THE SOLARSYSTEM

is created onAugust 24 of

this timescale.

BIG BANGoccurs on thefirst second ofthe first day ofthe year.

JANUARY DECEMBER

13.7 billion-454° F (-270° C)

The universe continues to expand. Countless galaxiesare surrounded by dark matter, which represents 22percent of the mass and energy in the universe. Theordinary matter, of which stars and planets are

made, represents just 4 percent of the total. The predominantform of energy is also of an unknown type. Called dark energy, itconstitutes 74 percent of the total mass and energy.

11

DARK MATTERThe visible objects in thecosmos represent only asmall fraction of the totalmatter within the universe.Most of it is invisible even tothe most powerfultelescopes. Galaxies and theirstars move as they dobecause of the gravitationalforces exerted by thismaterial, which astronomerscall dark matter.

THE UNIVERSE TODAY

TIMESCALE

The Transparent UniverseWith the creation of atoms and overall cooling, the once opaque anddense universe became transparent. Electrons were attracted by the

protons of hydrogen and helium nuclei, and together they formed atoms.Photons (massless particles of light) could now pass freely through theuniverse. With the cooling, radiation remained abundant but was no longer thesole governing factor of the universe. Matter, through gravitational force, couldnow direct its own destiny. The gaseous lumps that were present in thisprocess grew larger and larger. After 100 million years, they formed evenlarger objects. Their shapes not yet defined, they constituted protogalaxies.Gravitation gave shape to the first galaxies some 500 million years after thebig bang, and the first stars began to shine in the densest regions of thesegalaxies. One mystery that could not be solved was why galaxies weredistributed and shaped the way they were. The solution that astronomers havebeen able to find through indirect evidence is that there exists material calleddark matter whose presence would have played a role in galaxy formation.

9.1 billionTHE EARTH IS CREATEDLike the rest of the planets, the Earth is made ofmaterial that remained after the formation of the solarsystem. The Earth is the only planet known to have life.

EVOLUTION OF MATTERWhat can be observed in the universe today is a greatquantity of matter grouped into galaxies. But that was notthe original form of the universe. What the big bang initiallyproduced was a cloud of uniformly dispersed gas. Just threemillion years later, the gas began to organize itself intofilaments. Today the universe can be seen as a network ofgalactic filaments with enormous voids between them.

1 Gaseous cloudThe first gasesand dust resultingfrom the Big Bangform a cloud.

2 First filamentsBecause of thegravitational pull of darkmatter, the gases joinedin the form of filaments.

3 Filament networksThe universe haslarge-scale filamentsthat contain millionsand millions of galaxies.

9 billion-432° F (-258° C)

Nine billion years after the bigbang, the solar systememerged. A mass of gas anddust collapsed until it gave rise

to the Sun. Later the planetary system wasformed from the leftover material.

10


Everything Comes to an EndT

he big bang theory helped solve the enigma of the early moments of the universe. What has yet tobe resolved is the mystery surrounding the future that awaits. To unravel this mystery, the totalmass of the universe must be known, but that figure has not yet been reliably determined. The

most recent observations have removed some of this uncertainty. It seems that the mass of the universeis far too little to stop its expansion. If this is this case, the universe's present growth is merely the laststep before its total death in complete darkness.

Black hole

Universe 1

Universe 1Universe 4

Universe 3Blackhole

Universe 2

Object in threedimensions

Object that changeswith time

Universe 3

New universe

Inflection point

DISCOVERIESThe key discovery that led to the bigbang theory was made in the early1920s by Edwin Hubble, whodiscovered that galaxies were movingaway from each other. In the 1940s,George Gamow developed the ideathat the universe began with aprimordial explosion. A consequenceof such an event would be theexistence of background radiation,which Arno Penzias and RobertWilson accidentally detected in themid-1960s.

There is a critical amount of massfor which the universe wouldexpand at a declining rate without

ever totally stopping. The result of thiseternal expansion would be the existence ofan ever-increasing number of galaxies andstars. If the universe were flat, we couldtalk about a cosmos born from an explosion,but it would be a universe continuingoutward forever. It is difficult to thinkabout a universe with these characteristics.

Flat Universe

BIG BANG

BIGCRUNCH

HOW IT IS MADE UPDark energy is hypothesized to bethe predominant energy in theuniverse. It is believed to speed upthe expansion of the universe.

BLACK HOLESSome theorists believethat, by entering ablack hole, travelthrough space toother universes mightbe possible because ofantigravitationaleffects.

1 The universeexpands violently. 2 The universe's

growth slows. 3 The universe collapsesupon itself, forming adense, hot spot.

1 The universecontinuouslyexpands andevolves.

TIME

2 The universe'sexpansion isunceasing butever slower.

2 Expansion iscontinuous andpronounced.

3 Gravity is notsufficient to bring acomplete stop to theuniverse's expansion.

4 The universeexpands indefinitely.

1Self-generatedUniverses

A less widely accepted theory aboutthe nature of the universe suggeststhat universes generate themselves.

If this is the case, universes would becreated continuously like the branches of atree, and they might be linked bysupermassive black holes.

According to this theory, universescontinuously sprout other universes. Butin this case, one universe would be

created from the death or disappearance ofanother. Each dead universe in a final collapse, or

Big Crunch, would give rise to a supermassiveblack hole, from which another universe wouldbe born. This process could repeat itselfindefinitely, making the number of universesimpossible to determine.

Baby Universes

5

Closed UniverseIf the universe had more thancritical mass, it would expanduntil reaching a point where

gravity stopped the expansion. Then,the universe would contract in the BigCrunch, a total collapse culminating inan infinitely small, dense, and hot spotsimilar to the one from which theuniverse was formed. Gravity's pull onthe universe's excess matter would stopthe expansion and reverse the process.

2

Open UniverseThe most accepted theory aboutthe future of the cosmos saysthat the universe possesses a

mass smaller than the critical value. Thelatest measurements seem to indicate thatthe present time is just a phase before thedeath of the universe, in which it goescompletely dark.

41 After the originalexpansion, theuniverse grows.

3 reaches a point whereeverything grows darkand life is extinguished.

3

74%dark energy

22%dark matter

4%visible matter

GALACTIC EXPANSIONBy noting a redshift toward the red end of thespectrum, Hubble was able to demonstrate thatgalaxies were moving away from each other.

1920sGAMOW'S SUSPICION

Gamow first hypothesized the big bang,holding that the early universe was a

“cauldron” of particles.

1940sBACKGROUND RADIATION

Penzias and Wilson detected radio signalsthat came from across the entire sky—the

uniform signal of background radiation.

1965

THE HAWKING UNIVERSEThe universe was composed originally of fourspatial dimensions without the dimension oftime. Since there is no change without time,one of these dimensions, according to Hawking,transformed spontaneously on a small scaleinto a temporal dimension, and the universebegan to expand.


The Forces of the UniverseT

he four fundamental forces of nature are those that are not derived from basic forces. Physicistsbelieve that, at one time, all physical forces functioned as a single force and that during theexpansion of the universe, they became distinct from each other. Each force now governs different

processes, and each interaction affects different types of particles. Gravity, electromagnetism, strongnuclear interactions, and weak nuclear interactions are essential to our understanding of the behavior ofthe many objects that exist in the universe. In recent years, many scientists have tried with little successto show how all forces are manifestations of a single type of exchange.

The universe, if it were empty, could bepictured in this way.

The universe is deformed by the massof the objects it contains.

MOLECULAR MAGNETISMIn atoms and molecules, the electromagnetic force isdominant. It is the force that causes the attractionbetween protons and electrons in an atom and theattraction or repulsion between ionized atoms.

NEWTON'S EQUATION

BENDING LIGHTLight also bends because of the curvature of space-time.When seen from a telescope, the real position of an objectis distorted. What is perceived through the telescope is afalse location, generated by the curvature of the light. Itis not possible to see the actual position of the object.

The biggest contribution to our comprehension of the universe's internalworkings was made by Albert Einstein in 1915. Building on Newton's

theory of universal gravitation, Einstein thought of space as linked to time. ToNewton, gravity was merely the force that attracted two objects, but Einsteinhypothesized that it was a consequence of what he called the curvature of space-time. According to his general theory of relativity, the universe curves in thepresence of objects with mass. Gravity, according to this theory, is a distortion ofspace that determines whether one object rolls toward another. Einstein's generaltheory of relativity required scientists to consider the universe in terms of a non-Euclidian geometry, since it is not compatible with the idea of a flat universe.In Einsteinian space, two parallel lines can meet.

General Theory of Relativity

UNIVERSAL GRAVITATIONThe gravitation proposed by Newton isthe mutual attraction between bodieshaving mass. The equation developed byNewton to calculate this force statesthat the attraction experienced by twobodies is directly proportional to theproduct of their masses and inverselyproportional to the square of thedistance between them. Newtonrepresented the constant ofproportionality resulting from thisinteraction as G. The shortcoming of

Newton's law, an accepted paradigmuntil Einstein's theory of generalrelativity, lies in its failure to make timean essential component in theinteraction between objects. Accordingto Newton, the gravitational attractionbetween two objects with mass did notdepend on the properties of space butwas an intrinsic property of the objectsthemselves. Nevertheless, Newton's lawof universal gravitation was afoundation for Einstein's theory.

The strong nuclear force holds the protons and neutronsof atomic nuclei together. Both protons and neutrons aresubject to this force. Gluons are particles that carry the

strong nuclear force, and they bind quarks together to formprotons and neutrons. Atomic nuclei are held together byresidual forces in the interaction between quarks and gluons.

Strong Nuclear Force

3

The weak nuclear force is not as strong as the otherforces. The weak nuclear interaction influences the betadecay of a neutron, which releases a proton and a

neutrino that later transforms into an electron. This force takespart in the natural radioactive phenomena associated with certaintypes of atoms.

Weak Nuclear Force

4

Electromagnetism is the force that affectselectrically charged bodies. It is involved in thechemical and physical transformations of the

atoms and molecules of the various elements. Theelectromagnetic force can be one of attraction or repulsion,with two types of charges or poles.

Electromagnetism

2

Gravity was the first force tobecome distinguishable from theoriginal superforce. Today

scientists understand gravity in Einstein'sterms as an effect of the curvature ofspace-time. If the universe were thought ofas a cube, the presence of any object with

mass in space would deform the cube.Gravity can act at great distances (just aselectromagnetism can) and always exerts aforce of attraction. Despite the manyattempts to find antigravity (which couldcounteract the effects of black holes), ithas yet to be found.

Gravity

1

E=mc2In Einstein's equation, energy and mass areinterchangeable. If an object increases itsmass, its energy increases, and vice versa.

F=Gxm1xm2d2

SUN

EARTH

Whatwe see

LUM

INO

US TRA

JECTORY

Realposition

Two bodies with mass attract each other. Whicheverbody has the greatest mass will exert a greater forceon the other. The greater the distance between theobjects, the smaller the force they exert on each other.

d

Fm1 m2

Quark

Force

Positivepole

Positivepole

Negativepole

Negativepole

Nucleus

Electron

Helium

Hydrogen

Neutron WIMP

Nucleus

Proton

HYDROGEN ATOM

HELIUM ISOTOPE

GluonForce

Proton

Electron

Electron

1 Quarks and gluonsThe strong nuclear interactiontakes place when the gluoninteracts with quarks.

AttractionTwo atoms are drawn together,and the electrons rotatearound the new molecule.

1 HydrogenA hydrogen atom interactswith a weak, light particle(WIMP). A neutron'sbottom quark transformsinto a top quark.

2 UnionQuarks join and formnuclear protons andneutrons.

2 HeliumThe neutron transformsinto a proton. An electronis released, and the heliumisotope that is formed hasno nuclear neutrons.

THE FINAL DARKNESS 30-31

ANATOMY OF GALAXIES 32-33

ACTIVE GALAXIES 34-35

STELLAR METROPOLIS 36-37

What Is in the Universe?

The universe is populated on agrand scale by strands ofsuperclusters surroundingvacant areas. Sometimes thegalaxies collide with each

other, triggering the formation of stars.In the vast cosmos, there are alsoquasars, pulsars, and black holes.Thanks to current technology, we canenjoy the displays of light and shadow

that make up, for example, the EtaCarinae Nebula (shown), which iscomposed of jets of hot, fluorescentgases. Although not all the objects in theuniverse are known, it can be said

without a doubt that most of the atomsthat make up our bodies have been bornin the interior of stars.

LUMINOUS 20-21

STELLAR EVOLUTION 22-23

RED, DANGER, AND DEATH 24-25

GAS SHELLS 26-27

SUPERNOVAE 28-29

ETA CARINAE NEBULAWith a diameter of more than 200 light-years, it isone of biggest and brightest nebulae of our galaxy.This young, supermassive star is expected to becomea supernova in the near future.

100,000

SUNMainsequence

SupergiantsRed giants

White dwarfs

10,000

1,000

100

10

1

0.1

0.01

0.001

0.0001

INTRINSIC LUMINOSITY (SUN = 1)

SPECTRAL CLASSES

O B A F G K M

UNIVERSE 2120 WHAT IS IN THE UNIVERSE?

LuminousF

or a long time stars were a mystery to humans, and it was only as recentlyas the 19th century that astronomers began to understand the true natureof stars. Today we know that they are gigantic spheres of incandescent

gas—mostly hydrogen, with a smaller proportion of helium. As a star radiateslight, astronomers can precisely measure its brightness, color, and temperature.Because of their enormous distance from the Earth, stars beyond the Sun onlyappear as points of light, and even the most powerful telescopes do not revealany surface features.

COLORS The hottest stars arebluish-white (spectral classesO, B, and A). The coolest starsare orange, yellow, and red(spectral classes G, K, and M).

Calcium Hydrogen Hydrogen HydrogenSodium

Wavelength longest on the red side

When a star moves toward or away from an observer, itswavelengths of light shift, a phenomenon called the Doppler effect.If the star is approaching the Earth, the dark lines in its spectrumexperience a blueshift. If it moves away from the Earth, the linesexperience a redshift.

DOPPLER EFFECT

SCORPIUS REGION

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 26 27 28 29 30PARSECS

PRINCIPAL STARS WITHIN 100 LY FROM THE SUN

SUN(G2)

The H-R diagram plots the intrinsicluminosity of stars against their

spectral class, which corresponds to theirtemperature or the wavelengths of lightthey emit. The most massive stars are

those with greatest intrinsic luminosity.They include blue stars, red giants, andred supergiants. Stars spend 90 percentof their lives in what is known as themain sequence.

Hertzsprung-Russell (H-R) Diagram

In measuring the great distancesbetween stars, both light-years (ly)

and parsecs (pc) are used. A light-year isthe distance that light travels in a year—5.9 trillion miles (10 trillion km). A light-

year is a unit of distance, not time. A parsecis equivalent to the distance between thestar and the Earth if the parallax angle is ofone second arc. A pc is equal to 3.26 light-years, or 19 trillion miles (31 trillion km).

Light-years and Parsecs

When the Earth orbits the Sun, the closest starsappear to move in front of a background of more

distant stars. The angle described by the movement of astar in a six-month period of the Earth's rotation is calledits parallax. The parallax of the most distant stars are toosmall to measure. The closer a star is to the Earth, thegreater its parallax.

Measuring Distance

The electromagnetic waves that make up light havedifferent wavelengths. When light from a hot

object, such as a star, is split into its differentwavelengths, a band of colors, or spectrum, is obtained.Patterns of dark lines typically appear in the spectrum ofa star. These patterns can be studied to determine theelements that make up the star.

Spectral Analysis

Dark lines deviate toward the blue end of the spectrum.

BLUESHIFT of a star moving toward the Earth.

OPEN CLUSTERThe Pleiades are a formation ofsome 400 stars that will eventuallymove apart.

GLOBULAR CLUSTERMore than a million stars aregrouped together into a sphericalcluster called Omega Centauri.

ALPHACENTAURI(G2, K1, M5)

SIRIUS (A0 and

dwarf star)

PROCYON (F5 and

dwarf star)

ALTAIR (A7)

VEGA (A0)

POLLUX (K0 giant)

ARCTURUS (K2 giant)

CAPELLA (G6 and G2

giants)

CASTOR(A2, A1, and M1)

ALDEBARAN(K5 giant)

ALIOTH (A0 giant)

MENKALINAN (A2 and A2)

ALGOL (B8 and K0)

REGULUS(B7 and K1)

25

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

LIGHT-YEARS

GACRUX (M4 giant)

TYPE O52,000-72,000° F(29,000-40,000° C)

TYPE B17,500-52,000° F(9,700-29,000° C)

TYPE A13,000-17,500° F(7,200-9,700° C)

TYPE F10,500-13,000° F(5,800-7,200° C)

TYPE G8,500-10,500° F(4,700-5,800° C)

TYPE K6,000-8,500° F(3,300-4,700° C)

TYPE M4,000-6,000° F(2,100-3,300° C)

Star Earth

Wavelength is compressed bythe movement of the star.

Because the parallaxof star A is small, wesee that it is distantfrom the Earth.

Position ofthe Earth in

January

Position ofthe Earth inJuly

PARALLAX

SUN

A

The parallax of star Bis greater than that ofstar A, so we see thatB is closer to theEarth.

B

A CLOUD OF GAS AND DUST collapsesbecause of gravitational forces. In doingso it heats up and divides into smallerclouds. Each one of these clouds willform a protostar.

PROTOSTAR A protostar has adense, gaseouscore surrounded bya cloud of dust.

1.

RED SUPERGIANT The star swells and heats up.Through nuclear reactions, aheavy core of iron is formed.

3.

NEUTRON STARIf the star's initial mass is betweeneight and 20 solar masses, it ends upas a neutron star.

BLACK HOLE If the star's initialmass is 20 solar masses or more, itsnucleus is denser and it turns into ablack hole, whose gravitational forceis extremely strong. 5.

5.

PROTOSTARA protostar is formedby the separation of gasand dust. Gravitationaleffects cause its core torotate.

1.

PLANETARY NEBULA Whenthe star's fuel is depleted, itscore condenses, and its outer

layers detach, expellinggases in an expanding shellof gases.

WHITE DWARFThe star remainssurrounded bygases and is dim.

5.

22 WHAT IS IN THE UNIVERSE? UNIVERSE 23

Stars are born in nebulae, which are giant clouds of gas (mainly hydrogen)and dust that float in space. Stars can have a life span of millions,or even billions, of years. The biggest stars have the

shortest lives, because they consume their nuclearfuel (hydrogen) at a very accelerated rate. Otherstars, like the Sun, burn fuel at a slower rate andmay live some 10 billion years. Many times, astar's size indicates its age. Smaller stars are theyoungest, and bigger stars are approachingtheir end, either through cooling or byexploding as a supernova.

Stellar Evolution

Nebula

Small starLess than 8 solar masses

STARA star is finally born. Itfuses hydrogen to formhelium and lies alongthe main sequence.

2.

The evolution of a star depends on its mass. Thesmallest ones, like the Sun, have relatively long and

modest lives. Such a star begins to burn helium when itshydrogen is depleted. In this way, its external layersbegin to swell until the star turns into a red giant. Itends its life as white dwarfs, eventually fading awaycompletely, ejecting remaining outer layers, and forminga planetary nebula. A massive star, because of its higherdensity, can form elements heavier than helium from itsnuclear reactions. In the final stage of its life, its corecollapses and the star explodes. All that remains is ahyperdense remnant, a neutron star. The most massivestars end by forming black holes.

Life Cycle of a Star

end their lives as white dwarfs. Other (larger)stars explode as supernovae, illuminatinggalaxies for weeks, although their brightness isoften obscured by the gases and dust.

STAR The star shines andslowly consumes itshydrogen. It begins to fusehelium as its size increases.

2.

RED GIANT The star continues toexpand, but its mass remainsconstant and its core heats up.When the star's helium is depleted,it fuses carbon and oxygen.

3.

Massive starMore than 8 solar masses

95% of stars

BLACK DWARFIf a white dwarffades outcompletely, itbecomes a blackdwarf.

6.

4.

4. SUPERNOVA When the star can no longerfuse any more elements, its core collapses,causing a strong emission of energy.

Redgiant

3 4

5

6

7

23

4

6

7

LIFE CYCLE OF ASTAR

5

When a star exhausts its hydrogen, it begins to die. Thehelium that now makes up the star's core begins toundergo nuclear reactions, and the star remains

bright. When the star's helium is depleted, fusion ofcarbon and oxygen begins, which causes the star'score to contract. The star continues to live, though itssurface layers begin to expand and cool as the star turnsinto a red giant. Stars similar to the Sun (solar-type stars)follow this process. After billions of years, they end up aswhite dwarfs. When they are fully extinguished, they willbe black dwarfs, invisible in space.


Red, Danger, and Death

DIAMETER

All stars go through a red-giantstage. Depending on a star's

mass, it may collapse or it may simplydie enveloped in gaseous layers. Thecore of a red giant is 10 times smallerthan it was originally since it shrinks

from a lack of hydrogen. A supergiantstar (one with an initial mass greaterthan eight solar masses) lives a muchshorter life. Because of the high densityattained by its core, it eventuallycollapses in on itself and explodes.

Red Giant

HERTZSPRUNG-RUSSELLWhen a white dwarf leaves thered-giant stage, it occupies thelower-left corner of the H-Rdiagram. Its temperature may bedouble that of a typical red giant.A massive white dwarf cancollapse in on itself and end itslife as a neutron star.

HYDROGENHydrogen continues undergoingnuclear fusion in the exterior ofthe core even when the innercore has run out of hydrogen.

HELIUMHelium is produced by the fusionof hydrogen during the mainsequence.

CARBON AND OXYGENCarbon and oxygen are producedby the fusion of helium within thecore of the red giant.

1

2

3

SUN

ConvectionCellsConvection cells carry heat towardthe surface of a star. The ascendingcurrents of gas eventually reachthe surface of the star, carryingwith them a few elements thatformed in the star's core.

Hot SpotsHot spots appear when largejets of incandescent gasreach the star's surface.They can be detected on thesurface of red giants.

REGION OF THE CORE

TEMPERATUREAs the helium undergoes fusion,the temperature of the corereaches millions of degreesFahrenheit (millions of degreesCelsius).

4

Venus's orbit

Mercury's orbit

Earth's orbit

Mars's orbit

Jupiter's orbit

Saturn's orbit

Red supergiant. Placedat the center of thesolar system, it wouldswallow up Mars andJupiter.

Red giant. Placed atthe center of the solarsystem, it could reachonly the nearer planets,such as Mercury,Venus, and the Earth.

1%The scale of thediameter of the Sunto the diameter ofa typical red giant.

After going through the red-giant stage, a solar-type star loses itsouter layers, giving rise to a planetary nebula. In its center remains a

white dwarf—a relatively small, very hot (360,000° F [200,000° C]), densestar. After cooling for millions of years, it shuts down completely andbecomes a black dwarf.

White Dwarf

On leaving the main sequence,the star enlarges to 200 timesthe size of the Sun. When thestar begins to burn helium, its

size decreases to between 10 and100 times the size of the Sun.The star then remains stable untilit becomes a white dwarf.

SPECTACULAR DIMENSIONS

HERTZSPRUNG-RUSSELLWhen the star exhausts itshydrogen, it leaves the mainsequence and burns heliumas a red giant (or asupergiant). The smalleststars take billions of years toleave the main sequences.The color of a red giant iscaused by its relatively coolsurface temperature of3,600° F (2,000° C).

Sun

NEBULa NGC 6751

After the nuclear reaction in the star's core ceases, the starejects its outer layers, which then form a planetary nebula.

WHITE DWARF

Mars Venus

Sun

Earth Mercury

Mars Venus

Sun

Earth Mercury

Mars Venus

Sun

Earth

Like any typical star, the Sun burns hydrogenduring its main sequence. After takingapproximately five billion years to exhaust itssupply of hydrogen, it will begin itstransformation into a red giant, doubling in

brightness and expanding until it swallowsMercury. At its maximum size, it may evenenvelop the Earth. Once it has stabilized, it willcontinue as a red giant for two billion years andthen become a white dwarf.

THE FUTURE OF THE SUN

Dust Grains Dust grains condense in the star's outeratmosphere and later disperse in the form ofstellar winds. The dust acquires a darkappearance and is swept into interstellarspace, where new generations of stars willform. The outer layer of the star mayextend across several light-years ofinterstellar space.

RED GIANTThe radius of theSun reaches theEarth's orbit.

Earth

1

2

Gas Shells

HOURGLASS

HELIX

SPIROGRAPH

When a small star dies, all that remains is an expandinggas shell known as a planetary nebula, which hasnothing to do with the planets. In general,

planetary nebulae are symmetrical or sphericalobjects. Although it has not been possible todetermine why they exist in such diversity, the reasonmay be related to the effects of the magnetic field of thedying central star. Viewed through a telescope, severalnebulae can be seen to contain a central dwarf star, a mereremnant of its precursor star.

UNIVERSE 27

The two rings of coloredgas form the silhouette of

this hourglass-shapednebula. The red in the

photograph corresponds tonitrogen, and the green

corresponds to hydrogen.This nebula is 8,000 light-

years from the Earth.

MYCN 18

NGC 7293

BUTTERFLY

The density of a white dwarf is a milliontimes greater than the density of water.In other words, each cubic meter of awhite dwarf star weighs a million tons.

The mass of a star is indirectlyproportional to its diameter. A whitedwarf with a diameter 100 times smallerthan the Sun has a mass 70 times greater.

M2-9The Butterfly Nebula containsa star in addition to a whitedwarf. Each orbits the otherinside a gas disk that is 10times larger than Pluto'sorbit. The Butterfly Nebulais located 2,100 light-yearsfrom Earth.

IC 418The Spirograph Nebula has ahot, luminous core thatexcites nearby atoms,causing them to glow. TheSpirograph Nebula is about0.1 light-year wide and islocated 2,000 light-yearsfrom Earth.

WhiteDwarfThe remains of the redgiant, in which thefusion of carbon andoxygen has ceased, lieat the center of thenebula. The star slowlycools and fades.

HydrogenThe continuously expandingmasses of gas surrounding thestar contain mostly hydrogen,with helium and lesser amountsof oxygen, nitrogen, and otherelements.

Concentriccirclesof gas, resembling the inside of an onion, form amultilayered structure around the white dwarf.Each layer has a mass greater than the combinedmass of all the planets in the solar system.

TWICE THETEMPERATURE OFTHE SUNis reached at the surface of awhite dwarf, causing it toappear white even though itsluminosity is a thousand timesless than that of the Sun.

is the weight of a singletablespoon of a white

dwarf. A white dwarf isvery massive in spite of

the fact that itsdiameter of 9,300 miles

(or 15,000 km) iscomparable to the

Earth's.

The astrophysicistSubrahmanyanChandrasekhar, winner of theNobel Prize for Physics in1983, calculated the maximummass a star could have so thatit would not eventually collapseon itself. If a star's mass exceedsthis limit, the star will eventuallyexplode in a supernova.

CHANDRASEKHARLIMIT

1.44 SOLAR MASSESis the limit Chandrasekharobtained. In excess of this value,a dwarf star cannot support itsown gravity and collapses.

NGC 6542 CAT'S EYE

26 WHAT IS IN THE UNIVERSE?

SMALLER DIAMETERMore massive white dwarf

LARGER DIAMETERLess massive white dwarf

DENSITY OF A WHITE DWARF

The Helix is aplanetary nebula that

was created at theend of the life of a

solar-type star. It is650 light-years from

the Earth and islocated in the

constellation Aquarius.

Planetarynebula

1

2

3 4

5

6

7

23 4

6

7

LIFE CYCLE OFA STAR

5

3 tons

28 WHAT IS IN THE UNIVERSE?

SupernovaeA

supernova is an extraordinary explosion of a giant star atthe end of its life, accompanied by a sudden increasein brightness and the release of a great amount

of energy. In 10 seconds, a supernova releases 100times more energy than the Sun will release in itsentire life. After the explosion of the star that givesrise to a supernova, the gaseous remnant expands andshines for millions of years. It is estimated that, in our MilkyWay galaxy, two supernovae occur per century.

Supernova

GAS AND DUSTGas and dust that have

accumulated in the two visible lobesabsorb the blue light and ultraviolet

rays emitted from its center.FUSION The nuclearreactions in adying star occurat a faster ratethan they do in ared giant.

GASEOUS FILAMENTSGaseous filaments are ejectedby the supernova at 620 miles

(1,000 km) per second.

THE ENDEither a neutron staror a black hole mayform depending on theinitial mass of the starthat has died.

Stellar RemnantWhen the star explodes as a supernova, itleaves as a legacy in space the heavy

elements (such as carbon, oxygen, and iron) thatwere in the star's nucleus before its collapse. TheCrab Nebula (M1) was created by a supernovaseen in 1054 by Chinese astronomers. The CrabNebula is located 6.5 light-years from Earth andhas a diameter of six light-years. The star thatgave rise to the Crab Nebula may have had aninitial mass close to 10 solar masses. In 1969, apulsar radiating X-rays and rotating 33 times persecond was discovered at the center of thenebula, making the Crab Nebula a very powerfulsource of radiation.

The explosion that marks theend of a supergiant's life occurs

because the star's extremely heavycore has become incapable ofsupporting its own gravity any longer.In the absence of fusion in its interior,the star falls in upon itself, expelling

its remaining gases, which will expandand shine for hundreds—or eventhousands—of years. The explosion ofthe star injects new material intointerstellar space and contributesheavy atoms that can give rise to newgenerations of stars.

The Twilight of a Star

SupergiantThe diameter of the star mayincrease to more than 1,000times that of the Sun. Throughnuclear fusion, the star canproduce elements even heavierthan carbon and oxygen.

When a star's iron coreincreases in density to 1.44solar masses, the star canno longer support its ownweight and it collapses

upon itself. The resultingexplosion causes theformation of elements thatare heavier than iron, suchas gold and uranium.

Other Elements

CoreA star's core can be seen tobe separated into distinctlayers that correspond tothe different elementscreated during nuclearfusion. The lastelement createdbefore the star'scollapse is iron.

DENSECORE

CRAB NEBULA

ExplosionThe star's life ends in an immenseexplosion. During the weeksfollowing the explosion, greatquantities of energy are radiatedthat are sometimes greater than theenergy emitted by the star's parentgalaxy. A supernova may illuminateits galaxy for weeks.

ETA CARINAESUPERMASSIVE The mass of Eta Carinaeis 100 times greaterthan that of the Sun.Astronomers believethat Eta Carinae isabout to explode,but no one knowswhen.

The image at left shows a sector ofthe Large Magellanic Cloud, anirregular galaxy located 170,000 light-

years from the Earth, depicted beforethe explosion of supernova 1987A. Theimage at right shows the supernova.

BEFORE AND AFTER

FEBRUARY 23, 1987After the supernovaexplosion, increasedbrightness isobserved in theregion near the star.

FEBRUARY 22,1987This star is in its lastmoments of life. Because itis very massive, it will endits life in an explosion. Thegalaxy exhibits only its usualluminosity.

LIFE CYCLE OFA STAR1

2

3 4

5

6

7

23 4

6

7

5


The Final DarknessT

he last stage in the evolution of a star's core is itstransformation into a very dense, compact stellar body.Its particulars depend upon the amount of mass

involved in its collapse. The largest stars become blackholes, their density so great that their gravitationalforces capture even light. The only way to detect thesedead stars is by searching for the effects of theirgravitation.

Discovery of Black HolesThe only way of detecting the presence ofa black hole in space is by its effect on

neighboring stars. Since the gravitational forceexerted by a black hole is so powerful, the gasesof nearby stars are absorbed at great speed,spiraling toward the black hole and forming astructure called an accretion disk. The frictionof the gases heats them until they shinebrightly. The hottest parts of the accretion diskmay reach 100,000,000° C and are a source of

X-rays. The black hole, by exerting suchpowerful gravitational force, attracts everythingthat passes close to it, letting nothing escape.Since even light is not exempt from thisphenomenon, black holes are opaque andinvisible to even the most advanced telescopes.Some astronomers believe thatsupermassive black holes might havea mass of millions, or evenbillions, of solar masses.

When a star's initial mass is between10 and 20 solar masses, its final mass

will be larger than the mass of the Sun.Despite losing great quantities of matterduring nuclear reactions, the star finisheswith a very dense core. Because of its intensemagnetic and gravitational fields, a neutronstar can end up as a pulsar. A pulsar is arapidly spinning neutron star that gives off abeam of radio waves or other radiation. Asthe beam sweeps around the object, theradiation is observed in very regular pulses.

tons is what one tablespoon of aneutron star would weigh. Its smalldiameter causes the star to have acompact, dense core accompanied byintense gravitational effects.

1 billion

RED GIANTA red giant leavesthe main sequence.Its diameter is 100times greater thanthe Sun's.

1

SUPERGIANTA supergiant growsand rapidly fusesheavier chemicalelements, formingcarbon, oxygen, andfinally iron.

2

EXPLOSIONThe star's iron corecollapses. Protonsand electronsannihilate each otherand form neutrons.

3

DENSE COREThe core's exactcomposition ispresently unknown.Most of itsinteracting particlesare neutrons.

4

PulsarsThe first pulsar (a neutron star radiatingradio waves) was discovered in 1967.

Pulsars rotate approximately 30 times per secondand have very intense magnetic fields. Pulsarsemit radio waves from their two magnetic poleswhen they rotate. If a pulsar absorbs gas from aneighboring star, a hot spot that radiates X-raysis produced on the pulsar's surface.

Devouring gas froma supergiantLocated within a binary system, the pulsar canfollow the same process as a black hole. The pulsar'sgravitational force causes it to absorb the gas ofsmaller, neighboring stars, heating up the pulsar'ssurface and causing it to emit X-rays.

CURVED SPACE

THE SUN forms a shallowgravitational well.1

A WHITEDWARFgenerates adeepergravitationalwell, drawingin objects at ahigher speed.

2

Accretion Disk An accretion disk is a gaseous accumulationof matter that the black hole draws fromnearby stars. In the regions of the diskvery close to the black hole, X-rays areemitted. The gas that accumulatesrotates at very high speeds. Whenthe gases from other starscollide with the disk, theycreate bright, hot spots.

Bright gases Since the accretion disk is fed by gases

spinning at high speed, it shinesintensely in the region closest to its core

but at its edges is colder and darker.

Rotation axis

Radio-wave beam

Magnetic field

Possiblesolid core

Neutronstar

Strong GravitationalAttractionThe gravitational force of the black hole attractsgases from a neighboring star. This gas forms alarge spiral that swirls faster and faster as it getscloser to the black hole. The gravitation field thatit generates is so strong that it traps objects that

pass close to it.

LIGHT RAYS

BLACK HOLEThe objects that approach the black hole too closely

are swallowed by it. The black hole'sgravitational well is infinite and trapsmatter and light forever. The eventhorizon describes the limit of what is,and is not, absorbed. Any object thatcrosses the event horizon follows a spiralpath into the gravitational well. Somescientists believe in the existence of so-called wormholes—antigravity tunnels,through which travel across the universe is hypothesized to be possible. By takingadvantage of the curvature of space, scientiststhink it could be possible to travel from theEarth to the Moon in a matter of seconds.

4

WORMHOLE

ENTRANCE

EXIT

Neutron Star

STRUCTURE OF A PULSAR

X-RAYSAs gases enter theblack hole, they areheated and emit X-rays.

Total escapeRays of light thatpass far from thecenter of a blackhole continueunaffected.

A NEUTRON STARattracts objects atspeeds approaching halfthe speed of light. Thegravitational well is evenmore pronounced.

3

The theory of relativity suggeststhat gravity is not a force but adistortion of space. This distortioncreates a gravitational well, the

depth of which depends on themass of the object. Objects areattracted to other objects throughthe curvature of space.

Close to the limitSince the rays of lighthave not crossed theevent horizon, theystill retain theirbrightness.

DarknessRays of light thatpass close to thecore of a black holeare trapped.

1

2

3 4

5

7

23

4

6

7

LIFE CYCLE OFA STAR

5

6

Black hole

Neutron star

CROSS SECTION

HOTGASES

ACCRETIONDISK

X-RAYS

BLACKHOLE

LOSS OF MASSToward the end ofits life, a neutronstar loses more than90 percent of itsinitial mass.

Star CitiesThe first galaxies formed 100 million yearsafter the big bang. Billions of these great

conglomerates of stars can be found throughoutspace. The two most important discoveriesconcerning galaxies are attributed to theastronomer Edwin Hubble. In 1926, he pointed outthat the spots, or patches, of light visible in the

night sky were actually distant galaxies. Hubble'sdiscovery put an end to the view held by astronomersat the time that the Milky Way constituted theuniverse. In 1929, as a result of various observations

of the spectrum of light radiated by the stars in thegalaxies, Hubble noted that the light from the galaxiesshowed a redshift (Doppler effect). This effectindicated that the galaxies were moving away fromthe Milky Way Galaxy. Hubble concluded that the

32 WHAT IS IN THE UNIVERSE? UNIVERSE 33

Galactic ClustersGalaxies are objects that tend to form groups or clusters.Acting in response to gravitational force, they can form

clusters of galaxies of anywhere from two to thousands ofgalaxies. These clusters have various shapes and are thought toexpand when they join together. The Hercules cluster, shown here,was discovered by Edmond Halley in 1714 and is locatedapproximately 25,100 light-years from Earth. Each dot representsa galaxy that includes billions of stars.

Anatomy of Galaxies

COLLISION300 million light-yearsfrom the Earth, thesetwo colliding galaxiesform a pair. Togetherthey are called “TheMice” for the large tailof stars emanatingfrom each galaxy. Withtime, these galaxies willfuse into a single, largerone. It is believed thatin the future theuniverse will consist ofa few giant stars.

MILKY WAYSeen from its side, the Milky Way looks like aflattened disk, swollen at the center. Aroundthe disk is a spherical region, called a halo,containing dark matter and globular clustersof stars. From June to September, the MilkyWay is especially bright, something thatwould make it more visible viewed from abovethan from the side.

1.2 BILLIONYEARS ago, the Antennae(NGC 4038 andNGC 4039) weretwo separatespiral galaxies.

1300 MILLIONYEARS later, thegalaxiescollided atgreat speed.

2300 MILLIONYEARS go by until thecollision takesplace and theshapes of thegalaxies aredistorted.

3300 MILLIONYEARS later, thestars in thespiral armsare expelledfrom bothgalaxies.

4NOW two jets ofexpelledstars stretchfar from theoriginalgalaxies.

5

universe is expanding. But the expansion of theuniverse does not imply that galaxies are growing innumbers. On the contrary, galaxies can collide and

merge. When two galaxies collide, they can distorteach other in various ways. Over time, there are fewerand fewer galaxies. Some galaxies exhibit very peculiar

ELLIPTICALThese galaxies are elliptical inshape and have little dust andgas. Their masses fall within awide range.

SPIRALIn a spiral galaxy, a nucleus ofold stars is surrounded by a flatdisk of stars and two or morespiral arms.

Galaxies are subdivided intodifferent categories according totheir tendency toward roundshape (in the case of ellipticalgalaxies), as well as by thepresence of an axis and the lengthof their arms (in the case of spiral

and barred spiral galaxies). An E0galaxy is elliptical but almostcircular, and an E7 galaxy is aflattened oval. An Sa galaxy has alarge central axis and coiled arms,and an Sc galaxy has a thinner axisand more extended arms.

IRREGULARIrregular galaxies have no definedshape and cannot be classified.They contain a large amount ofgases and dust clouds.

ngc 4676

CLASSIFYING GALAXIES ACCORDING TO HUBBLE

SUBCLASSIFICATIONS

ngc 6205 hercules

shapes. The Sombrero Galaxy, shown in the center ofthe page, has a bright white core surrounded by thinspiral arms.

Galaxies are rotating groups of stars,gas, and dust. More than 200 years ago,philosopher Immanuel Kant postulated

that nebulae were island-universes of distantstars. Even though astronomers now know thatgalaxies are held together by gravitational force,they have not been able to decipher what reasonsmight be behind galaxies' many shapes. The varioustypes of galaxies range from ovals of old stars to spiralswith arms of young stars and bright gases. The center of agalaxy has the greatest accumulation of stars. The Milky WayGalaxy is now known to be so big that rays of light, which travel at186,000 miles (300,000 km) per second, take 100,000 years to crossfrom one end to the other.

1

Asmall number of galaxies differ from the rest by emitting high amounts ofenergy. The energy emission might be caused by the presence of black holes inits core that were formed through the gravitational collapse accompanying the

death of supermassive stars. During their first billion years, the galaxies might haveaccumulated surrounding gaseous disks with their corresponding emissions of radiation. Itis possible that the cores of the first galaxies are the quasars that are now observed at verygreat distances.

The Forceof GravityGravitational force begins tounite vast quantities of hot,gaseous clouds. The cloudsattract one another and collide,forming stars. A large amount ofgas accumulates at the center ofthe galaxy, intensifyinggravitational forces until amassive black hole comes intobeing in the galaxy's core.

2The Quasarin the CoreThe quasar in the core ejects twojets of particles that reachspeeds approaching the speed oflight. The quasar stage is thoughtto have been the most violentstage in the formation ofgalaxies. The gases and starsarising from the jets areintroduced as spirals into theblack hole, forming a type ofaccretion disk known as a quasar.

4Stable GalaxyNine billion years after its formation, with asupermassive black hole at its core, the galaxydrastically slows its energetic activity, forming a low-energy core. The stabilization of the galaxy allowedthe formation of stars and other heavenly bodies.

Active Galaxies

Astronomers believe that active galaxies area direct legacy from the beginning of the

universe. After the big bang, these galaxies wouldhave retained very energetic levels of radiation.Quasars, the brightest and most ancient objects inthe universe, make up the core of this type ofgalaxy. In some cases, they emit X-rays or radiowaves. The existence of this high-energy activityhelps support the theory that galaxies could be

born from a supermassive black hole with aquasar that became inactive as stars formed andit was left without gas to feed it. This processof formation might be common to manygalaxies. Today quasars represent thelimit of what it is possible to see,even with specializedtelescopes. Quasars aresmall, dense, and bright.

Energetic Activity

A theory of galaxy formationassociated with active galaxies

holds that many galaxies, possiblyincluding the Milky Way, were formedfrom the gradual calming of a quasarat their core. As the surroundinggases consolidated in the formation ofstars, the quasars, having no moregases to absorb, lost their energetic

fury and became inactive. Accordingto this theory, there is a naturalprogression from quasars to activegalaxies to the common galaxies oftoday. In 1994, astronomers studyingthe center of the Milky Waydiscovered a region that may containa black hole and could be left overfrom early galactic activity.

Galaxy Formation

GASAs two jets are expelled from

the core, radio waves areemitted. If the waves

collide with clouds ofintergalactic gas, they

swell and formgigantic clouds that

can emit radiowaves or X-rays.

CENTRAL RINGThe core of an activegalaxy is obscuredby a ring of dustand gas that isdark on theoutside andbright within.It is a powerfulsource ofenergy.

GASEOUS CLOUDSGaseous clouds appeared from thegravitational collapse of immense massesof gas during the early stages of the

universe. Later, in the clouds' interior,stars began to form.

3Black HoleA black hole swallows the gas that begins tosurround it. A hot, gaseous spiral forms,emitting high-speed jets. The magnetic fieldpours charged particles into the region aroundthe black hole, and the exterior of the diskabsorbs interstellar gas.


The classification of an active galaxy dependsupon its distance from Earth and the perspectivefrom which it is seen. Quasars, radio galaxies, andblazars are members of the same family of objectsand differ only in the way they are perceived.

CLASSIFICATION

QUASARS The mostpowerful objects in theuniverse, quasars are so distantfrom Earth that they appear to usas diffuse stars. They are thebright cores of remote galaxies.

RADIO GALAXIES Radio galaxies arethe largest objects in the universe. Jets ofgases come out from their centers thatextend thousands of light-years. The coresof radio galaxies cannot be seen.

BLAZARS Blazars may beactive galaxies with jets of gasthat are aimed directly towardEarth. The brightness of a blazarvaries from day to day.

PARTICLESejected from the black holehave intense magnetic fields.The jets of particles travel at

speeds approaching the speed oflight when they leave the core.

Dark clouds of gasand dust on theouter edge of ablack hole aregraduallyswallowed up.

ACCRETIONDISKFormed by interstellargas and star remnants,the accretion disk canradiate X-rays because ofthe extreme temperature ofits center.

100MILLION DEGREESCelsius is the temperature that thecore of a black hole can reach.

4 The center of theblack hole radiatescharged particles.

3 The stronggravitational force ofthe disk attracts anddestroys stars.

2 As the gasesmove inward,their temperatureincreases.

1

INCREASINGGRAVITY


Stellar MetropolisF

or a long time, our galaxy (called the Milky Way because ofits resemblance to a stream of milk in the night sky) was atrue enigma. It was Galileo Galilei who, in 1610, first pointed

a telescope at the Milky Way and saw that the weak whitish stripwas composed of thousands and thousands of stars that appearedto almost touch each other. Little by little, astronomers began torealize that all these stars, like our own Sun, were part of the enormousensemble—the galaxy that is our stellar metropolis.

GASES SWIRLoutward because of forces in theSagittarius A region. Because thegas rotates at high speed butremains concentrated, it could betrapped by gravitational forcesexerted by a black hole.

BRIGHT STARSBright stars are

born from gas thatis not absorbed by

the black hole. Mostof them are young.

Central RegionBecause the Milky Way is full of clouds of dust and rockparticles, its center cannot be seen from outside the galaxy.The Milky Way's center can be seen only through telescopesthat record infrared light, radio waves, or X-rays, which canpass through the material that blocks visible light. Thecentral axis of the Milky Way contains ancient stars, some 14billion years old, and exhibits intense activity within itsinterior, where two clouds of hot gas have been found:Sagittarius A and B. In the central region, but outside thecore, a giant dark cloud contains 70 different types ofmolecules. These gas clouds are associated with violentactivity in the center of our galaxy and contain the heart ofthe Milky Way within their depths. In general, the stars inthis region are cold and range in color from red to orange.

The core of the Milky Way galaxy is markedby very intense radio-wave activity thatmight be produced by an accretion diskmade up of incandescent gas surrounding amassive black hole. The region of SagittariusA, discovered in 1994, is a gas ring thatrotates at very high speed, swirling withinseveral light-years of the center of the

galaxy. The speed of its rotation is anindication of the powerful gravitationalforce exerted from the center of the MilkyWay, a force stronger than would beproduced by the stars located in the region.The hot, blue stars that shine in the centerof the Milky Way may have been born fromgas not yet absorbed by the black hole.

The Exact Center

The brightest portion of the Milky Waythat appears in photographs taken with

optical lenses (using visible light) is in theconstellation Sagittarius, which appears to lie inthe direction of the center of the Milky Way. Thebright band in the nighttime sky is made up ofstars so numerous that it is almost impossible tocount them. In some cases, stars are obscuredby dense dust clouds that make some regions of

the Milky Way seem truly dark. The objects thatcan be found in the Milky Way are not all of onetype. Some, such as those known as the halopopulation, are old and are distributed within asphere around the galaxy. Other objects form amore flattened structure called the diskpopulation. In the spiral arm population, we findthe youngest objects in the Milky Way. In thesearms, gas and interstellar dust abound.

A Diverse Galaxy

ROTATIONThe speeds of the rotation of the various parts of theMilky Way vary according to those parts' distances fromthe core of the galaxy. The greatest number of stars isconcentrated in the region between the Milky Way'score and its border. Here the speed of rotation is muchgreater because of the attraction that the objects in thisregion feel from the billions of stars within it.

THE MILKY WAY IN VISIBLE LIGHT

THE CONSTELLATIONSAGITTARIUSClose to the center ofthe Milky Way,Sagittarius shinesintensely.

SECTORS Many differentsectors make up theMilky Way.

STARSSo many

starscompose the

Milky Way that itis impossible for us

to distinguish them all.

DARK REGIONSDark regions are

produced by dense clouds that obscure

the light of stars.

HOT GASESThe hot gases originating

from the surface of thecentral region may be the

result of violent explosionsin the accretion disk.

BLACK HOLEMany astronomers believe that

a black hole occupies thecenter of the Milky Way. Its

strong gravitational forcewould trap gases inorbit around it.

MAGNETISMThe center of the MilkyWay is surrounded bystrong magnetic fields,perhaps from a rotatingblack hole.

SAGITTARIUS B2The largest dark cloud inthe central region of theMilky Way, Sagittarius B2contains enough alcohol tocover the entire Earth.

OUTER RINGA ring of dark clouds of dust andmolecules that is expanding as aresult of a giant explosion. It issuspected that a small object inthe central region of the MilkyWay might be its source.

AndromedaGalaxy

MILKY WAY

The Milky Way, containing more than 100 billion stars, has two spiral armsrotating around its core. The Sagittarius arm, located between the Orion arm

and the center of the Milky Way, holds one of the most luminous stars in the galaxy,Eta Carinae. The Perseus arm, the main outer arm of the Milky Way, contains youngstars and nebulae. The Orion arm, extending between Perseus and Sagittarius,houses the solar system within its inner border. The Orion arm of the MilkyWay is a veritable star factory, where gaseousinterstellar material can give birth tobillions of stars. Remnants ofstars can also be foundwithin it.

100,000 LIGHT-YEARSThe diameter of the Milky Way islarge in comparison with othergalaxies but not gigantic.

Centralprotuberance

TriangleGalaxy

EagleNebula

Cassiopeia A

EtaCarinae

6,000 light-years

CrabNebula

OrionNebula

Large Magellanic Cloud

Small MagellanicCloud

Structure of the Milky Way

300

600

900

1200

1500

1800

2100

2400

2700

360000

SOLAR SYSTEM

120 miles per hour (200 km/h)




PLUTO: NOW A DWARF 58-59

DISTANT WORLDS 60-61

CONSTRUCTION DEBRIS:ASTEROIDS AND METEORITES 62-63

THOSE WITH A TAIL 64-65

The Solar System

Among the millions andmillions of stars that form theMilky Way galaxy, there is amedium-sized one located inone of the galaxy's arms—the

Sun. To ancient peoples, the Sun was agod; to us, it is the central source ofenergy that generates heat, helping lifeexist. This star, together with the planetsand other bodies that spin in orbits

around it, make up the solar system,which formed about 4.6 billion years ago.The planets that rotate around it do notproduce their own light. Instead, theyreflect sunlight. After the Earth, Mars is

the most explored planet. Here we see aphoto of Olympus Mons, the largestvolcano in the entire solar system. It isalmost two-and-a-half times as high as thetallest peak on the Earth, Mount Everest.

JUPITER, GAS GIANT 50-51

THE LORD OF THE RINGS 52-53

URANUS WITHOUT SECRETS 54-55

NEPTUNE: DEEP BLUE 56-57

ATTRACTED BY A STAR 40-41

A VERY WARM HEART 42-43

MERCURY, AN INFERNO 44-45

VENUS, OUR NEIGHBOR 46-47

RED AND FASCINATING 48-49

OLYMPUS MONS, ON MARSOlympus Mons is the largestvolcano of the solar system. Itis about two-and-a-half timesas high as Mount Everest.

Outer PlanetsPlanets located outside the asteroid belt. They are enormous gas spheres withsmall solid cores. They have very low temperatures because of their great

distance from the Sun. The presence of ring systems is exclusive to these planets. Thegreatest of them is Jupiter: 1,300 Earths could fit inside of it. Its mass is 2.5 times asgreat as that of the rest of the planets combined.

Inner PlanetsPlanets located inside the asteroidbelt. They are solid bodies in which

internal geologic phenomena, such asvolcanism, which can modify their surfaces,are produced. Almost all of them have anappreciable atmosphere of some degree ofthickness, according to individualcircumstances, which plays a key role inthe surface temperatures of each planet.

Asteroid BeltThe border between the outer and innerplanets is marked by millions of rocky

fragments of various sizes that form a bandcalled the asteroid belt. Their orbits areinfluenced by the gravitational pull exerted onthem by the giant planet Jupiter. This effect alsokeeps them from merging and forming a planet.

ORIGINRemains from theformation of the Suncreated a disk of gasand dust around it,from which theplanetesimals formed.

Early ideas suggested that the planets formed gradually,beginning with the binding of hot dust particles. Todayscientists suggest that the planets originated from thecollision and melding of larger-sized bodies calledplanetesimals.

1

COLLISIONThrough collisionsamong themselves,planetesimals ofdifferent sizes joinedtogether to becomemore massive objects.

2

HEATThe collisionsproduced a largeamount of heat thataccumulated in theinterior of the planets,according to theirdistance from the Sun.

3

BUILDING PLANETS

ORBITS

NEPTUNEDIAMETER

MOONS

30,775 MILES (49,528 KM)

13

URANUSDIAMETER

MOONS

31,763 MILES (51,118 KM)

27

SATURNDIAMETER

MOONS

74,898 MILES (120,536 KM)

50+

JUPITERDIAMETER

MOONS

88,846 MILES (142,984 KM)

60+

S U N

EARTHDIAMETER

MOONS

7,926 MILES

(12,756 KM)

1

VENUSDIAMETER

MOONS

7,520 MILES

(12,103 KM)

0

Venus'sorbit

Jupiter'sorbit

Saturn'sorbit

Uranus'sorbit

Neptune'sorbit

Earth'sorbit

Mercury'sorbit

Mars'sorbit

Mainbelt

In general, theplanets orbit in onecommon planecalled the elliptic.

The rotation of most planets around their ownaxes is in counterclockwise direction. Venus andUranus, however, revolve clockwise.

Triton

Titania Oberon

Titan Rhea Iapetus Tethys

Ganymede Callisto Io Europa

Umbriel Ariel Miranda Puck

Nereid

MOON

Phobos Deimos

SOLARGRAVITY

MARSDIAMETER

MOONS

4,217 MILES

(6,786 KM)

2

DIAMETER

MOONS

3,031 MILES

(4,878 KM)

0

MERCURY

Proteus

Planets and their satellites, asteroids and other rockyobjects, and an incalculable number of cometlike objects,some more than 1 trillion miles (1.6 trillion km) from the

Sun, make up the solar system. In the 17th century, astronomerJohannes Kepler proposed a model to interpret the dynamicproperties of the bodies of the solar system. According to this

interpretation, the planets complete elliptical trajectories, calledorbits, around the Sun. In every case, the movement is producedby the influence of the gravitational field of the Sun. Today, aspart of a rapidly developing field of astronomy, it is known thatplanet or planetlike bodies also orbit other stars.

Attracted by a Star

40 THE SOLAR SYSTEM UNIVERSE 41

The gravitational pull of theSun upon the planets notonly keeps them insidethe solar system butalso influences thespeed with whichthey revolve in theirorbits around the Sun.Those closest to the Sunrevolve in their orbits muchfaster than those fartherfrom it.


A Very Warm Heart

SUNSPOTSare regions of gases that aregenerally colder (7,232° F [4,000°C]) than the photosphere (10,112° F[5,600° C]). For that reason, theyappear dark.

MACROSPICULESThis type of vertical eruption is similar to aspicule, but it usually reaches up to 25,000miles (40,000 km) in height.

COREThe core occupies only 2 percentof the total volume of the Sun,but in it is concentrated abouthalf the total mass of the Sun.The great pressures andtemperatures in the core producethermonuclear fusion.

27,000,000º F(15,000,000º C)

Very GassyThe Sun is a giant ball of gases with very high density andtemperature. Its main components are hydrogen (90%) and

helium (9%). The balance of its mass is made up of trace elements,such as carbon, nitrogen, and oxygen, among others. Because of theconditions of extreme temperature and pressure on the Sun, theseelements are in a plasma state.

Surface and AtmosphereThe visible portion of the Sun is a sphere oflight, or photosphere, made of boiling gases

emanating from the solar core. The gas flares formplasma, which passes through this layer. Later thegas flares enter a vast gas layer called the solaratmosphere. The density of this layer decreases

toward its outermost region. Above thephotosphere lies the solar atmosphere—thechromosphere and the corona. The energygenerated at the core moves through the surface ofthe photosphere and solar atmosphere forthousands of years in search of an exit into space.

CONVENTIONALPLANETSYMBOL

*In both cases, Earth = 1

Equatorialdiameter

Averagetemperature

Density

Mass*Gravity*

Orbitalspeed

Average distancefrom Earth

93 million miles(150 million km)

864,000 miles(1,391,000 km)

332,900

7,456 miles persecond (12,000 km/s)

28

0.81 ounce per cubicinch (1.4 g per cu cm)

9,932° F(5,500° C)

Atmosphere Dense

Moons None

CHARACTERISTICS

NUCLEAR COLLISIONTwo hydrogen nuclei (twoprotons) collide and remainjoined. One changes into aneutron, and deuteriumforms, releasing a neutrino,a positron, and a lot of energy.

1.

PHOTOSPHERE

The visible surface of the Sun, a boiling tide,is thick with gases in a plasma state. In itsuppermost layer, its density decreases and itstransparency increases, and the solar radiationescapes from the Sun as light. Thespectrographic study of this layer has allowedscientists to confirm that the main components ofthe Sun are hydrogen and helium.

CHROMOSPHERE

Above the photosphere, and of less density,lies the chromosphere, a layer 3,110 miles(5,000 km) thick. Its temperature rangesfrom 8,100° F (4,500° C) to 900,000° F(500,000° C) with increasing altitude. Thetemperature of the corona can reach1,800,000° F (1,000,000° C).

CORONALocated above thechromosphere, it extendsmillions of miles into space andreaches temperatures nearing1,800,000° F (1,000,000° C).It has some holes, or low-density regions, through whichgases flow into the solar wind.

MAXIMUM TEMPERATUREOF THE CHROMOSPHERE

THE TEMPERATURE IN THE CORONA

900,000º F(500,000º C)

1,800,000º F(1,000,000º C)

SOLAR FLARESThese eruptions come out of the solaratmosphere and can interfere with radiocommunications on Earth.

SOLAR WINDConsists of a flux of ions emitted by the solaratmosphere. The composition is similar to that ofthe corona. The Sun loses approximately 1,800pounds (800 kg) of matter per second in theform of solar wind.

SPICULESVertical jets of gas that spew from the chromosphere, usuallyreaching 6,200 miles (10,000 km) in height. They originate inupper convection cells and can rise as high as the corona.

SOLAR PROMINENCESClouds and layers of gas fromthe chromosphere travelthousands of miles until theyreach the corona, where theinfluence of magnetic fieldscauses them to take on theshape of an arc or wave.

T he Sun at the center of the solar system is a source of light andheat. This energy is produced by the fusion of atomic hydrogen nuclei,which generate helium nuclei. The energy that emanates from the

Sun travels through space and initially encounters the bodies thatpopulate the solar system. The Sun shines thanks to thermonuclearfusion, and it will continue to shine until its supply of hydrogenruns out in about six or seven billion years.

UMBRACentral region.It is thecoldest anddarkestpart.

ESSENTIAL DATA

10,112º F(5,600º C)

PENUMBRAPeripheral region. It isthe hottest and brightestpart of the Sun.

PHOTONSThe deuterium formedcollides with a proton.This collision releases onephoton and gamma rays.The high-energy photonneeds 30,000 years toreach the photosphere.

2.

Proton

Neutrino

Photon

Positron

Neutron

Deuterium

Proton 2Proton 1

Deuterium 2

Deuterium 1

HELIUMNUCLEUS

CONVECTIVE ZONEextends from the base of thephotosphere down to a depth of15 percent of the solar radius.Here energy is transported uptoward the surface by gascurrents (through convection).

3. HELIUM NUCLEIThe group of two protonsand a neutron collides withanother such group. Ahelium nucleus forms, and apair of protons is released.

NUCLEAR FUSION OF HYDROGENThe extraordinary temperature ofthe nuclear core helps the hydrogennuclei join. Under conditions oflower energy, they repel each other,but the conditions at the center ofthe Sun can overcome the repulsiveforces, and nuclear fusion occurs.For every four hydrogen nuclei, aseries of nuclear reactions produceone helium nucleus.

RADIATIVE ZONEThis portion of the Sun istraversed by particles comingfrom the core. A proton can takea million years to cross this zone.

14,400,000º F(8,000,000º C)

A Scar-Covered SurfaceThe surface of Mercury is very similar to that of the Moon. It is possible to findcraters of varying sizes. The largest one has a diameter of some 810 miles (1,300

km). There are also hills and valleys. In 1991, radio telescopes were able to detectpossible evidence of the presence of frozen water in Mercury's polar regions,information that Mariner 10 had been unable to gather. Mariner 10, the onlymission sent to Mercury, flew by the planet three times between 1974 and 1975.The polar ice was found at the bottom of very deep craters, which limit theice's exposure to the Sun's rays. The spacecraft Messenger, launched in2004, is scheduled to orbit the planet Mercury in 2011 and is expected toprovide new information about Mercury's surface and magnetic field.

UNIVERSE 4544 THE SOLAR SYSTEM

Like the Earth, Mercury has a magnetic field, although a muchweaker one. The magnetism results from its enormous core

made of solid iron. The mantle that surrounds the core is believed tobe a fine layer of iron and sulfur.

Composition and Magnetic Field

Mercury rotates slowly on its axis and takes approximately 59Earth days to complete a turn, but it only needs 88 days to

travel in its orbit. To an observer in Mercury, these two combinedmotions would give a combined interval of 176 days between two

sunrises. A person observing the sunrise from position 1 would have towait for the planet to make two orbits around the Sun and make threerotations on its own axis before seeing the next sunrise.

Rotation and Orbit

Baked by its neighbor the Sun, Mercury is the planet withthe greatest thermal fluctuations in the solar system. Itsaverage temperature is 333° F (167° C), but when it getscloser to the Sun, the temperature can climb to 842° F(450° C). At night, it drops to -297° F (-183° C).

COREDense, large, and madeof iron, its diametermay be as great as2,240 to 2,300 miles(3,600-3,800 km).

MANTLEMade up mostly ofsilica-based rocks

BEETHOVENis the second largest crater onMercury. It is 400 miles (643km) in diameter. Its floor wasflooded by lava and latermarked by meteorite impacts.

The craterwasfloodedwith lava.

When the projectile that formed thecrater struck, Mercury was stillforming. The extensive waves thatextended from the site of impactformed hills and mountains ranges.

29%Sodium

6%Helium

43%Others

22%Hydrogen

CRUSTMade of silicate rocks,Mercury's crust is similarto the crust and mantleof the Earth. It has athickness of 310 to 370miles (500-600 km).

Mercury, an InfernoM ercury is the planet nearest to the Sun and is therefore the one that has to

withstand the harshest of the Sun's effects. Due to its proximity to the Sun, Mercurymoves at great speed in its solar orbit, completing an orbit every 88 days. It has almost no

atmosphere, and its surface is dry and rugged, covered with craters caused by the impact ofnumerous meteorites; this makes it resemble the Moon. Numerous faults, formed during the cooling ofthe planet when it was young, are also visible on the surface. Constantly baked by its neighbor, theSun, Mercury has an average surface temperature of 333° F (167° C).

CONVENTIONALPLANET SYMBOL

ESSENTIAL DATA


One rotationlasts 59 days.

Equatorialdiameter

Average temperature

Solar orbit(Mercurian year)

Density

Mass*Gravity*

Orbital speed

Average distancefrom the Sun

36,000,000 miles(57,900,000 km)

3,032 miles(4,880 km)

0.06

29.75 miles persecond (47.87 km/s)

0.38

3.14 ounces per cubicinch (5.43 g/cu cm)

332° F (167° C)

Atmosphere Almost nonexistent

Lunas

88 days00 hours

0.1°

EXTREMELY THIN ATMOSPHEREMercury's atmosphere is almost nonexistent and consistsof a very thin layer that cannot protect the planet eitherfrom the Sun or from meteorites. During the day, whenMercury is closer to the Sun, the planet's temperature cansurpass 842° F (450° C). At night, temperatures canplummet to -297° F (-183° C).

During theday, the Sundirectly heatsthe rock.

During the night, theheat of Mercury'srocks is lost rapidly,and the planet'stemperature drops.

Mariner 10

2,240

miles

(3,60

0 K

m)

310 m

iles

(500

Km

)

Messenger

The probe will passby Mercury twice in2008 and onceagain in 2009before beginning toorbit the planet.

CALORIS CRATERThe largest impact crater in thesolar system, it has a diameterof 810 miles (1,300 km).

The space probe Mariner 10 was the first to reach Mercury. Between1974 and 1975, the craft flew by the planet three times and came

within about 200 miles (320 km) of the surface. Messenger, a space probescheduled to study Mercury between 2008 and 2011, was launched in 2004.

Missions to Mercury

SUN1

23

4

56 7 Rises and

increases its size2Recedesa bit4 Stops

again5

Resumes its originalpath toward thehorizon

Each numbercorresponds to a positionof the Sun in the sky asseen from Mercury.

6

Reaches the zenith(noon) and stops3

The Sunrises.1Decreases toward

the sunset7

HORIZON OF MERCURY

VIEW FROMMERCURY

ORBIT OF MERCURYAROUND THE SUN

AXIS INCLINATION333º F(167º C)

-297º F(-183º C)

883º F (473º C)

CHARACTERISTICS


SurfaceThe Venusian surface has not remained the same throughout its life. Thecurrent one is some 500 million years old, but the rocky landscape visibletoday was formed by intense volcanic activity. Volcanic rock covers 85

percent of the planet. The entire planet is crisscrossed by vast plainsand enormous rivers of lava, as well as a number of mountains. The

lava flows have created a great number of grooves, some of whichare very wide. The brightness of Venus's surface is the result of

metallic compounds.

The overwhelming presence of carbon dioxide in theVenusian atmosphere induces a greenhouse effect,

increasing the surface temperature to 864° F (462° C). Becauseof this, Venus is hotter than Mercury, even though Venus is

farther from the Sun and reflects all but 20 percent of the Sun'slight. The surface temperature of Venus is relatively constant,averaging 860° F (460° C). The atmospheric pressure on Venus is90 times greater than that on the Earth.

Composition

VENUS'S PHASES

As Venus revolves around theSun, its solar illumination variesas is seen from the Earthdepending upon its position inrelation to the Sun and theEarth. Thus Venus has phasessimilar to the Moon's. During itselongations, when Venus isfarthest from the Sun in the sky,Venus appears at its brightest.

14,400º F(8,000º C)

864º F(462º C)

COREIt is believed that Venus's core issimilar to the Earth's, containingmetallic elements (iron andnickel) and silicates. Venus has nomagnetic field—possibly becauseof its slow axial rotation.

The surface of Venus isrocky and dry. Most ofthe planet is formed byvolcanic plains andother, elevated regions.

CRUSTMade up ofsilicates, it isthicker than theEarth's crust.

Venus lacks water. A U.S.robot probe sent to Venus in1978 found some evidencethat water vapor could haveexisted in the atmospherehundreds of millions of yearsago, but today no trace ofwater remains.

MANTLEMade of molten rock, itconstitutes most of theplanet. It traps the solarradiation and is between37 and 62 miles (60 and100 km) thick.

GREENHOUSE EFFECTOnly 20 percent of the Sun'slight reaches the surface ofVenus. The thick clouds ofdust, sulfuric acid, and carbondioxide that constitute Venus'satmosphere reflect theremaining light, leaving Venusin permanent darkness.

SOLAR RADIATIONVenus is kept hot by its thickatmosphere, which retains theenergy of the Sun's rays.

INFRARED RAYSThe surface of Venus radiatesinfrared radiation. Only 20 percentof the Sun's rays pass throughVenus's thick clouds of sulfuric acid.

ISHTAR TERRAOne of the raised plateaus of Venus, itis similar in size to Australia and islocated close to Venus's north pole. Ithas four main rocky mountain rangescalled Maxwell Montes, Freyja Montes,Akna Montes, and Dam Montes.

APHRODITE TERRALarger than Ishtar Terra, it is the sizeof South America. Aphrodite Terralies near the equator and consistsmostly of mountainous regions to theeast and west, which are separatedby a low-lying region.

ATMOSPHEREVenus's glowing appearanceis caused by the planet's thick,suffocating atmosphere,which is made up of carbondioxide and sulfuric cloudsthat reflect sunlight.

97%

Carbondioxide

Venus, Our NeighborV enus is the second closest planet to the Sun. Similar in size to the Earth, it

has a volcanic surface, as well as a hostile atmosphere governed by the effects of carbondioxide. Although about four billion years ago the atmospheres of the Earth and Venus were

similar, the mass of Venus's atmosphere today is 100 times greater than the Earth's. Its thickclouds of sulfuric acid and dust are so dense that stars are invisible from the planet's surface.Viewed from the Earth, Venus can be bright enough to be visible during day and second only to themoon in brightness at night. Because of this, the movements of Venus were well-known by mostancient civilizations.


ESSENTIAL DATA


Rotates on itsown axis every243 days

Equatorialdiameter

Average temperature

Solar orbit (Venusian year)

Density

Mass*Gravity*

Orbital speed


67,000,000 miles(108,000,000 km)

7,520 miles(12,100 km)

0.8

22 miles per second(35 km/s)

0.9


860° F (460° C)

Atmosphere Very thick

NoneMoons

224 days17 hours

117°

Nitrogen andtraces ofother gases

3%

MAGELLANVenus was explored by theMagellan spacecraftbetween 1990 and 1994. Theprobe was equipped with aradar system to observe thesurface through its denseatmosphere.

VENUS'S PHASESAS SEEN FROMEARTH

WANINGCRESCENT

LASTQUARTER

WANINGGIBBOUS

WAXINGGIBBOUS

FIRSTQUARTER

WAXINGCRESCENT

SULFURICACID

3,70

0 m

iles

(6,0

00

km

)

3,70

0 m

iles

(6,0

00

km

)

EARTH

THE NEW AND FULLPHASES ARE NOTVISIBLE FROM EARTH.

VENUS

SUN

AXIS INCLINATION

CHARACTERISTICS

50 miles(80 km)IS THE THICKNESS OFTHE ATMOSPHERE.


Red and Fascinating OLYMPUS MONSThis gigantic, inactive volcano is not only thelargest on Mars but also in the solar system.

1965 MARINER 4The first mission sentto Mars, it made onlybrief flyovers.

1969 MARINER 6AND 7 studied thesouthern hemisphereand equator of Mars.

1971 MARINER 9photographed theOlympus volcanofor the first time.

1973 MARS 4, MARS 5,MARS 6, AND MARS 7Russian spacecraftsuccessfully sent to Mars

1976 VIKING 1 AND 2searched for traces of life.They were the first spacecraftto land on Martian soil.

1997 MARSPATHFINDER wasthe third successfulMars landing.

1997 MARS GLOBALSURVEYOR took morethan 100,000 photosof the planet.

2003 MARS EXPRESSOrbiting probe. Firstspacecraft sent by theEuropean Space Agency.

2001 MARS ODYSSEYmapped the mineralogyand morphology ofMars's surface.

2004 SPIRIT ANDOPPORTUNITYsurveyed many squaremiles of the surface.

2006 MARS RECONNAISSANCEORBITER made a detailed studyof the Martian surface whileorbiting the planet.

Because Mars's orbit is moreelliptical than that of Earth,

Mars's distance from the Sun varieswidely. At its perihelion, or closestapproach to the Sun, Mars receives45 percent more solar radiation thanat its aphelion, or farthest point.Temperatures on Mars range from-220° F to 62° F (-140° C to 17° C).

CompositionMars, a rocky planet, has an iron-rich core. Marsis almost half the size of the Earth and has a

similar period of rotation, as well as clearly evidentclouds, winds, and other weather phenomena. Its thinatmosphere is made up of carbon dioxide, and its redcolor comes from its soil, which is rich in iron oxide.

SurfaceIt is a place of geologic extremes, shaped byvolcanic activity, meteorite bombardment,

windstorms, and floods (though there is little or nowater on Mars today). Mountains dominate thesouthern hemisphere, but lowlands are common inthe northern hemisphere.

DIAMETERDISTANCEFROM MARS

9 MILES (15 KM)14,627 MILES(23,540 KM)

DEIMOSPHOBOS

DIAMETER

DISTANCEFROM MARS

17 MILES (27 KM)5,840 MILES(9,400 KM)

Martian Orbit

MoonsMars has two moons, Phobos and Deimos.Both have a lower density than Mars and

are pitted with craters. Phobos has a diameter of17 miles (27 km), and Deimos has a diameter ofnine miles (15 km). Deimos orbits Mars in 30

hours at an altitude of 14,627 miles (23,540 km),and Phobos orbits Mars in eight hours at analtitude of 5,840 miles (9,400 km). Astronomersbelieve that the moons are asteroids thatwere captured by Mars's gravity.

IN SUMMER

62º F(17º C)

IN WINTER

-220º F(-140º C)

SUN

Val

les

Mar

ine

ris

Terra

Sirenum

South Pole

Tharsis Mons

Solis Lacus

OlympusMons

EARTH MARS

OLYMPUS72,200 FEET

(22,000 METERS)

MANTLEIt is made ofmolten rock ofgreater densitythan the Earth'smantle.

CORESmall and likelycomposed of ironCRUST

is thin and made up ofsolid rock. It is 31miles (50 km) thick.

Nitrogen

Carbondioxide

M ars is the fourth planet from the Sun. Of all the planets, Mars mostclosely resembles the Earth. It has polar ice caps, and the tilt of its axis,period of rotation, and internal structure are similar to those of the Earth. Known as the

Red Planet because of the reddish iron oxide that covers its surface, Mars has a thin atmospherecomposed essentially of carbon dioxide. Mars does not have water, though it did in the past, andthere is evidence some water might exist underground. Many spacecraft have been sent to exploreMars, in part because it is the planet other than Earth most likely to have developed some form oflife, and it will probably be the first planet humans leave the Earth to visit.

2,00

0 m

iles

(3,294

km)

1,00

0 m

iles

(1,700

km)

2.6%

95.3%

VALLES MARINERIS

The canyon system of theValles Marineris was likelycaused naturally, primarilyby water erosion.

MISSIONS TO MARSAfter our own Moon, Mars has been a more attractive target forexploratory missions than any other object in the solar system.

ATMOSPHEREThin and continuouslythinning as solar windsdiminish atmosphere


ESSENTIAL DATA

AXIS INCLINATION


One rotationlasts 1.88 years.

Equatorialdiameter

Averagetemperature

Solar orbit (Martian year)

Density

Mass*Gravity*

Orbitalspeed

Averagedistance fromthe Sun

141,600,000 miles

(227,900,000 km)

4,222 miles(6,794 km)

0.107

15 miles persecond (24 km/s)

0.38


-81° F (-63° C)

Atmosphere Very thin

Moons 2

1.88 years

CHARACTERISTICS

25.2°

Oxygen, carbon monoxide,water vapor, and other gases2.1%

EVEREST29,000 FEET(8,848 METERS)

26 302 322 335/8

Jupiter has more than 60 moons. Many ofthem have not been officially confirmed and do

not even have names. Jupiter's rotation is graduallyslowing because of the moons' tidal effects.

LYSITHEA ELARA ANANKE CARME PASIPHAE SINOPEHIMALIALEDA

Its size is similar to thatof the Earth's core.

CORE

CompositionJupiter is a giant ball ofhydrogen and helium

that have been compressedinto liquid in the planet'sinterior and into metallic rockin its core. Not much is knownabout Jupiter's core, but it isbelieved to be bigger than theEarth's core.


Jupiter, Gas Giant

WindsThe winds on Jupiter blow in contiguous bandsand opposing directions. The bands' small

differences in temperature and chemical compositiongive the planet its multicolored appearance. Jupiter'sinclement environment, in which winds blow at morethan 370 miles per hour (600 km/h), can cause largestorms, such as the Great Red Spot in the southernhemisphere of the planet. The Great Red Spot, which is16,155 miles (26,000 km) long, is believed to becomposed mainly of ammonia gas and clouds of ice.

The Moons of Jupiter

Jupiter's magnetic field is 20,000 times strongerthan the Earth's. Astronomers believe the field iscaused by the electrical currents that are createdby the rapid rotation of metallic hydrogen. Jupiter

is surrounded by a huge magnetic bubble, themagnetosphere. The magnetosphere's tail reachesmore than 370,000,000 miles (600,000,000km)—beyond the orbit of Saturn.

JUPITER'S MAGNETISM

Of Jupiter's 63 moons, four are visible from Earth withbinoculars. These are called the Galilean moons in honorof their discoverer, Galileo Galilei. Astronomers believethat Io has active volcanoes and that Europa has anocean underneath its icy crust.

GALILEAN MOONS

RINGSJupiter's rings are made of dust from the planet's fourinner moons. These rings were first seen in 1979 by thespace probe Voyager 1 and later by Voyager 2.

RING MATERIAL

Jupiter's magnetosphere isthe largest object in the solarsystem. It varies in size andshape in response to the solarwind, which is composed ofthe particles continuouslyradiated from the Sun.

surrounds the innerliquid layers and thesolid core. It is 620miles thick (1,000 km).

INNER MANTLESurrounds the core. It is made ofliquid metallic hydrogen, an elementonly found under hot, high-pressureconditions. The inner mantle is asoup of electrons and nuclei.

ATMOSPHEREmeasures 620miles (1,000 km).

OUTER MANTLEMade of liquidmolecular hydrogen.The outer mantlemerges with theatmosphere.

J upiter is the largest planet in the solar system. Its diameter is 11 times thatof the Earth, and its mass is 300 times as great. Because the speed of Jupiter's rotationflattens the planet at its poles, its equatorial diameter is greater than its polar diameter.

Jupiter rotates at 25,000 miles per hour (40,000 km/hr). One of the most distinctive elementsof Jupiter's atmosphere is its so-called Great Red Spot, a giant high-pressure region ofturbulence that has been observed from the Earth for more than 300years. The planet is orbited by numerous satellites and hasa wide, faint ring of particles.

9,00

0 m

iles

(14,0

00

km)

89.8%

10.2%

With tracesof methaneand ammonia

Helium

Hydrogen

ATMOSPHERE

17,00

0 m

iles

(27,00

0 km

)

HALO

MAIN RING

INNER GOSSAMER RING

OUTER GOSSAMER RING

2,000 MILES(3,200 KM)

EUROPA3,270 MILES(5,268 KM)

GANYMEDE

2,986 MILES(4,806 KM)

CALISTO2,264 MILES(3,643 KM)

IO


ESSENTIAL DATA

AXIS INCLINATION


One rotationlasts 9 hoursand 55 minutes.

Equatorialdiameter

Averagetemperature

Solar orbit (Jovian year)

Density

Mass*Gravity*

Orbital speed

Averagedistance fromthe Sun

483,000,000miles

(778,000,000 km)

88,700 miles(142,800 km)

318


2.36

0.77 ounce per cubicinch (1.33 g/cu cm)

-184° F (-120° C)

Atmosphere Very dense

Moons More than 60

11 years312 days

3.1°

METIS

ADRASTEA

GANYMEDE

CALLISTO

THEBEAMALTHEA

1 radius 6 7 8 9 152 3 4 5

EUROPA

IO

160/63/67

Enlargedregion

23,00

0 m

iles

(37,00

0 km

)

54,000º F(30,000º C)

400,000,000miles(650,000,000 km)

16,160miles

(26,000 km)GREAT RED SPOT

CHARACTERISTICS

RADIUS 38,470 MILES(61,911 KM)

RingsSaturn's rings, the brightest rings in the solarsystem, are made of rock and ice and orbit

Saturn's equator. The rings are probably remains ofdestroyed comets that were trapped by Saturn'sgravitational field.


WINDSSaturn's winds generallyreach speeds of about 220miles per hour (360 km/h),causing strong storms.

COMPONENTSThe maincomponents ofSaturn's atmosphereare hydrogen (97%)and helium (2%).The rest is composedof sulfur, methane,and other gases.

ATMOSPHEREMainly hydrogenand helium

OUTER MANTLEThis layer is formedby liquid molecularhydrogen.

INNER MANTLEIt is made up of liquidmetallic hydrogen.

21,600ºF(12,000º C)

COREComposed of rock andmetallic elements, such assilicates and iron

ENCKE DIVISIONA small gap that separatesring A into two parts

RINGS GAND E

CASSINI DIVISION3,100 miles (5,000 km) wide, it islocated between the A and B rings.

THICKNESS AND WIDTHAlthough Saturn'srings are verywide, theirthickness is sometimes lessthan 33 feet (10 m).

Titan has a largerdiameter thanMercury. It has anatmosphere that ismostly made ofnitrogen.

D RINGThe closest ring to thesurface of Saturn—sonear that it almosttouches the planet

C RINGSaturn's onlytransparentring

S aturn is the solar system's second largest planet. Like Jupiter, it is a largeball of gas surrounding a small, solid core. Saturn was the most distant planetdiscovered before the invention of the telescope. To the naked eye, it looks like a yellowish star, but

with the help of a telescope, its rings are clearly visible. Ten times farther from the Sun than the Earth,Saturn is the least dense planet. If an ocean could be found large enough to hold it, Saturn would float.

Gaseous ExteriorSaturn and Jupiter differ very little in their composition. Bothare gaseous balls surrounding solid cores. What sets Saturn

apart are its rings, formed by clustered pieces of ice that range insize from small particles to large chunks. Each particle in a ring is asatellite orbiting Saturn. From the Earth, the massed debris seemsto form large structures, but each discrete piece actually has itsown orbit.

SurfaceLike Jupiter, Saturnhas a surface of clouds

that form bands because ofthe planet's rotation.Saturn's clouds are lessturbulent and less colorfulthan Jupiter's. The higher,white clouds reachtemperatures of -220° F(-140° C). A layer of hazeextends above the clouds. <1% 2% 97%

HydrogenHelium

ATMOSPHERE

DEEP ANDORANGE CLOUDS

WHITECLOUDS

HAZE

BLUISHCLOUDS

15,5

00

mile

s(2

5,0

00

km

)

9,30

0 m

iles

(15,

00

0 k

m)

9,30

0 m

iles

(15,

00

0 k

m)

310 miles(500 km) 9,100 miles(14,600 km)

15,800 miles(25,500 km)

10,900 miles(17,500 km)5,300 miles(8,500 km) 2,200 miles(3,500 km)

3,200 MILES(5,150 KM)DIAMETER

B RINGSaturn'sbrightest andwidest ring

F RING The farthestvisible ring

A RINGSaturn'souter ring

DAPHNIS

PAN

RADIUS =37,500 MILES(60,300 KM)

PROMETHEUS

PANDORA

MIMAS

METHONE PALLENE

ATLAS

1 radius

25 61 220

6 7 8 202 3 4 5

The Moons of SaturnSaturn has more than 45 moons, making Saturn'sfamily of moons one of the largest in the solar system.

The sizes of the moons vary from Titan's 3,200 miles (5,150 km)to tiny Calypso's 10 miles (16 km).

EPIMETHEUSJANUS

ENCELADUS POLYDEUCES

RHEA TITANTETHYSTELESTOCALYPSO

DIONE HELENE

IAPETUSHYPERION PHOEBE


ESSENTIAL DATA

AXIS INCLINATION



Equatorialdiameter

Average temperature

Solar orbit(Saturnine year)

Density

Mass* Gravity*

Orbital speed


887,000,000 miles(1,427,000,000 km)

74,940 miles(120,600 km)

95

6 miles per second(10 km/s)

0.92

0.4 ounce per cubicinch (0.7 g/cu cm)

-193° F (-125° C)

Atmosphere Very dense

Moons More than 45

29 years154 days

26.7°

The Lord of the Rings

Enlargedregion

Sulfur gives it ayellowish appearance.

CHARACTERISTICS


Uranus Without Secrets

CompositionUranus's core is made ofabundant amounts of silicates

and ice. The planet is almost fourtimes larger than the Earth, and itsatmosphere is made up of hydrogen,helium, and methane. Uranus has analmost horizontal tilt, causing it tohave very long seasons.

Uranus has small, dark moons, discovered by Voyager 2, as well as biggermoons, such as Miranda, Ariel, Umbriel, Oberon, and Titania. These lasttwo are approximately 930 miles (1,500 km) in diameter.

Miranda, only 293 miles(472 km) in diameter, isthe smallest of Uranus'sfive main moons. It hasan irregular surfacewith grooves and abright mark.

MOONS

In Uranus, sunlight is reflected by acurtain of clouds that lie underneatha layer of methane.

REFRACTION OF RAYS

Uranus generates a magneticfield 50 times more powerfulthan Earth's. This field is notcentered on the planet, but isoffset and tilted 60 degreesfrom Uranus's axis. If this werethe case on Earth, the magneticnorth pole would be located inMorocco. Unlike other planets,Uranus's magnetic fieldoriginates in the planet's mantle,not its core.


ESSENTIAL DATA

AXIS INCLINATION

* In both cases, Earth = 1


Equatorialdiameter

Average temperature

Solar orbit(Uranian year)

Density

Mass* Gravity*

Orbital speed


1,780,000,000 miles(2,870,000,000 km)

32,200 miles(51,800 km)

14.5


0.89

0.8 ounce per cubic inch(1.3 g/cu cm)

-346° F (-210° C)

Atmosphere Less dense

Moons 27

84 years4 days

CHARACTERISTICS

97.9°

COREMade up of silicatesand ice

6,20

0 M

ILES

(10,

000

KM

)

6,20

0 M

ILES

(10,

000

KM

)

10,6

00 M

ILES

(17,

000

KM

)

-346° F (-210° C)AVERAGE TEMPERATURE

T o the unaided eye, Uranus looks like a star at the limit of visibility. It is theseventh farthest planet from the Sun and the third largest planet in the solar system.One peculiarity distinguishing it from the other planets is its anomalous axis of

rotation, tilted nearly 98 degrees around the plane of its orbit, so that one or the other ofUranus's poles points toward the Sun. Astronomers speculate that, during its formation,Uranus may have suffered an impact with a protoplanet, which could have altered Uranus'stilt. Uranus's orbit is so large that the planet takes 84 years to completely orbit the Sun.Uranus's period of rotation is 17 hours and 14 minutes.

1986U2R

6

5

4

ALPHA

BETA

ETA

GAMMA

DELTA

LAMBDA

EPSILON

UMBRIEL730 MILES(1,170 KM)

OBERON946 MILES(1,522 KM)

TITANIA980 MILES(1,578 KM)

ARIEL720 MILES(1,158 KM)

INNER MANTLEProbably icy water, methane,and ammonia. (According tosome models, the materials ofthe mantle and core do notform layers.)

OUTER MANTLEComposed primarily ofhydrogen and helium, as wellas a small amount of methane

ATMOSPHEREUranus's atmosphere is made upof hydrogen, methane, helium,and small amounts of acetyleneand other hydrocarbons.

For a long time, Uranuswas believed to have a

smooth surface. The HubbleSpace Telescope, however,showed that Uranus is adynamic planet that has thesolar system's brightestclouds and a fragile ringsystem that wobbles like anunbalanced wheel.

Like all giant planets of the solar system, Uranus has a ringsystem, but it is much darker than Saturn's and more difficult

to see. The planet's 11 rings, which orbit the planet's equator, werediscovered in 1977. In 1986, they were explored by Voyager 2.

Rings

1.When sunlight passes through this layer, the methaneabsorbs the red light waves and lets the blue lightwaves pass through, producing the planet's hue.2.

URANUS

ATMOSPHERE

URANUS

SUNLIGHTATMOSPHERE

SUNLIGHT

CORDELIA

OPHELIA PORTIA

ROSALIND BELINDA

RADIUS=15,882 MILES(25,559 KM)

CRESSIDABIANCA

MIRANDA UMBRIEL

1 radius

169 283 314 339 482 580 654 698

6 7 8 9 16 212 3 4 5

Uranus has 27 moons. The first four were discoveredin 1787, and another 10 were identified in 1986 by

the space probe Voyager 2. Uranus's moons were named inhonor of characters from the works of William

Shakespeare and Alexander Pope, a naming convention thatdistinguishes them from the other moons in the solarsystem. Some of Uranus's moons are large, but mostmeasure only dozens of miles.

PUCK

JULIET

DESDEMONA

2001U3(FRANCISCO) TRINCULO SYCORAX

2003U3(MARGARET) PROSPERO SETEBOSSTEPHANOCALIBAN

ARIEL

TITANIA

OBERON

MIRANDA293 MILES(472 KM)

MAGNETIC FIELDSome scientists suggestthat Uranus's anomalousmagnetic field may indicatethat the convection ofUranus's core has stoppedbecause of cooling—or,perhaps, that the planet iscurrently undergoing amagnetic inversion, ashas happened on theEarth. Enlarged

region

Satellites

Surface

85%Hydrogen

3%Methane

12%Helium

Magnetic envelope

Magnetopause

Cusp

Capture region

Probably icy water,methane, and ammonia

INNER MANTLE


Neptune: Deep Blue

ATMOSPHEREBanded, like the atmospheresof the other gas giants,Neptune's atmosphere forms acloud system at least as activeas Jupiter's.

COMPOSITIONNeptune's rings are dark,like those of Uranus andJupiter. Their compositionis unknown, and they arebelieved to be unstable.Liberty, which makes uppart of the outer ring,could vanish before the22nd century.

TRITONIts diameter is 1,681 miles(2,706 km). Triton orbitsNeptune in a directionopposite that of the othermoons. Its surface has darkstripes formed by thematerial spewed from itsgeysers and volcanoes.

is its temperature, makingTriton one of the coldestbodies in the solar system.

COREMade up ofsilicates and ice

THE GREAT SPOTThis giant storm, calledthe Great Dark Spot, wasfirst seen on the surfaceof Neptune in 1989 andwas as large as the Earth.By 1994, it haddisappeared.

OUTER MANTLEComposed primarily ofhydrogen and helium, as wellas a small amount of methane

S een from our planet, Neptune appears as a faint, blue point invisible tothe naked eye. Images sent to Earth by Voyager 2 show the planet as aremarkably blue sphere, an effect produced by the presence of methane in the outer part

of Neptune's atmosphere. The farthest of the gaseous planets, Neptune is 30 times farther fromthe Sun than the Earth is. Its rings and impressive clouds are noteworthy, as is its resemblanceto Uranus. Neptune is of special interest to astronomers because, before its discovery, itsexistence and location were predicted on the basis of mathematical calculations.

89.8%

10.2%Helium

Hydrogen

Moons

Rings

Neptune has 13 natural satellites, nine of which arenamed. Triton and Nereid were the first moons

observed by telescope from Earth. The 11 remaining moons

were observed from space by the U.S. space probe Voyager 2.All the names of Neptune's satellites correspond to ancientGreek marine deities.

Uranus has faint rings of dust. When theywere discovered from the Earth,

astronomers thought the rings formedincomplete arcs. The ring nameshonor the first scientiststo study Neptune.

SurfaceWhite methane clouds surround Neptune,circulating at some of the fastest speeds in

the solar system. Neptune's winds reach 1,200miles per hour (2,000 km/h) from east to west,swirling against the direction of the planet'srotation.

According to some models, Neptune has a rocky silicate core, covered by amantle of icy water, ammonia, hydrogen, and methane. According to some

models, however, the materials of the mantle and core do not form layers.

ADAMSLocated 39,000 miles (63,000 km) fromthe planet's core, this ring has threeprominent arcs, or sections, namedLiberty, Fraternity, and Equality.

ARAGO

LASSELLLE VERRIER

GALLE

3,700

miles

(6,0

00

km)

8,70

0 m

iles

(14,0

00

km)

4,50

0 m

iles

(7,200

km)


ESSENTIAL DATA


Equatorialdiameter

Average temperature

Solar orbit(Neptunian year)

Density

Mass*

Gravity*

Orbital speed


2,800,000,000 miles(4,500,000,000 km)

30,800 miles (49,500 km)

17.2

3.4 miles per second(5.5 km/s)

1.121 ounce per cubic inch

(1.6 g/cu cm)

-330° F (-200° C)

Atmosphere Dense

Moons 13

164 years 264 days

CHARACTERISTICS

NAIAD

THALASSA DESPINA TRITON NEREID

GALATEA

LARISSA

NEPTUNE'SRADIUS=15,388 MILES(24,764 KM)

1 radius 6 7 8 9 10 11 12 13 14 2222 3 4 5

PROTEUS

Ascendingwinds

Descendingwinds

Hard Heart

AXIS INCLINATION

One rotation lasts16 hours and 36minutes.

28.3°

1,200miles

per hour

(2,000 km/h)

-391° F (-235° C)

The core ismade of iron,nickel, andsilicates.

CORE

Charon's density is between0.7 and 0.8 ounce per cubicinch (1.2 and 1.3 g/cu cm),indicating that its compositiondoes not include much rock.

DENSITY


Pluto: Now a Dwarf

ATMOSPHEREPluto's very thin atmosphere freezes andfalls to the dwarf planet's surface asPluto moves toward its aphelion.

The orbital arrangement of Pluto and Charon is unique. Eachalways faces the other, making the two seem connected by aninvisible bar. The synchronization of the two bodies is such that

an observer on one side of Pluto would be able to see Charon,but another observer standing on the other side of the planetcould not see this moon due to the curvature of the planet.

CRUSTThe crust of thisdwarf planet ismade of methaneand water frozenon the surface.

MANTLEThe mantle isa layer offrozen water.

P Pluto stopped being the ninth planet of the solar system in 2006 whenthe International Astronomical Union decided to change the classification of cold, distant Pluto tothat of dwarf planet. This tiny body in our solar system has never had an imposing profile, and it

has not yet been possible to study it closely. All that is known about Pluto comes through observationsmade from the Earth or Earth orbit, such as those made by the Hubble Space Telescope. Despite the lackof information gathered about Pluto, it is notable for its unique orbit, the tilt of its axis, and its locationwithin the Kuiper belt. All these characteristics make Pluto especially intriguing.

98%

2%With sometraces of carbonmonoxide

Methane

Nitrogen

A Double World

Moons

Pluto and its largest satellite,Charon, have a very special

relationship. They have been calleddouble planets—the diameter of Charonis about that of Pluto. One theoryhypothesizes that Charon was formedfrom ice that was torn from Pluto whenanother object collided with the dwarfplanet.

In addition to Charon, which wasdiscovered in 1978, Pluto is orbited by

two additional moons, Nix and Hydra, firstobserved in 2005. Unlike the surface of Pluto,which is made of frozen nitrogen, methane,and carbon dioxide, Charon appears to becovered with ice, methane, and carbondioxide. One theory holds that the matter thatformed this satellite was ejected from Plutoas a result of a collision, an origin similar tothat ascribed to Earth's moon.

CompositionScientific calculations have deduced that 75percent of Pluto consists of a mixture of rocks and

ice. This frozen surface is made up of 98 percentnitrogen, as well as traces of solidified carbon monoxideand methane. Recently scientists have concluded that

Pluto is an object that belongs to the Kuiper belt, a groupof objects left over from the formation of the outerplanets. In addition to large amounts of frozen nitrogen,Pluto has simple molecules containing hydrogen andoxygen, the building blocks of life.

SurfaceOnly a little is known about Pluto,but the Hubble Space Telescope

showed a surface covered by a frozenmixture of nitrogen and methane. Thepresence of solid methane indicates thatits temperature is less than -333° F (-203° C), but the dwarf planet'stemperature varies according to itsplace in orbit, ranging between 30 and50 astronomical units from the Sun.

A PECULIAR ORBITPluto's orbit is noticeably elliptical, and it is tilted 17°from the plane of the planets' orbits. The distancebetween Pluto and the Sun varies from 2,500,000,000to 4,300,000,000 miles (4,000,000,000 to7,000,000,000 km). During each 248-year orbit, Plutoorbits closer to the Sun than Neptune for nearly 20years. Although Pluto appears to cross paths withNeptune, it is impossible for them to collide.

570 m

iles

(920 km

)

270 m

iles

(434 km

)


ESSENTIAL DATA

* In both cases, Earth = 1

Equatorialdiameter

Average temperature

Solar orbit(Plutonian year)

Density

Mass*Gravity*

Orbital speed 3miles per second


3,700,000,000 miles(5,900,000,000 km)

1,400 miles(2,247 km)

0.002

(4.8 km/s)

0.067

1.2 ounces per cubic inch(2.05 g/cu cm)

-380° F (-230° C)

Atmosphere Very thin

Moons 3

247.9 years

CHARACTERISTICS

730 miles(1,172 km) terrestrial days is the time

Pluto takes to completeone rotation.

6,387

NEW HORIZONS MISSIONThe first space probe to be sent to Pluto was launched on January 19,2006. It is to reach the dwarf planetin July 2015 and achieve the firstflyby of Pluto and Charon.

BEST VIEW OF PLUTOAVAILABLE

SYNCHRONIZED ORBITS

ROTATIONAXIS

PLUTO

CHARON

AXIS INCLINATION

One rotationlasts 6.387Earth days.

122°

Charon's diameterΩhalfof Pluto's


Distant Worlds

Kuiper BeltExtending outward from the orbit of Neptune are many frozen worlds similar in some ways toplanets but much smaller. They are located in the Kuiper belt, the frozen boundary of our solar

system. So far, almost a thousand objects have been cataloged, including Quaoar, which has adiameter of 810 miles (1,300 km). The Kuiper belt, estimated to contain more than 100,000 bodies ofice and rock larger than 60 miles (100 km) in diameter (including Pluto), spreads out in the shape of awide ring. Many of the comets that approach the Sun come from the Kuiper belt.

Comparable SizesThe discovery of Quaoar in 2002allowed scientists to find the link

they had long looked for between theKuiper belt and the origin of the solarsystem. Quaoar's almost circular orbithelped prove that some objects both belongto the Kuiper belt and orbit the Sun. At theofficial meeting of the InternationalAstronomical Union, on August 24, 2006,Pluto was reclassified from a planet to adwarf planet. For the time being, anyfurther objects discovered in the Kuiper beltwill be classified in the same category.

is the diameter of Pluto—750 miles (1,200km) smaller than the Earth's Moon. Becauseof its size and orbit, Pluto is considered adwarf planet instead of a planet.

THE FARTHEST ONE

Eris is 97 astronomical units(9,040,000,000 miles [14,550,000,000km]) from the Sun, making it the mostdistant object observed in the solarsystem. This dwarf planet follows anoval, eccentric orbit that takes 560 yearsto complete. The dwarf planet measuresabout 1,900 miles (3,000 km) indiameter, and traces of methane ice havebeen detected on its surface.

F arther even than Neptune, the eighth planet, we find frozenbodies smaller than the Earth's Moon—the more than 100,000objects forming the Kuiper belt, the frozen boundary of our solar system.

Recently astronomers of the International Astronomical Union decided to reclassify Pluto asa dwarf planet because of its size and eccentric orbit. Periodic comets (comets that appearat regular intervals) originate in the Kuiper belt. Nonperiodic comets, on the other hand,come from the Oort cloud, a gigantic sphere surrounding the entire solar system.

SATURN'SORBIT

URANUS'SORBIT

NEPTUNE'SORBIT

PLUTO'SORBIT

ERIS

SEDNA Its diameter isestimated at 1,000miles (1,600 km).

QUAOAR has a diameterof 810 miles(1,300 km).

ERIS Larger than Pluto, itsdiameter is about 1,900miles (3,000 km).

PLUTO possesses adiameter of 1,400miles (2,300 km).

OR MORE, POSSIBLEEXTRASOLAR PLANETSHAVE BEEN DETECTED.200

1,410 miles(2,274 km)

43 m

iles

per

seco

nd

(70

km/s

)

contain similarquantities of iron,nickel, and silicates.

MESOSIDERITES

completes a solar orbitevery 14 Earth years.

HIDALGO

The Trojans trace an orbitsimilar to Jupiter's, onegroup in front of the planetand another behind it.

APOLLO

ATEN

AMOR

MAIN ASTEROID BELT

Mars'sOrbit

Jupiter'sOrbit

An asteroid 35 miles (56 km) long, its surfaceis marked by collisionswith other bodies.

IDA

15%

meteorites contain ahigh percentage ofiron and nickelcompounds. They arecreated in the ruptureof asteroids.

IRON

meteorites containsilicate minerals.They are subdividedinto chondrites andachondrites.

STONY


Construction Debris:Asteroids and Meteorites

E ver since the formation of the solar system, the melting, collision, and rupture ofvarious materials played an essential role in the formation of the planets. Remnantsof this process remain in the form of rock debris, which serves as witnesses to the

formation of the solar system. These objects are also associated with episodes thatinfluenced subsequent evolutionary processes on Earth. They are a possible cause of themass extinction of dinosaurs more than 60 million years ago.

ExtraterrestrialOne of the main goals of scientist who study meteorites isto understand their nature. Meteoric material holds

extraterrestrial solids and gases. Scientific tests have confirmedthat some meteorites are from the Moon or Mars, but mostmeteorites are associated with asteroids. The samples obtainedfrom meteorites are analyzed and classified by their composition.

AsteroidsAlso called minor planets, they are the millions of rock andmetal fragments of various shapes and sizes that orbit the

Sun. They are mostly located in a belt between the orbits of Marsand Jupiter, but a few, such as those that belong to the Amor,Apollo, and Aten asteroid groups, orbit closer to Earth.

Tighten Your BeltMore than a million asteroids at least amile in diameter are distributed in the

main asteroid belt. Ceres was the first asteroiddiscovered (in 1801). It is the largest knownasteroid, with a diameter of 580 miles (932 km).

7 miles per second(12 km/s) IMPACT VELOCITY

TYPES OF METEORITES

EXPLOSIONThe friction created as ameteorite falls through the airincreases its temperature.This is how an ignitionprocess is started.

1.

DIVISIONThe fragmentation ofa meteorite causes avisual effect called ashooting star.

2.

IMPACTThe collision of themeteorite compressesand excavates theground, leaving a crater.

3.

TYPES OF ASTEROIDS

Despite a great variety in size and shape, three types ofminor planets, or asteroids, are known. Classified bytheir composition, they are grouped as siliceous,carbonaceous, and metallic.

A HUGE METEORITE STRIKES

A meteorite is an object from space that does not completelyvaporize as it penetrates the Earth's atmosphere. Largermeteorites can form a crater when they strike the Earth. Shown isthe impact of an exceptionally large meteorite, such as the onethat many scientists believed might have led to the extinction of dinosaurs and many other species about 65 million years ago.

Ferrous-type rocksdominate itscomposition.

The percentage of thetotal mass of theasteroids compared withthe mass of the Moon

The Kirkwood gapsare the open areasin the main asteroidbelt that are devoidof asteroids.

KIRKWOOD

Launched in 1986, it passed the nucleusof Halley's comet at adistance of 310 miles(500 km) in 1992.

GIOTTO

Frozen water,methane, carbondioxide, ammonia,rock, and dust

NUCLEUS


Those with a Tail

DUST TAILThe suspended dustparticles trail behind thecomet, reflecting sunlightand making the luminoustail visible.

ION TAILThe trail of suspendedgases generates a low-intensity, luminous regionwith a bluish color. The gasmolecules lose an electronand therefore have anelectrical charge.

Envelops the nucleus.It is formed by thegases and dust that it releases.

Comets are small, deformed objects a few miles in diameter that arenormally frozen and dark. Made of dust, rock, gases, and organicmolecules rich in carbon, comets are usually found in orbits beyond that

of Neptune in the Kuiper belt or in the Oort cloud. Occasionally a comet, such asHalley's comet, veers toward the interior of the solar system, where its ice isheated and sublimates, forming a head and long, spectacular tails of gases and dust.

Types of CometsComets with short periodshave orbits around the Sun

that are shorter than 200 years.Those with a period of more than200 years travel tens or hundredsof times farther from the Sunthan Pluto.

Deep Impact MissionOn January 12, 2005, as part of theDiscovery program, NASA launched

the space probe Deep Impact, which, inturn, sent a projectile on a collision coursetoward the comet 9P/Tempel 1, where itobtained samples to be studied on Earth.

Periodic cometsComets that leave their originalorbits and approach the Sun

generally settle into new trajectories.Halley's comet, for example, completesits elongated orbit in 76 years.

22,370miles per hour(36,000 km/h)

COMA

Formed by the nucleusand the coma. Thefront part is called theimpact front.

HEAD

PROBE LAUNCHDeep Impact launches the770-pound (350-kg)copper projectile that willcollide with the comet.

1.

IN POSITIONBy means ofinfrared camerasand spectrometers,the ship follows thecomet to analyzethe impact on thenucleus.

The projectilesearches theimpact front.

NUCLEUS

SOLAR

WIND

2.

IMPACT WITH THE COMETtook place on July 4, 2005.The projectile generated acrater the size of a footballfield and seven stories deep.

3.

PREVIOUS MISSIONS

NASA has sent other unmanned missions tocomets. The first was the international craftISEE-3/ICE. Launched in 1978, its missionended after crossing the tail of the Giacobini-Zinner comet in September 1985.

STARDUST

THE HEADThe head of a cometcan measure 62,000miles (100,000 km) ormore in diameter.

SOLARSYSTEM

OORTCLOUD

LONG-PERIODCOMET

SHORT-PERIODCOMET

KUIPERBELT

FORMATION OF THE TAIL AND HEAD

Because of the effects of solar radiation andthe solar wind, gases and dust are releasedfrom an accelerating comet. The dustparticles tend to form a curving trail, whichis less sensitive to the pressure of the solarwind. As the comet leaves the confines ofthe solar system, its tails coincide oncemore, but they disappear as the nucleuscools down and ceases releasing gases.

Close to the Sun,the tails reach

maximum length.

As the cometmoves away fromthe Sun, its tailsdisappear.

Sun Earth

Comet orbit

Mars

Jupiter

The NASA spacecraftapproached thecomet Borrelly in 2001.

DEEP SPACE

In 2004, it tooksamples of thecomet Wild 2and sent them to Earth.

VELOCITY OF THECOMET IMPACT

THE MOON AND TIDES 76-77

ECLIPSES 78-79The Earth and the Moon

In the beginning, the Earth was anincandescent mass that slowlybegan to cool, allowing thecontinents to emerge and acquiretheir current form. Although

many drastic changes took place duringthese early eras, our blue planet has stillnot stopped changing. It must berecognized that life on Earth would beimpossible without the presence of the

atmosphere—the colorless, odorless,invisible layer of gases that surroundsus, giving us air to breathe andprotecting us from the Sun's harmfulradiation. Although the atmosphere is

approximately 435 miles (700 km) thick,it has no clear boundary and fades intospace until it finally disappears.

THE BLUE PLANET 68-69

JOURNEY TO THE CENTER OF THE EARTH 70-71

ONCE UPON A TIME 72-73

MOVEMENTS AND COORDINATES 74-75

AERIAL VIEW OF THE EARTHIn this partial image of the Earth, we can seeBora-Bora, an island that forms part of theLeeward Islands, located in French Polynesia.

68 THE EARTH AND THE MOON UNIVERSE 69

The Blue Planet

The Phenomenon of LifeWater, in liquid form, makes it possible for life toexist on the Earth, the only planet where

temperatures vary from 32° F to 212° F (0° C to 100° C),allowing water to exist as a liquid. The Earth’s averagedistance from the Sun, along with certain other factors,allowed life to develop 3.8 billion years ago.

Above 212° F(100° C)

ONLY ICEMars is so far fromthe Sun that all itswater is frozen.

3 STATESOn the Earth, water isfound in all three of itspossible states.

ONLY STEAMOn Mercury or Venus, whichare very close to the Sun,water would evaporate.

GRAVITY ANDWEIGHT

Magnetism and GravityThe Earth’s magnetic field originates in the planet’s outer core,where turbulent currents of molten iron generate both electric and

magnetic fields. The orientation of the Earth’s magnetism varies overtime, causing the magnetic poles to fluctuate.

WHAT IT DOES

The magnetic fieldprotects the Earthfrom the radiationof the solar wind.

Weight is the force ofthe gravity that actson a body.

Solar wind

The Van Allen belts trapthe particles from the solarwind, causing phenomenalike the auroras.

Some particles areattracted to the poles.

Earth

Axis

Magneticfield lines

Magnetic field tail

Van Allen belt

EARTH MOVEMENTS

The Earth moves,orbiting the Sun androtating on its own axis.

ROTATION: The Earthrevolves on its axis in 23hours and 56 minutes.

REVOLUTION: It takes the Earth 365days, 5 hours, and 57 minutes to travelonce around the Sun.

The Moon, our only naturalsatellite, is four times smallerthan the Earth and takes27.32 days to orbit the Earth.

93,500,000 miles(149,503,000 km)


ESSENTIAL DATA

Revolution aroundthe Sun (Earth year)

Mass*

Orbitingspeed

Gravity*

Diameter atthe equator

Average distanceto the Sun

93 million miles(150 million km)

365.25 days

1

7,930 miles(12,756 km)

17 miles per second(27.79 km/s.)

1

CHARACTERISTICS

The Earth is known as the blue planet because of the colorof the oceans that cover two thirds of its surface. This planet,the third planet from the Sun, is the only one where the right

conditions exist to sustain life, something that makes the Earthspecial. It has liquid water in abundance, a mild temperature, andan atmosphere that protects it from objects that fall fromouter space. The atmosphere also filters solar radiationthanks to its ozone layer. Slightly flattened at its polesand wider at its equator, the Earth takes 24 hours torevolve once on its axis.

23.5°

of the Earth’s surfaceis water. From space,the planet looks blue.

70%

Magnetosphere

NORTHPOLE

ROTATIONAXIS

AXISINCLINATION

1.EVAPORATIONBecause of the Sun’senergy, the waterevaporates, entering theatmosphere from oceansand, to a lesser extent,from lakes, rivers, andother sources on thecontinents.

2. CONDENSATIONThe Earth’s winds transportmoisture-laden air untilweather conditions cause thewater vapor to condense intoclouds and eventually fall to theground as rain or other formsof precipitation.

3. PRECIPITATIONThe atmosphere loseswater throughcondensation. Gravitycauses rain, snow, andhail. Dew and frostdirectly alter the state ofthe surface they cover.

AXIS INCLINATION


One rotationlasts 23.56hours.

Averagetemperature

Density 3.2 ounces per cubicinch (5.52 g/cu cm)

59° F (15° C)

23.5°

SOUTH POLE

This is the inclination of theEarth’s axis from thevertical. As the Earth orbitsthe Sun, different regionsgradually receive more orless sunlight, causing thefour seasons.

Magneticforce

The liquidouter core isin constantmotion.

Mantle

The Earth’score works asa magnet.

The Earth’s magneticfield is created byconvective currentsin its outer core.

Solid core

SUN

24ON JÚPITERJupiter has 300 times moremass than the Earth andtherefore more gravity.

ON EARTHThe object is drawntoward the Earth’s center.

ON THE MOONThe Moon has less massthan the Earth and, as aresult, less gravity.

-76° F (-60° C)

32° to 212° F(0° to 100° C)

pounds(11 kg) 154 pounds

(70 kg) 390 pounds(177 kg)

UNIVERSE 7170 THE EARTH AND THE MOON

Journey to the Centerof the Earth

Internal StructureWe live on a rocky surface similar to a shell. Therocks we live with are made mostly of oxygen and

silicon, but underneath them is the mantle, made of muchheavier rocks. The mantle also surrounds the inner andouter cores with a region of constantly boiling liquid metals,creating the convective currents that generate the Earth’smagnetic field. The inner core, solid because of the greatpressure put upon it, is the densest part of the planet.

The hydrosphere, the liquid part of the Earth,includes the oceans, lakes, rivers, underground

waters, snow, and ice. It almost completely covers thecrust, surrounds the shores of the continents, and covers

755 miles (1,216

km)

Mount Everest5.5 miles (8.85 km)

Continentalpenetration

Oceanicpenetration

OUTER COREThe outer layer of the core isliquid, consisting of molten ironand nickel. Its temperature islower than that of the innercore and it is under lesspressure. The motion of themolten material produces thegeomagnetic field.

INNER MANTLEThe solid, intermediatelayer between the coreand the crust. High-temperature S and Pwaves pass through itbecause of its contactwith the core.

HOW FAR HUMANS HAVE GONE

OUTER MANTLEAs a result of the hightemperatures, the materialsdilate and produce acontinuous ascendingmovement that generatesconvection currents and theforces that cause thechanges to the Earth’s crust.

Hydrosphere andLithosphereThe lithosphere includes thecrust and the upper portionof the mantle, and thehydrosphere includes liquidwater, covering 71 percent ofthe Earth’s surface in lakes,rivers, and five oceans.

TROPOSPHERE

STRATOSPHERE

MESOSPHERE

THERMOSPHERE

EXOSPHERE

7.5 miles

(12 km)

1 mile

(1.7 km)

W e live on the Earth, but do we know what we arestanding on? The planet is made up of layers ofvarious materials, such as solid and molten

rock, which in turn are composed of such elementsas iron, nickel, and silicon. Our atmosphere is thelayer of gases surrounding our planet. Oneof these gases, oxygen, does a very specialjob—it permits life to exist.

71 percent of the Earth’s surface. The lithosphereis a superficial, elastic region that is 4 to 7 miles(6 to 11 km) thick under the oceans and up to 43miles (70 km) thick under mountain ranges.

3,965 miles(6,380 km)from the Earth’s surface toits center.

INNER COREis made of the samemetals as the outer core,but, despite its hightemperature, its center issolid because of theenormous pressure thatcompresses it.

620 MILES

(1,000 KM)

3,965 MILES

(6,380 KM)

29.2%soil

70.8%water

94 %salt

water

6 %freshwater 0.03%

surface andatmosphere

1.7 %ice

WATER AND EARTH TOTAL VOLUME OF WATER FRESHWATER

4.3 %under-ground

Above the SurfaceThe existence of life on our planet would beimpossible without the atmosphere that provides

the air we breathe and the water we drink; it alsoprotects us from the Sun’s harmful radiation, whilesimultaneously maintaining mild temperatures byretaining the Sun’s warmth. The atmosphere is about435 miles thick (700 km) but lacks defined limits.

WITHOUTATMOSPHEREDirect solar radiation.Differences intemperature betweenthe equator and thepoles would be farmore pronounced.

WITH ATMOSPHERE The sunlight filters intothe atmosphere. Windsdistribute the heat,cooling the tropics andwarming the poles.

0 miles(0 km)

30 miles(50 km)

50 miles(80 km)

370 miles(600 km)

620 miles(1,000 km)

Air is very rarefied.

The ozone layer, locatedhere, absorbs the Sun’sultraviolet rays.

Vegetable and animallife.

It fades into outer space.

Orbit of an artificialsatellite

7 miles(11 km)

435 miles

(700

km)

1,410

miles (2,270

km)

1,800

miles (2,9

00

km)

Lithosphere and Hydrosphere


Once Upon a TimeT

he Earth probably formed from material in the solar nebula—the cloud of gas and dustthat led to the formation of the Sun. This material gradually grew into a larger and largerbody that became a red-hot ball of rock and metal. Later the rocky crust formed, its

surface cooling enough to allow the continents to appear. Even later the oceans arrived, as wellas the tiny organisms that released oxygen into the atmosphere. Although much of this gas wasinitially consumed in chemical reactions, over time, it allowed the development of multicellularorganisms and an explosion of life that took place at the start of the PaleozoicEra, 542 million years ago.

Chronology Geology is the study of rocks in the Earth’scrust. It divides the Earth’s history into

different eras, periods, and epochs lasting millionsof years. Geology also helps us catalog theprocesses of evolution—changes in generations asspecies adapt to their environment and theircompetitors.

Through the study of fossils—remains of creaturesburied in the Earth’s various sedimentary layersand consequently at different times in the past–geology helps us trace the timeline of evolutionaryhistory.

Continental DriftWe live on the continents, which are part of movable plates thatdrift across the Earth’s surface at the rate a fingernail grows. 250

million years ago, India, Africa, Australia, and Antarctica were partof the same continent. When tectonic plates rub against eachother, land and oceanic crust earthquakes occur. Where theplates separate, a rift forms. The mid-ocean ridges thatrun beneath the oceans are formed by lava thatemerges from the rifts between tectonic plates. Whereplates collide, a process called subduction takesplace, in which the rocks of the oceanic floor aredrawn under the continent and melt, reemerging inthe form of volcanoes.

Origin of the EarthThe Earth was formed 4.6 billion years ago from acloud of dust and gas. In the beginning, it was a

molten, constantly active, mass. As time passed, the Earthbegan to cool, and the atmosphere began to clear as rainfell, creating the oceans.

ICYTHYOSTEGA(amphibian)

CONIFERS

MARINE REPTILE

DINOSAUR

SMALLMAMMAL

LARGEMAMMAL

HOMOSAPIENS

Ball of fire The Earth wascreated from smallparticles thatcoalesced in thesolar nebula.

A The planet cooledThe atmospherewas created as theplanet cooled andbegan to emitgases and steam.

B The crust formsLava poured acrossthe surface of theEarth. As it cooled,it formed theEarth’s crust.

C Water appeared on theEarth 3.9 billion yearsago. Water-rich Earth isthe only planet in thesolar system known tohave life.

D

250 MILLION YEARS AGOThe Tethys Sea slowly split Pangea,creating two continents, known asLaurasia and Gondwana.

2

163 MILLION YEARS AGOGondwana split, forming Africa and SouthAmerica as the southern Atlantic Oceanwas created.

3

60 MILLION YEARS AGOThe northern Atlantic Ocean slowlyseparated, completing the formation ofEurope and North Africa.

4

PANGEA

GONDWANA

LAURASIA

LAURASIA

EURASIA

AFRICA

AFRICA

ANTARCTICA

ANTARCTICA

AUSTRALIAINDIA

INDIA

SOUTHAMERICA

AMERICA

Panthalassa

Tethys Sea

PRECAMBRIC TIME

PALEOZOIC ERA

MESOZOIC ERA

CENOZOIC ERA

Tectonic PlatesThe surface of the Earth is shaped by tectonicplates. There are eight major plates, some of

which even encompass entire continents. The plates’borders are marked by ocean trenches, cliffs, chainsof volcanoes, and earthquake zones.

THE EARTH IN ONE DAYIf the Earth’s history were compressed intoa normal day, Homo sapiens would appear

at just one minute to midnight.Metamorphicrocks

Hardsediment At a certain

depth, thepressuredestroysthe fossils.

The majorityare marineshells.

Softsediment

FOSSILSare remains of living beings preserved inthe rocks as a record of the Earth’s history.

WaterUNICELLULARORGANISM

TRILOBITE

CRINOID(sea animal)

COOKSONIA(plant)

290 MILLION YEARS AGOThe supercontinent called Pangeaformed. An immense ocean calledPanthalassa surrounded it.

1

74 THE EARTH AND THE MOON

Movements and Coordinates

UNIVERSE 75

Yes, it moves. The Earth rotates on its axis while simultaneously orbiting the Sun. Thenatural phenomena of night and day, seasons, and years are caused by these movements.To track the passage of time, calendars, clocks, and time zones were invented. Time

zones are divided by meridians and assigned a reference hour according to their location. Whentraveling east, an hour is added with each time zone. An hour is subtractedduring west-bound travel.

The Earth’s MovementsNight and day, summer and winter, new year andold year result from the Earth’s various movements

during its orbit of the Sun. The most important motionsare the Earth’s daily rotation from west to east on its ownaxis and its revolution around the Sun. (The Earth followsan elliptical orbit that has the Sun at one of the foci of theellipse, so the distance to the Sun varies slightly over thecourse of a year.)

JET LAG

The human body’s biological clock responds to the rhythms of light and dark based on thepassage of night and day. Long air flights east or west interrupt and disorient the body’sclock, causing a disorder known as jet lag. It can cause fatigue, irritability, nausea,headaches, and difficulty sleeping at night.

Time ZonesThe Earth is divided into 24 areas, or timezones, each one of which corresponds to

an hour assigned according to the CoordinatedUniversal Time (UTC), using the Greenwich,

June 20 or 21Summer solstice in the Northern Hemisphereand winter solstice in the Southern Hemisphere.Solstices exist because of the tilt of the Earth’s axis.The length of the day and the height of the Sun in thesky are greatest in summer and least in winter.

REVOLUTION1 YEARThe Earth’s orbitaround the Sun lasts365 days, 5 hours,and 57 minutes.

ROTATION1 DAYThe Earth revolvesonce on its axis in 23hours and 56 minutes.We see this as dayand night.

NUTATION18.6 YEARSis a sort of nod madeby the Earth, causingthe displacement ofthe geographic polesby nine arc seconds.

PRECESSION25,800 YEARSA slow turning of thedirection of the Earth’saxis (similar to that ofa top), caused by theEarth’s nonsphericalshape and thegravitational forces ofthe Sun and the Moon

About 30 days

12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00

THE EARTH’SORBITAbout 365 days

APHELIONThe point in theEarth’s orbit where itis farthest from theSun (94 million miles[152 million km]). Thisoccurs at thebeginning of July.

THE MONTHSEach period of time,between 28 and 31days, into which a yearis divided

PERIHELIONThe point where the orbiting Earthmost closely approaches the Sun(91 million miles [147 million km])

Temperatezone

Tropical zone

Polar zone

1 day

September21 or 22Autumn equinox in theNorthern Hemisphereand spring equinox in theSouthern Hemisphere.The Sun passes directly abovethe equator, and day andnight have the same length.

SUN

March20 or 21Spring equinox in the NorthernHemisphereand autumnequinox in theSouthernHemisphere.The Sun passesdirectly above theequator, and dayand night have thesame length.

Geographic CoordinatesThanks to the grid formed by the lines of latitude and longitude, theposition of any object on the Earth’s surface can be easily located by

using the intersection of the Earth’s equator and the Greenwich meridian(longitude 0°) as a reference point. This intersection marks the midpointbetween the Earth’s poles.

December 21 or 22Winter solstice in the NorthernHemisphere and summer solstice inthe Southern Hemisphere.Solstices exist because of the tilt of theEarth’s axis. The length of the day and theheight of the Sun in the sky are greatest insummer and least in winter.

93MILLION MILES

(149 MILLION KM)

23.5ºTILT OF THEEARTH’S AXIS

0 ° EQUATOR

23.5° S Tropic of Capricorn

66.5° S Antarctic Circle

66.5° N Arctic Circle

23.5° N Tropic of Cancer

SouthernHemisphere

NorthernHemisphere

0° GREENWICHMERIDIAN

PARALLELS

23º

S

N

3º

47º

MEASUREMENT OF TIMEMonths and days are charted by calendars and clocks,but the measurement of these units of time is neither acultural nor an arbitrary construct. Instead, it isderived from the natural movements of the Earth.

Every year, around June 21, the Northern Hemisphere reachesits maximum inclination toward the Sun (a phenomenonreferred to as the summer solstice in the Northern Hemisphereand the winter solstice in the Southern Hemisphere). The NorthPole receives sunlight all day, while the South Pole is covered indarkness. Between one solstice and another the equinoxesappear, which is when the axis of the Earth points toward theSun and the periods of daylight and darkness are the same allover our planet.

Equinox and Solstice

THE DAYS Period of time it takesthe Earth to rotate onits axis

12:00 A.M.Departuretime

12:00 P.M.Arrival time

England, meridian as the base meridian. Onehour is added when crossing the meridian inan easterly direction, and one hour issubtracted when traveling west.

12:00 A.M.

12:00 P.M.

3:00 P.M.9:00 A.M.

6:00 A.M. 6:00 P.M.

9:00 P.M.3:00 A.M.

WEST EAST

N

NorthernHemisphere


The Moon and Tides

The Lunar LandscapeObserving the Moon, the ancient astronomersdecided that, as on the Earth, its plainly visible dark

spots must be seas. These dark regions of the Mooncontrast against the bright ones, the highlands with themost impact craters.


ESSENTIAL DATA

AXIS INCLINATION

*Earth = 1

One rotationlasts 27.32Earth days.

Revolutionaround the Earth

Temperature

Volume*

Density

Mass*Gravity*

Orbiting speed

Average distancefrom the Earth

226,400 miles(364,400 km)

27.3 days

Diameter atthe equator

2,160 miles(3,476 km)

0,01

0.6 miles persecond (1.02 km/s)

0,172 ounces per cubic

inch (3.34 g/cu cm)

302° F (150° C) (day)-148° F (-100° C) (night)

0.02

CHARACTERISTICS

5.14°

R omance and terror, mystery and superstition–all theseemotions are responses to the Moon, the Earth’s one naturalsatellite, which always hides one of its two faces. However,

whatever symbolic meanings are attributed to the Moon, itsgravitational pull has a concrete effect on the Earth—it is a cause ofthe tides. Depending on the distance of the Moon from the Earth, thegravitational pull exerted by the Moon varies in strength and so canhigh tides and low tides. To reach full height, tides need large openareas of ocean. For this reason, tides in closed or small bodies ofwater are much lower.

OUTERCOREPartiallymelted

100

0 km

100

km

INNER CORECentraltemperatureof 2,730° F(1,500° C)

INNER STRUCTUREVarious seismic analyses of the Moon suggest that itscore is solid or semisolid.

THE MOON’S MOVEMENTS

THE PHASES OF THE MOON

CRUSTSurface made of rocks, such as granite, coveredby 65 feet (20 m) of lunar dust called regolith

The most widely acceptedtheory of the Moon’s originsuggests that an object the sizeof Mars collided with the Earthduring its formation.

ORIGIN OF THE MOON

The diameter of the Moon is onefourth of the Earth’s.

2,160 miles(3,476 km)

The Moon is the Earth’sonly natural satellite.

UniqueThe Sun’s gravityalso influencesthe tides.

46,6 %

MaguinusSchickard

Tycho100 millionyears old

NewMoon

Waxingcrescent

Firstquarter

Waxinggibbous

FullMoon

Waninggibbous

Thirdquarter

Waningcrescent

Gassendi

Grimaldi

SEAScover almost 16 percent ofthe Moon’s surface andwere formed by flowinglava. Today the Moon hasno volcanic activity.

Montes ApenninusOne of the mostnotable mountainranges

HumboldtCrater named inhonor of theGerman naturalist

Rupes AltaiMountain chain5,900 feet(1,800 m) high

Mare CrisiumMeasures 280miles by 370miles (450 kmby 563 km)and has largecraters.

MareTranquillitatisThe seas areflatlands with fewcraters.

Mare Imbriumis 3.85 billionyears old.

VISIBLE FACESpotted with darkareas, it alwaysfaces the Earth.

NEW MOONSPRING TIDEWhen the Sun and the Moonare aligned, the highest hightides and lowest low tidesare produced. FIRST QUARTER

NEAP TIDEThe Moon and the Sun are atright angles to the Earth,producing the lowest high tidesand the highest low tides.

FULL MOONSPRING TIDEThe Sun and the Moonalign once again, and theSun augments the Moon’sgravitational pull, causinga second spring tide.

THIRD QUARTERNEAP TIDEThe Moon and the Sunagain form a rightangle, causing asecond neap tide.

The TidesThe water on the side of the Earthclosest to the Moon feels the Moon’s

gravitational pull most intensely, and viceversa. Two tides are formed, and they trackthe Moon in its orbit around the Earth.However, they precede the Moon instead ofbeing directly in line with it.

Sun

3

2

1

4

Earthorbit

Gravitationalpull of the Sun

Gravitationalpull of the Moon

Influence onthe tide by thegravitationalpull of theMoon

Influence onthe tide by thegravitationalpull of the Sun

KEY

HIDDENFACE

Invisible from the Earth, thisside of the Moon was a mystery

until 1959, when the Russian probeLuna 3 managed to photograph the

hidden zone. Because of thegreater thickness of the Moon’s

crust on this side, it hasfewer seas.

CRATERScan be from 40 inches (1 m)to 620 miles (1,000 km) indiameter and are formed bymeteorites that strike theMoon’s surface withincredible force.

MOUNTAIN RANGESWhen a meteorite strikesthe lunar surface, amountain range formsfrom the material ejectedduring the crateringprocess.

As the Moon orbits theEarth, it revolves on its ownaxis in such a way that italways shows the Earth thesame side.

Hiddenface

Moon

Earth

LUNAR MONTHIt takes 29.53days to completeits phases.

SIDEREAL MONTHIt takes 27.32 daysto orbit the Earth.

Lunar orbit

Visible face

Lunar orbit

Moon

The ejected materialscattered into spacearound the Earth, andover time, it coalescedinto the Moon.

Mare Morum

Mare Nubium

AristarchusBrightest spoton the Moon

OceanusProcellarumThe largest sea,it is not wellpreserved.

Copernicus60 miles(93 km) indiameter

ROCKYMANTLELess than halfthe thicknessof the Earth’smantle

78 THE EARTH AND THE MOON UNIVERSE 79

EclipsesT ypically four times a year, during the full or new

moon, the centers of the Moon, the Sun, and theEarth become aligned, causing one of the most

marvelous celestial phenomena: an eclipse. At these times,the Moon either passes in front of the Sun or passes throughthe Earth’s shadow. The Sun—even during an eclipse—is notsafe to look at directly, since it can cause irreparable damageto the eyes, such as burns on the retina. Special high-quality filters or indirect viewing by projecting the Sun’simage on a sheet of paper are some of the ways in whichthis celestial wonder can be watched. Solar eclipsesprovide, in addition, a good opportunity for astronomers toconduct scientific research.

Penumbracone

Shadowcone

Shadowcone

Earthorbit

Lunarorbit

FULLMOON

NEWMOON

EARTH

TOTALECLIPSE

TOTALECLIPSE

PARTIALECLIPSE

PENUMBRALECLIPSE

ALIGNMENT

During an eclipse, the Moon is not completelyblack but appears reddish.

times greater thanthe distance from theEarth to the Moon

DISTANCE FROM THE SUN TOTHE EARTH

400times largerthan theMoon

SUN’S APPARENT SIZE

400

Lunar EclipseWhen the Earth passes directly between thefull Moon and the Sun, a lunar eclipse (which

could be total, partial, or penumbral) occurs.Without the Earth’s atmosphere, during each lunareclipse, the Moon would become completely invisible(something that never happens). The totally eclipsedMoon’s characteristic reddish color is caused by lightrefracted by the Earth’s atmosphere. During a partialeclipse, on the other hand, part of the Moon falls inthe shadow cone, while the rest is in the penumbra,the outermost, palest part. It is not dangerous tolook at a lunar eclipse directly.

TOTALThe Moon iscompletely inthe shadowcone.

MoonEarthSun

PARTIALThe Moon isonly partiallyinside theshadow cone.

PENUMBRALThe Moon is inthe penumbralcone.

ALIGNMENT TYPES OF ECLIPSES

During a solar eclipse, astronomers takeadvantage of the blocked view of the Sun inorder to use devices designed to study theSun’s atmosphere.

Solar EclipseSolar eclipses occur when the Moonpasses directly between the Sun and the

Earth, casting a shadow along a path on theEarth’s surface. The central cone of the shadowis called the umbra, and the area of partialshadow around it is called the penumbra.Viewers in the regions where the umbra falls onthe Earth’s surface see the Moon’s diskcompletely obscure the Sun—a total solareclipse. Those watching from the surroundingareas that are located in the penumbra see theMoon’s disk cover only part of the Sun—apartial solar eclipse.

TOTALThe Moon isbetween the Sunand the Earth andcreates a cone-shaped shadow.

ANNULARThe Sun appearslarger than theMoon, and itremains visiblearound it.

PARTIALThe Moon doesnot cover the Suncompletely, so theSun appears as acrescent.

THE ECLIPSE CYCLEEclipses repeat every 223 lunations—18 years and 11 days.These are called Saros periods.

OBSERVATION FROM EARTHare different for eachlocal observer.

MAXIMUM DURATION

ECLIPSES IN 2006 AND BEYOND

3/29Total

2006

OF THESUN

OF THEMOON

SOLAR ECLIPSES

9/22Total

3/03Total

8/28Total

3/19Partial

9/11Partial

2/9Partial

7/7Partial

2/07Total

1/26Total

7/22Total

1/15Total

7/11Total

1/4Partial

11/25Partial

6/15Total

12/10Total

5/20Annular

11/13Annular

6/4Partial

12/28Partial

5/10Annular

11/3Hybrid

4/25Partial

10/18Partial

4/29Annular

10/23Partial

4/15Total

10/08Total

3/20Total

9/13Partial

4/4Total

9/28Total

3/14Partial

9/07Partial

8/16Partial

2/21Total

6/26Partial

12/21Total

2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

SUNLIGHT

8 minutes

are the same for allobservers.

MAXIMUM DURATION

LUNAR ECLIPSES

100 minutes

Moon EarthSun

Minimum

2of theSun

41of theMoon

29

A black, polymer filter, withan optical density of 5.0,produces a clear orange imageof the Sun.

Preventsretinal burns

70Maximum

7Average

4ECLIPSES IN A YEAR ECLIPSES IN A SAROS

Total

ANNUAL ECLIPSEOF THE SUN, SEENFROM THE EARTH

TOTAL LUNARECLIPSE, SEEN

FROM THE EARTHThe orange color comes from

sunlight that has been refractedand colored by the Earth’s

atmosphere.

TYPES OF ECLIPSES

Observing the Universe

Astronomy was born out ofhumankind's need to measuretime and seasons, marking thebest times to plant. In ancienttimes, the study of the stars

was mixed with superstition and ritual.The megalithic monument Stonehenge,found in southern England, is an exampleof this. Today, thanks to advances in newtechnologies, such as the giant telescopes

installed in various locations around theplanet, we have discovered many newthings about the universe. The VLT (VeryLarge Telescope), astronomy's newmonster telescope located in Chile, is

part of an attempt to find planetsbeyond the solar system, becausemany astronomers suspect that life isnot exclusive to the Earth.

ASTRONOMICAL THEORIES 82-83

SPRINKLED WITH STARS 84-85

CELESTIAL CARTOGRAPHY 86-87

FROM THE HOME GARDEN 88-89

A FOUR-EYED GIANT 90-91

STONEHENGELocated in Wiltshire (England), it was built in severalphases over some 600 years—between 2200 and1600 BC. The placement of most of its large stoneshas a relationship to the Moon and the Sun.

UNIVERSE 8382 OBSERVING THE UNIVERSE

For a long time, it was believed that the Earth wasstationary. The Sun, the Moon, and the planets werethought to orbit it. To study the sky and calculate

its movements, people began to build instruments, suchas the astrolabe, armillary sphere, and telescope. Thetelescope revolutionized the conception of theuniverse. Instead of the Earth being at the centerof the universe, it was suggested that the Earthand other planets travel around the Sun. TheRoman Catholic Church opposed the idea and,for a time, persecuted dissident astronomersand banned their theories.

In 1543, a few months before his death,Nicolaus Copernicus published the book Derevolutionibus orbium coelestium, inaugurating

what is now known as the CopernicanRevolution. The Polish astronomer

developed the heliocentric theory(from helios, the Greek word for “the

Sun”), which contradicted thegeocentric theory. Copernicus’s

new postulate inverted thetraditional relationship of

the Sun and the Earth,identifying the Sun as

the center of theuniverse and the

Earth as one ofmany solarsatellites.Copernicus

argued that spheres moved in endless, circular orbits. Sincethe universe and all the celestial bodies were thought to bespherical, he argued that their movements must also becircular and uniform (the Ptolemaic system considered theplanets’ circuits to be irregular). Copernicus reasoned that,since the movements of the planets appeared to beirregular, the Earth must not be the center of the universe.These discoveries were contrary to the views promulgatedby the Roman Catholic Church. In fact, bothRoman Catholics andProtestants suppressedany writingsadvocating these beliefs.When Galileo Galilei wasbrought to trial by the RomanCatholic Church foradvocating the Copernicantheory, he was forced torenounce his views.

Heliocentric Model

GALILEO’S TELESCOPEThe telescope is thought to have been invented in1609 by the Dutch optician Hans Lippershey buthad no real scientific application until GalileoGalilei improved and adapted it to observecelestial bodies. Galileo’s first telescope, made of aleather tube covered by a lens at each end (onelens convex and the other concave), magnifiedobjects up to 30 times. Using the telescope, Galileodiscovered that the Sun’s surface had imperfections(sunspots), that the Moon had mountains andcraters, and that there were four moons, orsatellites, that traveled around Jupiter.

MEASUREMENTSNoticing that the Sun, the Moon, and the starsmoved in cycles, ancient civilizations found theycould use the sky as both a clock and a calendar.However, ancient astronomers haddifficulties performing the complexcalculations needed to predict thepositions of stars accuratelyenough to create a truly precisecalendar. A useful tool developedto perform this task was theastrolabe. Its engraved platesreproduce the celestial sphere intwo dimensions, allowing theelevations of the celestial bodiesto be measured.

THE TRAVELERSAfter many years and great advancements in

technology, scientists decided that spaceobservation conducted only from the Earth’s

surface was insufficient. In 1959, the first spaceprobe was launched, an automatic vehicle thatflew to the Moon and photographed its hidden

face. The space probes Voyager 1 and 2explored the planets Jupiter, Saturn, Uranus,

and Neptune, a milestone in space exploration.In 2005, Voyager 1 reached the region called

Termination Shock, the frontier of the solarsystem, representing the farthest region explored

by humanity. Both probes carried with them goldendiscs, named Sounds of Earth, containing sounds and

images portraying the diversity of life on Earth.

2nd CenturyClaudius Ptolemy100-170

Resurrected and compiled theworks of great Greek

astronomers into twobooks. His postulates held

undisputed authority forcenturies.

16th CenturyNicolaus Copernicus1473-1543

In his De revolutionibus orbiumcoelestium, the Polishastronomer postulated that theSun—not the Earth—was the

center of the universe. Thisconcept is the foundation of our own astronomy..

20th CenturyEdwin Hubble1889-1953

In 1929, he began to investigate the expansion of the galaxies,allowing scientists to obtain an idea of the true scale of the universe as well as refine the big bang theory.

17th CenturyIsaac Newton1642-1727

He built upon the ideas of Galileo anddeveloped the theory of universalgravitation, asserting that themovements of the Earthand the celestial bodies aregoverned by the samenatural laws.

Astronomical Theories

17th Century 17th CenturyGalileo Galilei1564-1642

Built the first telescope, aprimitive device with which hediscovered sunspots, four ofJupiter’s moons, the phasesof Venus, and craters on theMoon’s surface.

Before telescopes,binoculars, and modern

observatories existed, little wasknown about the Earth. Manybelieved that the Earth was fixedand that the Sun, the Moon, andthe five known planets orbited it incircles. This geocentric model waspromoted by the Egyptianastronomer Claudius Ptolemy, whoin the 2nd century AD compiledthe astronomical ideas of theancient Greek astronomers (in

particular, those of Aristotle, whohad proposed the Earth as thecenter of the universe, with thecelestial objects revolving aroundit). Although other ancientastronomers, such as Aristarchusof Samus, proposed that the Earthwas round and rotated around theSun, Aristotle’s ideas wereaccepted as true for 16 centuries,and at times Aristotle’s ideas weredefended and preserved by theRoman Catholic Church.

Geocentric Model

TIMEThisastrolabe wasused by ancientPersians. To them,astronomyfunctioned as a kindof agriculturalcalendar.

Johannes Kepler1571-1630

The German astronomer, believerin Copernicus’s heliocentric model,formulated three famous laws of

planetary movement, whichencouraged Galileo to publishhis research.

COSMIC CHARACTERS

84 OBSERVING THE UNIVERSE

C onstellations are groups of stars thought to representdifferent animals, mythological characters, and other figures.Constellations were invented by ancient civilizations to serve

as reference points in the Earth’s sky. There are 88 of thesecollections of stars. Although each star in a constellation appears

related to the others, it is actually very far from them. Notall the constellations are visible at the same time from

any one place on the Earth.

The brightest stars arethose of the head and back.Regulus is prominent.

is the least notableof the zodiac’sconstellations.

The stars Castor andPollux form the head ofthe twins.

is visible even withoutbinoculars. The brightest star,Aldebaran, is red.

Not a verynoteworthyconstellation, it hasno very bright stars.

has globular clustersand nebulae visible withbinoculars.

is one of the leastprominentconstellations.

lies in the direction of the Milky Way and its brightest star is Antares.

is a constellationwith several brightstars.

was at onepoint, part ofScorpius.

Although it is the 13th constellation ofthe zodiac, Ophiuchus is not a part ofthe zodiac. When astrology began3,000 years ago, the constellation was far from the zodiac.

The Babylonians conceived of the zodiac2,000 years ago as a way of measuringtime, using it as a symbolic calendar.

is located at the center ofthe Milky Way and is full ofnebulae and star groups.

has only one verybright star,Hamal, the Arabicword for “sheep.”

Mintaka

Heka

Betelgeuse

Pi3 Orionis

Pi4 Orionis

Pi5 Orionis

Chi1 Orionis

Pi6 Orionis

Pi2 Orionis

Omicron

Orionis

Bellatrix

23.5°

Earth Sun

Earth’s orbit

Starbackground

Alnilam

Alnitak

THE MYSTERY OF GIZAThe alignment of the threepyramids at Giza in Egyptappears to be related tothe alignment of the threestars of Orion’s belt.

Saiph

ORIGINThe history of western culture’sconstellations begins with thefirst astronomical observationsmade by ancientMesopotamian peoples.Because we inherited theconstellations from Greco-Roman culture, most of theconstellations are named afterfigures in classical mythology.All of the earliest constellationswere named by the 16thcentury. Constellationsdiscovered more recently bearnames drawn from science ortechnology or from exoticfauna discovered in variousplaces across the globe.

Xi Orionis

Sprinkled with Stars

Mu Orionis

Rigel

UNIVERSE 85

Mythological CharactersSince ancient times, animal figures have beenseen represented in the sky by groups of

stars. The constellation of Taurus takes its namefrom its resemblance to a bull. Orion, Cassiopeia,Andromeda, and Perseus were named aftercharacters of Greek tragedies.

The Sky ChangesThe Earth takes one year to orbit theSun. As the planet advances in its orbit,

the nighttime sky changes, allowing differentparts of it to be seen. This is why someconstellations can only be seen during certainseasons of the year. In addition, differentconstellations can be seen from differentlatitudes. Only near the equator is it possibleto see all 88 constellations.

Different CulturesIn antiquity, each culture recognized certainconstellations that other civilizations did not. The

Chinese see smaller, more detailed patterns in the stars,allowing for more precise positional information.Various cultures also tend to use varied names for thesame constellation. Scorpius is recognized by the peopleof Mesopotamia, Greece, Rome, Mesoamerica, andOceania—under different names.

The Constellations of the ZodiacThe 13 constellations located within the elliptical plane—through which the Sun passes as seen from Earth—are

called the constellations of the zodiac. Twelve of theseconstellations have long formed the foundation of astrology, butOphiuchus, the 13th, is ignored by astrologers as a new addition.

URSA MAJORThe bear represented bythis constellation isunusual because of itslong tail. Theconstellations’ shapesrarely agree perfectlywith their namesakes.

SCORPIUSIn Greco-Romanmythology, Orion andScorpius are closelylinked. Orion is the giant,handsome, seductivehunter.

THE CENTAURis a creature from Greekmythology, half man andhalf horse. The centauraccompanied Orionduring his quest torecover his sight.

OBSERVING THE CONSTELLATIONSObservers in both hemispheres can see theconstellations of the zodiac. In the NorthernHemisphere, the southern constellations, such asScorpius, are difficult to see, and in the SouthernHemisphere, northern constellations, such as Gemini, are difficult to study.

LEO CANCERGEMINI

TAURUS

ARIES

PISCES

AQUARIUS CAPRICORN

SAGITTARIUS

SCORPIUS

OPHIUCHUS

LIBRA

VIRGO

88constellations

Babylon13


Celestial CartographyA

s on the Earth, so in heaven. Just as terrestrial maps help us find locations on thesurface of the planet, star charts use a similar coordinate system to indicate variouscelestial bodies and locations. Planispheres, or star wheels, are based on the idea of

a celestial sphere (an imaginary globe onwhich the stars appear to lie and thatsurrounds the planet). Twocommon types are polar andbimonthly star maps.

MAPOF

THE

ST

ELL

AR

NO

RT

HP

OL

EM

AP

OF

TH

EST

ELLA

RSO

UTHPOLE

UNIVERSE 87

Different Types of ChartsThroughout the year, different constellations are visiblebecause the Earth moves along its orbit. As the Earth’s

place in its orbit changes, the night side of the planet facesdifferent regions of space. To compensate for this shiftingperspective, there are various kinds of planispheres: north andsouth polar maps and bimonthly equatorial maps.

Stellar MovementsThe visible regions of the celestial sphereand the ways in which stars move

through the sky depend upon the observer’slatitude. As an observer moves north or south,the visible portion of the celestial sphere willchange. The elevation of the north or southcelestial pole above the horizon determines theapparent motion of the stars in the sky.

Measuring DistancesOnce a star or constellation has beenlocated in the sky, hands and arms can

serve as simple measuring tools. A singleextended finger, shown in the firstillustration, can form a one-degree anglefrom the observer’s line of sight and is usefulfor measuring short distances between stars.The closed palm of the hand forms a 10°angle, and the open hand measures 20°.

THE CELESTIALSPHEREThe celestial sphere is imagined toextend around the Earth and formsthe basis for modern starcartography. The sphere is divided intoa network of lines and coordinatescorresponding to those used on the Earth,allowing an observer to locate constellationson the sphere. The celestial equator is aprojection of the Earth’s equator, the north andsouth celestial poles align with the axis of the Earth,and the elliptic coincides with the path along whichthe Sun appears to move.

POLARThe celestialsphere is generallydivided into twopolar maps: northand south.

AT THE POLESAt the North Pole, the starsappear to rotate around theobserver’s head. The effect isthe same at the South Polebut in the opposite direction.

IN MIDDLELATITUDESsome stars can be seenall year long, but othersare only visible duringcertain months.

AT THE EQUATORstars can be seenthroughout the year,rising in the east andsetting in the west.

EQUATORIALSix bimonthly maps depict all 88constellations, which can be seenover the course of the year.

ONE FINGER

FULL MOON

BIG DIPPER

THE SQUAREOF PEGASUS

CLOSED HAND OPEN HAND

Starmagnitudes

Constellations

Milky Way

HOW TO READ A MAP OF THE SKYAstronomers divide the celestial sphere into sections,allowing them to study the sky in detailed, systematicways. These maps can show a particular region observedfrom a certain place at a certain time, or they canmerely concentrate on a specific location. To specify theposition of a point on the surface of the Earth, thegeographic coordinates called latitude and longitude areused. With the celestial sphere, declination and rightascension are used instead. Observers located at the equator see the celestial equator pass directly over their heads.


From the Home GardenS

targazing is not difficult. After learning to locate celestial objects, manypeople find the hobby very gratifying. With the aid of a star map, you canrecognize galaxies, nebulae, star clusters, planets, and other objects. Some

of these treasures of the universe are visible with the unaided eye, but othersrequire binoculars or even more sophisticated telescopes. Familiarity with thenight sky is useful in many ways.

UNIVERSE 89

SATELLITESThe bigger ones arebrighter than some stars.Some take a while to crossthe sky.

VENUScan generally be seenabove the horizon atdusk or dawn.

How to Look at the MoonUnder various degrees of magnification, the Moon and starstake on different appearances. In some cases, you can makeobservations of the Moon with the unaided eye as well aswith binoculars or a telescope.

Normal view

Planisphere

View with telescope

Moon

Measurement methods A planisphere is a circular star chart that is used tolocate celestial bodies in the celestial sphere. To identify

a particular object, your own arms and body can be used tomeasure its direction and altitude in relation to the horizon.

MEASUREMENT OF ELEVATION

90º

Horizon

Starting at the horizon,extend one of your armsuntil it is perpendicular tothe other.

THE MOTION OFCONSTELLATIONSThe Earth’s rotation makes theplanets and stars appear tomove through the nighttimesky in a general east-to-westdirection. When the southernconstellation Orion, visible fromNovember through March, isviewed from the NorthernHemisphere, it appears to movefrom left to right.

JÚPITER

10 TIMESLARGER

ORIÓN

SOUTH

12:00 A.M.

3:00 A.M.9:00 P.M.

WESTEAST

View with binoculars

MOONThe illuminated face ofthe Moon can always beseen at some time duringthe night, at leastpartially, except aroundthe new moon.

COMETSvisible to the naked eyeappear every one to twoyears and are visible forweeks or even months.

CENTAURI

OMEGA

LIGHT-YEARSFROM EARTH

4.2

LIGHT-YEARS FROM EARTH

17,000

A star to the southwest couldbe located with your arms at45°. Combine the directionalangles with your handmeasurements for elevation.

The planisphere indicates theprincipal direction of a star.Place the arms at 90º, usingnorth or south as the base.

To measure a 45° angle, moveyour arm halfway up from thehorizon.

MEASURING DIRECTION

45º 90º 45º

SHOOTING STARSVery short flashes oflight lasting only afraction of a second

BARREL

OPTIC TUBE

ADJUSTMENTSCREW

OBJECTIVE

PRISM

FOCUSINGWHEEL

FOCUSINGEYEPIECE

TRIPODADAPTER

Flat PerspectiveA constellation is a group of stars that, whenviewed from a certain angle, seem to assume a

specific shape. However, these stars that seem closelyjoined together are, in fact, separated by great distances.

BasicsBefore stepping out to observe the night sky, make sure you have everything you need.If you collect all your supplies beforehand, you will avoid having to expose your eyes to

bright light once they have adjusted to darkness. In addition to binoculars, star maps, and anotebook, you should bring warm clothes, a comfortable seat, and something to drink.

Observable ObjectsThe sky is a very busy place. Not only are there starsand planets, but there are also satellites, airplanes,

comets, and meteorites. Fortunately all become recognizableby their appearance as well as their movement.

Compass Flashlight with red cellophane

50 TO 100TIMES LARGER


A Four-Eyed GiantT he Paranal Observatory, one of the most advanced in the world, is located in the

region of Antofagasta, Chile. It uses four identical telescopes to obtain enough light-gathering power that it could see the flame of a candle on the surface

of the Moon. This sophisticated collectionof digital cameras, reflecting mirrors,and other instruments is mounted inthe interior of four metallicstructures weighing hundreds oftons. The Very Large Telescope(VLT) is operated by a scientificconsortium drawn from eightEuropean countries. One of theirstated objectives is to discovernew worlds orbiting other stars.

UNIVERSE 91

435–455 BCCARACOLIt is located in the ruins of theMayan city of Chichén Itzá. Thestructure was used for veneratingthe Sun, the Moon, and Venus.

2500–2000 BCSTONEHENGELocated in Wiltshire, England, itis an observatory temple datingfrom the Neolithic Period.

ARMILLARYSPHEREInvented byEratosthenes in theyear 225 BC, it wasused as a teachingaid and becameespecially popular inthe Middle Agesthanks to Danishastronomer TychoBrahe.

1979MAUNA KEAAn international complexlocated in Hawaii, with largeBritish, French-American, andAmerican observatories

1897YERKESLocated in Wisconsin, itcontains the largestrefracting telescope inthe world.

1888LICKLocated on 4,265-foot- (1,300-m-) high Mount Hamilton. Itwas the first observatory tobe located on a mountain.

1726JAIPURLocated in India, it was builtby the maharajah Sawai JaiSingh and has a large sextantand a meridional chamber.

Cerro Paranal ObservatoryThe ESO’s Very Large Telescope is located to the north of the Atacama desert, on Cerro Paranal. Completed in

2006, it has four 26.9-foot- (8.2-m-) wide reflector telescopescapable of observing objects four billion times fainter than thosevisible to the unaided eye. It also has three 5.9-foot- (1.8-m-)wide movable auxiliary telescopes that are used in conjunctionwith the larger ones to simulate the light-gathering power of a52-foot- (16-m-) wide mirror (with the resolution of a 656-foot-[200-m-] long telescope). This is enough to see an astronaut onthe Moon. The above technique is called interferometry.

CLIMATIC CONDITIONSCerro Paranal is located in the driest part of theAtacama desert, where the conditions forastronomical observation are extraordinarilyfavorable. It is an 8,645-foot- (2,635-m-) tallmountain that has about 350 cloudless nights a year.

ADAPTIVE OPTICSTo prevent the primary mirror from deforming because ofgravitational effects, the VLT has an adaptive optics systemthat maintains the mirror in optimal shape, with150 supporting pistons that continually adjustthe shape of the mirror.

3.9-foot-(1.2-m-)diametersecondarymirror

Mechanicalstructure

YEPUN

Light tunnels forinterferometry

Rails totransportthe AT

MELIPAL

KUEYEN

ACTIVE OPTICS

ADAPTIVE OPTICS

150-piston cell

ANTU

(20,000 SQ M) TOTAL SURFACE

215,000 SQ FT

(2,365 M) ABOVE SEA LEVEL7,759 FEET

AUXILIARY TELESCOPE (AT)There are four, each 5.9 feet(1.8 m) in diameter. Theyassist with interferometry.

LB PER SQ IN (MBAR)Air pressure

10.9(750)

LB PER CU FT (KG/M3)Air density

0.06(0.96)

FAHRENHEIT (CELSIUS)Average temperature

18–77°(-8–25°)

PERCENTHumidity

5 to 20ACRONYM FORVery Large Telescope

VLT

Curved mirror

Uncorrectedvision

Correctedvision

Telescope units

Lightenters

Reflectedlight beam

DOMEIts protective coverperceives changesin the weather bymeans of thermalsensors.

The TelescopeThe main feature of the VLT is itsrevolutionary optical design. By

using adaptive and active optics, it achievesresolution similar to that possible from space.

92 GLOSSARY UNIVERSE 93

Glossary

AnnihilationTotal destruction of matter in a burst of energy,as when it encounters antimatter.

AntigravityHypothesized force, equal to gravity anddiametrically opposed to it.

AntimatterMatter formed from subatomic particles withshared properties. Its electrical charge isopposite that of normal matter.

ApertureDiameter of the main mirror of a telescope oreyepiece. The larger the aperture, the more lightthe device receives.

AphelionThe point in a celestial body's orbit farthestfrom the Sun. The Earth reaches aphelion on orabout July 4, when it is 95,000,000 miles (152,600,000 km) from the Sun.

ApogeeThe farthest position from the Earth reached bythe Moon or any of the artificial satellites thatorbit the planet.

AsteroidsMinor bodies of the solar system, formed byrock, metal, or a mixture of both. Most asteroidsorbit the Sun between the orbits of Mars andJupiter. Their size ranges from dozens of feet tohundreds of miles.

AstrolabeAncient astronomical instrument for measuringboth the positions and the movements ofcelestial objects.

AstronomyScience that studies the universe. It isconcerned with the physical characteristics,movements, distances, formation, and

interactions of galaxies, stars, planets, moons,comets, asteroids, and other celestial bodies.

AtmosphereLayer of gas retained around a planet by itsgravity. It is also the outer layer of matter in astar, where the energy produced in the star'sinterior is emitted in the form of radiation.

AtomThe smallest part of an element that partakes ofall the element's properties. It is generallycomposed of three subatomic particles: theneutron, the proton, and the electron.

AuroraLuminous phenomenon, with red and greenlayers, visible in the skies of the polar regions.The auroras are caused by the collision of solarparticles with the Earth's atmosphere.

AustralRelated to the Southern Hemisphere.

Big BangCosmological theory asserting that the universebegan to exist as a result of a great explosionthat occurred some 14 billion years ago.

Big CrunchCosmological theory asserting that the universewould undergo a final, complete collapse if itwere to begin to contract.

Black HoleCelestial body so dense that not even light canescape its gravity.

Black Hole, Stellar-MassBlack hole produced by the explosion of amassive star as a supernova. Its mass istypically about 10 times that of the Sun.

Black Hole, SupermassiveBlack hole located at the center of a galaxy and

a meteorite on the surface of a natural satelliteor a planet.

CrustRocky layer of the surface of a planet or naturalsatellite.

Curvature of LightDistortion of light rays when passing throughregions with strong gravitation.

DecayProcess by which radioactive elements andunstable particles become stable substances.Also the way in which black holes eventuallydisappear.

DensityDegree of solidity of a body (its mass divided byits volume).

EclipseVisual concealment of one celestial body byanother. A lunar eclipse occurs when the Moonpasses into the Earth's shadow, and a solareclipse takes place when the Earth passes intothe Moon's shadow.

EclipticImaginary line around the sky along which theSun moves during the year. The orbits of theEarth and the other planets generally lie alongthe ecliptic.

Electrical ChargeProperty of particles causing them to eitherattract or repel each other because of electricalforces. Electrical charges are either positive ornegative.

Electromagnetic RadiationRadiation composed of magnetic and electricfields moving at the speed of light. Itencompasses radio waves (long wavelengths),visible light, and gamma rays (very shortwavelengths).

ElementA basic substance of nature that cannot bediminished without losing its chemicalproperties. Each element (such as hydrogen,helium, carbon, oxygen) has its owncharacteristics.

Elliptical OrbitOrbit shaped like a flattened circle. All orbits areelliptical. A circle is a special form of an ellipse.

EnergyThe capacity to do work.

Event HorizonThe edge of a black hole.

ExtraterrestrialForeign to the Earth.

ForceSomething that changes the motion or shape ofa body.

Galactic FilamentStructure formed by superclusters of galaxiesstretching out through great portions of space.Filaments are the largest structures in theuniverse and are separated by great voids.

GalaxyCollection of billions of stars, nebulae, dust, andinterstellar gas held together by gravity.

Galaxy ClusterGroup of galaxies linked together by gravity.

Gamma RaysForm of electromagnetic radiation withgreatest energy and shortest wavelength. It isgenerated by only the most powerfulphenomena in the universe, such as supernovaeor the fusion of neutron stars.

General RelativityTheory formulated by Albert Einstein in 1915. Inpart, it holds that gravity is a naturalconsequence of the curvature of space-timecaused by the presence of a massive body. Ingeneral relativity, the phenomena of classicalmechanics (such as the orbit of a planet or thefall of an object) are caused by gravity and arerepresented as inertial movements withinspace-time.

Gravitational WaveWaves in space that travel at the speed of lightand are produced by the movements of verymassive bodies.

GravityAttractive force between bodies, such asbetween the Earth and the Moon.

Greenhouse EffectTemperature increase caused by gases (such ascarbon dioxide and methane) that prevent thesurface heat of a planet from escaping intospace.

HeliosphereThe region of space around the Sun in which itseffects are evident. It extends some 100astronomical units around the Sun.

HeliumThe second most common and second lightestelement in the universe. It is a product of thebig bang and of nuclear fusion of stars.

Hubble ConstantNumber that measures the rate of expansion ofthe universe. It is expressed in kilometers persecond per millions of parsecs. It is currentlyestimated at 70 km/s/Mpc.

HydrogenThe most common and lightest element in theuniverse; the main component of stars andgalaxies.

formed by material that falls into the centralregion of the galaxy. Its mass can be a billiontimes that of the Sun.

CarbonOne of the most common elements in theuniverse, produced by stars. All known life iscarbon-based.

ChromosphereThe lowest layer of the Sun's atmosphere. Itemits a pinkish-red light that can be seen onlywhen the brighter photosphere is obscuredduring a total eclipse.

Circumpolar StarAny star always visible to an observer on theEarth as it rotates about the celestial pole.

CometObject made of ice and rock dust. When a cometapproaches the Sun, the growing heat causesthe ice to evaporate, forming a gaseous headand a tail of dust and gas pointing away fromthe Sun.

ConstellationGroup of stars in the sky. Constellations tend tobear the names of mythological characters orcreatures. To astronomers, the constellationsdemarcate regions of the sky.

CoreIn a planet, a solid, high-pressure central mass;in a star, the central region undergoing nuclearfusion; in a galaxy, the innermost light-years.

CoronaUpper atmosphere of the Sun. It is visible as apearly halo during a total solar eclipse.

CosmosAnother name for the universe.

CraterCircular depression formed by the impact of

94 GLOSSARY UNIVERSE 95

HypernovaDestruction of a massive star, which emits awave of gamma rays extending great distancesacross the universe.

ImplosionCollapse of a body upon itself in response togreat external pressure.

Infrared RadiationHeat radiation, with a wavelength betweenvisible light and radio waves.

Intergalactic SpaceSpace between galaxies.

Interstellar SpaceSpace between the stars.

IonosphereRegion of the Earth's atmosphere that iselectrically charged and is located between 30and 370 miles (50 and 600 km) from theEarth's surface.

Kuiper BeltRegion of the solar system that is home tomillions of frozen objects, such as comets. Itstretches from the orbit of Neptune to the innerlimit of the Oort cloud.

LightElectromagnetic radiation with a wavelengthvisible to the human eye.

Light PollutionBrightness of the sky originating in streetillumination and other artificial lighting, whichimpedes the observation of dim celestial objects.

Light-YearStandard astronomical measurement unitequivalent to the distance traveled by light, orany form of electromagnetic radiation, in one

year. Equivalent to 6,000,000,000,000 miles(10,000,000,000,000 km).

Lunar MareThe large, dark regions of the surface of theMoon. They were originally thought to be seas,but they are actually great depressions coveredby lava.

Magnetic FieldThe area near a magnetic body, electriccurrent, or changing electric field. Planets,stars, and galaxies have magnetic fields thatextend into space.

MagnetosphereSphere that surrounds a planet with a magneticfield strong enough to protect the planet fromthe solar wind.

MantleLayer that lies between the crust and the coreof a planet.

MassMeasure of the amount of matter in an object.

MatterThe substance of a physical object, it occupies a portion of space.

MeteoriteRocky or metallic object that strikes the surface of a planet or satellite, where it canform a crater.

Milky WayThe galaxy to which the Sun and the solarsystem belong. It is visible as a pale band oflight that crosses our night sky.

MoleculeSmallest unit of a pure substance that has thecomposition and chemical properties of thesubstance. It is formed by one or more atoms.

January 4, when it is 92,000,000 miles(147,500,000 km) from the Sun.

PhotonElemental particle responsible forelectromagnetic radiation. Photons are the mostcommon particles in the universe.

PlanetRoughly spherical object made of rocks or gasorbiting a star. A planet cannot generate itsown light but reflects the light of its parent star.

PolestarPolaris, a star that lies near the celestial northpole. Polaris is commonly called the North Star. Over thousands of years, other stars willbecome the polestar.

ProtonSubatomic particle with positive electricalcharge. It forms part of the nucleus of an atom.

Radio GalaxyActive galaxy emitting energy as both radiowaves and light. Most of the radio emissionoriginates at the core of the galaxy.

Solar FlareImmense explosion produced on the surface ofthe Sun by the collision of two loops of the solarmagnetic field.

Solar MassStandard unit of mass against which otherobjects in the universe can be compared. The Sun has 333,000 times as much mass asthe Earth.

SpaceThe medium through which all celestial bodiesmove.

Space-TimeFour-dimensional conception of the universe inwhich length, width, and height constitute three

dimensions and time acts as the fourth.

Spectral AnalysisStudy of spectral lines that provide informationabout the composition of stars or galaxies andtheir redshifts.

SpectrumThe result of dispersing the electromagneticradiation of an object so that the wavelengthsof which it is composed can be seen. Dark lines that originate from elements thatare present and punctuate the spectrum atspecific wavelengths reveal the composition of the object.

Speed of LightThe distance traveled by light in a vacuum inone second (approximately 186,000 miles, or300,000 km). No object can move faster thanthe speed of light.

StarEnormous sphere of gas (generally hydrogen)that radiates light and heat. The Sun is a star.

Star ClusterGroup of stars linked together by gravity. Openclusters are scattered groups of several hundredstars. Globular clusters are dense spheres ofseveral million old stars.

SunspotsDark, relatively cool spots on the surface of theSun. They tend to be located on either side ofthe solar equator and are created by the solarmagnetic field.

SupernovaExplosion of a massive star at the end of its life.

TideThe effect of the gravitational pull of oneastronomical object upon the surface ofanother. Ocean tides on Earth are an example.

UnstableTendency to change from one state into anotherless energetic one. Radioactive elements decayinto more stable elements.

VacuumSpace occupied by little or no matter.

Van Allen BeltRadiation zone surrounding the Earth, wherethe Earth's magnetic field traps solar particles.

WavelengthDistance between the peaks of any wave ofelectromagnetic radiation. Radiation with ashort wavelength (such as X-rays) has moreenergy than radiation with a longer wavelength(such as radio waves).

ZenithPoint in the sky 90° above the horizon (that is,immediately above an observer).

ZodiacTwelve constellations through which the Sun,the Moon, and the planets appear to move.

MoonThe Earth's natural satellite is called the Moon.The natural satellites of other planets arecommonly known as moons and have their ownproper names.

NebulaeClouds of gas and dust in space. Nebulae can beseen when they reflect starlight or when theyobstruct light from sources behind them.

NeutronElectrically neutral subatomic particle. It makesup part of an atom's nucleus (with the exceptionof ordinary hydrogen).

Neutron StarCollapsed star consisting mostly of neutrons.

NovaStar that increases greatly in brightness forseveral days or weeks and then slowly fades.Most novae probably occur in binary-starsystems in which a white dwarf draws in matterfrom its companion star.

Nuclear FusionNuclear reaction in which relatively lightelements (such as hydrogen) form heavierelements (such as helium). Nuclear fusion is thesource of energy that makes stars shine.

OxygenChemical element vital to life and to theexpansion of the universe. Oxygen makes up 21percent of the Earth's atmosphere.

ParticleIn particle physics, a tiny, individual componentof matter with characteristic mass, electricalcharge, and other properties.

PerihelionThe point in a celestial body's orbit closest to theSun. The Earth reaches perihelion on or about

96 INDEX UNIVERSE 97

Index

Aaccretion disc, 30, 34, 35, 37active galaxy, 34-35adaptive optics system, 91annular solar eclipse, 78Antennae (NGC 4038 and NGC 4039),

galactic collisions, 32-33anti-gravity, 15, 16antimatter, 10antiparticle, 10, 11aphelion, 75Aristarchus of Samus, 82Aristotle, 82armillary sphere, 82, 90asteroid (minor planet), 5, 63asteroid belt, 40, 41, 63astrolabe, 82astrology, zodiac, 84-85astronomy, 40, 80-91

big bang theory, 10, 12, 13, 14, 15, 32, 34, 83geocentric model, 82heliocentric model, 83Roman Catholic Church opposition, 82, 83See also space exploration; stargazing

atmosphereEarth, 46, 66, 68, 70, 71Jupiter, 50lunar eclipse, 79Mars, 49Mercury, 45Neptune, 57Pluto, 59Saturn, 53solar, 42thickness, 71Uranus, 54, 55Venus, 46

atom, 10, 11, 12, 16, 17, 31attraction, forces of nature, 16, 17, 31

Bbaby universe, 15Babylon, 85background radiation, 11, 15big bang theory, 10, 12, 13, 14, 15, 32, 34, 83Big Crunch, 14, 15bimonthly star map, 86black dwarf (star), 23black hole, 4, 15, 19

active galaxies, 34anti-gravity, 15, 16discovery, 30formation, 22, 23, 29, 30gravitational force, 30, 31Milky Way, 37temperature, 35

blazar (galaxy), 35blueshift, Doppler effect, 21Butterfly Nebula (M2-9), 26

Ccalendar, 5, 74, 82carbon, 12, 24Cassini division, rings of Saturn, 52Cat's Eye Nebula (NGC 6542), 26-27celestial cartography, 86-87

See also star chartcelestial equator, 86, 87celestial sphere, 86, 87Ceres (asteroid), 63Cerro Paranal Observatory: See Paranal

Astronomical ObservatoryChandrasekhar limit, stellar collapse, 26Charon (moon of Pluto), 58chromosphere, 43clock, 74, 82closed universe, 14color, stars, 20-21comet, 5, 64-65, 89

formation, 65

Coordinated Universal Time (UTC), 75Copernican Revolution, 83Copernicus, Nicholas, 5, 82corona (Sun), 43cosmic inflation theory, 10, 11cosmic void, 8cosmos: See universeCrab Nebula (M1), 29crater

Earth, 62Mercury, 44Moon, 77

creation, 10-11big bang theory, 10, 12, 13, 14, 15, 32, 34, 83cosmic inflation, 10, 11time and temperature, 10-13

critical mass, 14, 15curvature of space-time, 16, 31

Ddark energy, 13, 14dark matter, 4, 6-7, 11, 12, 13declination, 87dew, 69dinosaur, mass extinction, 62Doppler effect, 21, 32double planet, 58dwarf planet, 5, 57, 58, 60, 61

See also Pluto

EE=mc2 (equation), 16Earth, 39, 41, 66-75

aerial view, 66-67aphelion, 75atmosphere, 46, 66, 68, 70, 71axis inclination, 69, 74, 75center of universe, 4, 5, 82chronology, 73

comparison to Mars, 48 composition, 70continental drift, 72continental penetration, 70cooling, 73distance from Sun, 68eclipses, 78-79equinox and solstice, 74, 75essential data, 69evolutionary process, 62formation, 13, 72-73geographical coordinates, 75gravity, 69hydrosphere, 71internal structure, 70-71lithosphere, 71magnetic field, 69, 70magnetic inversion, 54moon and tides, 68, 76-77movements, 74-75night sky, 84-85, 88-89nutation, 74oceanic penetration, 70oceans, 68, 73one day, 72orbit, 75origin, 8, 73ozone layer, 71perihelion, 74revolution, 68, 74rotation, 21, 68, 74solar radiation, 71thermosphere, 71tides, 76time zones, 75water, 68-69, 71, 73

earthquake, 72eclipse, 78-79

lunar, 79observation from Earth, 79solar, 78

Einstein, Albert, 16electromagnetism, 11, 16, 17, 69electron, 10, 11, 12elliptical galaxy, 33

energy, 16, 34, 42Enke division, 52equinox, autumn and spring, 74Eris, Kuiper belt objects, 61Eta Carinae Nebula, 18-19, 29, 36evaporation, 68event horizon, black holes, 31evolution, timeline, 72-73extrasolar planet, discovery, 61

Ffilament, 9, 12, 29flat perspective, 89flat universe, 14, 16forces of nature, 16-17

electromagnetism, 11, 16, 17, 69gravity, 11, 14, 16, 69, 76nuclear interactions, 11, 16, 17unified, 11, 16

fossil, 73frost, 69

GGacobini-Zinner comet, 64galactic cluster, 33galaxy, 8, 9

active, 34-35anatomy, 32-33 classification, 33, 35clusters, 33collision, 19, 32-33energy emission, 34expansion, 8, 83formation, 12, 32, 33, 35galactic clusters, 33Mice, The, 32Milky Way, 5, 8, 32, 33, 36-37number, 9shape, 12, 32

Gacobini-Zinner, 64Halley's, 64Kuiper belt, 6, 60, 64missions to, 64parts, 64, 65types, 64

condensation, 69Cone Nebula, 4constellation, 5, 84-85

Andromeda, 9, 85Aquarius, 85, 87Aries, 84, 86Cancer, 74, 86Capricorn, 85, 87Cassiopeia, 85celestial sphere, 86-87Centaur, 85cultural interpretations, 85discovery, 84flat perspective, 89Gemini, 74, 86Leo, 84, 86Libra, 85, 87locating, 86, 88-89measuring distances, 86motion, 88mystery of Giza, 85mythological characters, 85naming, 84near equator, 84number, 84observing, 84, 88-89Ophiuchus, 85origin, 84Orion, 85, 88Perseus, 85Pisces, 84, 86Sagittarius, 37, 85, 87Scorpius, 85, 87sky changes, 84Taurus, 74, 85, 86Ursa Major, 85Virgo, 85, 87zodiac, 84-85, 86-87

continental drift, 72-73

98 INDEX UNIVERSE 99

stabilization, 35subclassification, 33superclusters, 9, 19total number, 9types, 32

Galileo Galilei, 5, 36, 51, 83Gamow, George, 15gas, 12, 34, 37general theory of relativity, 16, 31geocentric model (astronomical theory), 82geology, 73Gliese (star), extrasolar planet, 61globular cluster, 21gluon, 10, 11, 17graviton, 10gravity (gravitation), 11, 12, 14, 16, 17

black holes, 30, 31dark matter, 7Earth, 69galaxy formation, 12, 34, 35Jupiter and asteroid belt, 41 Milky Way, 37moon, 76Newton's theory, 16, 17precipitation, 69Sun, 40, 41tides, 76weight, 69

Great Dark Spot (Neptune), 57Great Red Spot (Jupiter), 50, 51greenhouse effect, Venus, 46Greenwich meridian, 75Guth, Alan, 11

Hhail, 69Halley, Edward, 33Halley's comet, 64Hawking, Stephen, 5, 14heliocentric model (astronomical theory), 83helium, 11, 12, 17, 20, 22, 24, 42, 57Helix Nebula (NGC 7293), 27

Hercules Cluster (NGC 6205), 33Hertzsprung-Russell diagram (H-R diagram),

20, 24, 25Homo sapiens, first appearance, 13, 72, 73Hourglass Nebula (MYCN 18), 27Hubble, Edwin, 15, 32, 33, 83Hubble Space Telescope, 55, 58hydrogen, 11, 12, 17, 20, 21, 22, 24, 26, 42, 57

IIda (asteroid), 63impact crater, 44, 62, 77inner planet, 41interferometry, 91International Astronomical Union, 57, 60intrinsic luminosity, stars, 20iron meteorite, 62irregular galaxy, 33

Jjet lag, 75Jupiter, 41, 50-51

missions to, 51, 83

KKant, Immanuel, 32Kepler, Johannes, 40, 82, 83Kirkwood gap, asteroid belt, 63Kuiper belt, 5, 59, 60-61, 64

LLarge Magellanic Cloud (galaxy), 28, 36latitude, 75, 87

Nneap tide, 76nebula, 22

Butterfly, 26Cat's Eye, 26-27Cone, 4Crab, 29Eta Carinae, 18-19, 29, 36Helix, 27Hourglass, 27NGC 6751, 25planetary, 3, 22, 23, 25, 26solar, 72Spirograph, 26

Neptune, 40, 56-57missions to, 56, 83orbit, 60-61

neutrino, 11, 17, 42neutron, 11, 17, 42neutron star, 22, 23, 29, 31Newton, Isaac, 16, 17, 83NGC 6751 (nebula), 251987a supernova, 28nuclear interaction, 11, 16, 17

Oobservation: See stargazingobservatory

Caracol, 90Jaipur, 90-91Lick, 90Mauna Kea, 90Paranal, 90Stonehenge, 80-81, 90-91Yerkes, 90

ocean, creation, 73Olympus Mons (Mars), 38-39, 49Omega Centauri (star cluster), 21Oort cloud (region of Solar System), 60, 64open cluster, 21

open universe, 15orbit, 40

Earth, 75Eris, 60-61Mars, 48Mercury, 44, 45Neptune, 60Pluto, 58, 59, 60-61Saturn, 60Uranus, 60

outer planet, 40ozone layer, 68, 71

Pparallax, 21parallels, geographical coordinates, 75Paranal Astronomical Observatory, 90, 91

See also Very Large Telescopeparsec, 20partial eclipse

lunar, 79solar, 78

penumbra, 43, 78, 79penumbral lunar eclipse, 79Penzias, Arno, 15perihelion, 74phase

Moon, 77Venus, 47

photon, 10, 12, 42photosphere, Sun, 43planet, 5, 40-41

extrasolar discovery, 61formation, 41, 62inner, 41irregular movement, 83observation, 89orbits, 40outer, 40remnant materials, 62revolution, 40See also under specific name, for example

Earth; Marsplanetary nebula, 1, 22, 23, 25, 26planetesimal, 41planisphere (star wheel), 86, 88, 89plasma, 43plate tectonics, Earth, 73Pleiades, The (star formation), 21Pluto, 5, 58-59

diameter, 60, 61orbit, 58, 59, 60-61

polar star map, 86Pope, Alexander, 55positron, 11, 42precipitation, 69protogalaxy 12proton, 11, 12, 17, 42protostar, 22Ptolemy, Claudius, 4, 5, 82pulsar, 19, 29, 31

QQuaoar, Kuiper belt objects, 60, 61quark, 10, 11, 17quasar, 19, 34, 35

Rradiation, 10, 11, 12, 29

background, 11, 15solar, 41, 67, 71, 76

radio galaxy, 35rain, 69red giant (star), 23, 24, 25, 31red planet: See Marsred supergiant (star), 23, 24redshift, Doppler effect, 21, 32relativity, general theory of, 16, 31repulsion, forces of nature, 17rift, 72right ascension, 87

life, existence of, 5, 8, 12, 68, 72-73, 83light

bending, 17spectral analysis, 20, 21

light-year, 8, 20longitude, 75, 87luminosity, stars, 20-21, 24, 25lunar eclipse, 79

Mmacrospicule, 43magnetic field, 37, 45, 51, 54, 69magnetic inversion, 54magnetosphere, 51main sequence, stellar life cycle, 20map, celestial cartography, 86-87Mars, 39, 41, 48-49

exploration, 48-49Olympus Mons, 38-39, 49

mass, 16, 17 matter, 10, 11, 12, 13, 30Mercury, 41, 44-45mesosiderite (meteorite), 62meteorite, 62-63

See also asteroidmethane, 58Mice, The (NGC 4676), galactic collision, 32Milky Way, 5, 8, 32, 33, 35, 36-37moon

Earth: See MoonGalilean, 51Jupiter, 51Mars, 48Neptune, 56Pluto, 58Saturn, 52Uranus, 55

Moon (Earth's), 68, 76-77, 89eclipses, 79landscape, 77, 83looking at, 88

mythology, constellations, 85

100 INDEX

ring systemJupiter, 51Neptune, 56Saturn, 52Uranus, 55

SSagittarius A and B (gas clouds), 37Saros period, eclipses, 79satellite, 5, 55, 56, 59

See also moonSaturn, 40, 52-53

missions to, 83orbit, 60-61

Scorpius (region of space), 20-21Sedna, diameter, 61self-generated universe, 15Shakespeare, William, 55shooting star, 62, 89snow, 69solar eclipse, 78-79solar flare, 43solar nebula, 72solar prominence, 43solar radiation, 41, 67, 71, 76Solar System, 5, 38-65

components, 39, 40-41Earth, 39, 41, 66-67Eris, 61formation, 13inner planets, 41Jupiter, 40, 41, 50-51Mars, 39, 41, 48-49Mercury, 41, 44-45Neptune, 40, 56-57origin, 60outer planets, 40Pluto, 5, 58-59Saturn, 40, 52-53“Termination Shock” region, 83Uranus, 40, 54-55Venus, 41, 46-47

solar wind, 43, 49, 51, 69solstice, summer and winter, 74, 75Sombrero Galaxy, 32-33Sounds of Earth (Voyager recording), 83space exploration, 44, 47, 48, 49, 55, 56, 64,

76, 83space-time, 16spatial dimension, 14spectral class, stars, 20spectrum, 21spicule, 43spiral galaxy, 32, 33Spirograph Nebula (IC 418), 26spring tide, 76star, 84-85

black dwarf, 23, 24blue, 20brightness, 20calendar, 5, 82celestial sphere, 86-87chart: See star chartcolor, 20composition, 12, 20, 21, 29constellations, 37, 74, 84-85, 88convection cells, 24core, 24, 28, 31death, 24-25, 28, 29distance from Earth, 20, 21Doppler effect, 21dust grains, 25end, 29final darkness, 30-31formation, 12, 22-23, 28, 34gas shells, 26-27groups, 5, 21Hertzsprung-Russell diagram, 20, 24, 25hot spots, 25life cycle, 22-23, 24-25, 26, 28, 30luminosity, 20-21, 24, 25 main sequence, 20maximum mass, 26navigation, 5near, 8origin, 22-23parallax, 21

shooting stars, 89supplies, 88telescope, 88unaided eye, 88Venus, 89Very Large Telescope, 90-91

stellar map, 5, 86-87stellar wind, 25Stonehenge (United Kingdom), 80-81, 90-91stony meteorite, 62strong nuclear force (strong nuclear

interaction), 11, 16, 17subatomic particle, 10subduction, 72Sun (Earth's), 22, 42-43

ancient peoples, 38astronomical theories, 82, 83convective zone, 42core, 43corona, 43distance to earth, 78eclipses, 78-79 essential data, 42formation, 13, 72future, 25gravitational pull, 31, 40, 41luminosity, 20photosphere, 43radiation, 41, 67, 71, 76radiative zone, 42size, 31, 78surface and atmosphere, 43

sunspot, 43, 83supercluster (galaxy), 9, 19supergiant (star), 28, 31supernova, 12, 19, 22, 23, 26, 28-29

Ttectonic plate, 72, 73telescope, 4, 82

auxiliary, 90early, 81

Galileo, 5, 36, 83Hubble Space Telescope, 55object position, 17parts, 88-89Very Large Telescope, 90-91visible matter, 5, 12, 20, 30, 34, 52, 56

temperatureblack holes, 35 stars, 20, 24

temporal dimension, 14Termination Shock (solar system region), 83thermonuclear fusion, 42tide, 51, 76time, 16, 17

curvature of space-time, 16, 31Greenwich median, 75measurement, 74, 82zone, 75

total eclipselunar, 79solar, 78

Uumbra, 43, 78universal gravitation, theory of, 16, 17universe

background radiation, 11, 15composition, 8-9, 14cooling, 12creation, 10-11curvature of space-time, 16defined, 6-7, 18-19expansion, 7, 8, 10, 11, 13, 16, 32, 83expansion theories, 14-15forces, 11, 16-17future, 14geocentric model, 82heliocentric model, 83history, 13observation, 80-81, 90-91secrets, 4-5 size/immensity, 8

transparent, 12-13Uranus, 40, 54-55

missions to, 55, 83orbit, 60-61

VValles Marineris canyon, Mars, 49Van Allen belts, Earth, 69Venus, 41, 46-47, 89Very Large Telescope (VLT), 81, 90-91

auxiliary telescope, 90dome, 90optics, 90, 91photograph, 90-91

volcanism, 41, 47, 49, 77

Wwater, 44, 46, 48, 61, 68, 71weak, light particle (WIMP), 17weak nuclear force (weak nuclear

interaction), 11, 16, 17weight, relation to gravity, 69white dwarf (star), 23, 25, 26-27, 31Wilson, Robert, 15wind, 51, 53WMAP (Wilkinson Microwave Anisotropy

Probe) project, 11wormhole, 31

X-ZX-ray, black holes, 30zero time, 10zodiac, constellations, 84-85

fdf dsdfsdaf

planetary nebulae, 26principal, within 100 light-years from Sun,

20-21red giant, 20, 23, 24, 25, 31red supergiant, 20, 23, 24reference points, 84shooting, 62size, 22, 24, 26stellar movement, 87stellar remnant, 29study, 80supergiant, 24, 28, 31supernovae, 28-29superstition and ritual, 80temperature, 20, 24white dwarf, 23, 24, 25, 26-27, 31See also Sun

star chartcelestial sphere, 86equatorial, 87measuring distances, 86observer's latitude, 87polar, 87reading a map of the sky, 87stellar movements, 87stellar north pole, 86stellar south pole, 87types, 87

star wheel (planisphere), 86, 88, 89stargazing, 88-89

auxiliary telescope, 90basics, 88binoculars, 88comets, 89flat perspective, 89history, 82-83instruments for, 82measuring direction, 89moon, 88, 89motion of constellations, 88night sky, 88observable objects, 88observatories, 80-81, 90-91recognizing objects, 88, 89satellites, 89

UNIVERSE 101