Ch 1 Astronomy 9/20/04 2:41 PM Page 2 1 Astronomy …and .... 1 The... · The booklet should have a model of the solar system that will help citizens understand the relative sizes

Astronomy …and Your CommunityGetting Started

Throughout time, all systems in the universe are affected byprocesses and outside influences that change them in some way.This includes Earth and the solar system in which it exists. Youhave years of experience with life on the third planet from theSun, and you know a lot about your tiny corner of the universe.Think about the Earth in relation to its neighbors in the solarsystem.

• What objects make up the solar system?

• How far is the Earth from other objects in the solar system?

• Which objects in the solar system can influence the Earth?

• Can you think of any objects or processes outside the solarsystem that might affect the Earth?

Write a paragraph about Earth and its place in this solar system.After that, write a second paragraph about processes or events inthe solar system that could change Earth. Describe what they doand how Earth is, or might be, affected. Try to include answersto the questions above.

Scenario

Scientists recently announced that an asteroid 2-km wide, asteroid1997XF11, would pass within 50,000 km of Earth (about one-eighth the distance between the Earth and the Moon) in October2028. A day later, NASA scientists revised the estimate to800,000 km. News reports described how an iron meteoriteblasted a hole more than 1 km wide and 200 m deep, andprobably killed every living thing within 50 km of impact. Thatcollision formed Arizona’s Meteor Crater some 50,000 years ago.Such a collision would wipe out a major city today. These reports

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Ch 1 Astronomy 9/20/04 2:41 PM Page 2

how likely it is that one willcollide with Earth in your lifetime,and what would happen if onedid.

• Explain why extraterrestrialinfluences on your community area natural part of Earth systemevolution.

The booklet should have a model ofthe solar system that will help citizensunderstand the relative sizes of anddistances between solar-system bodies.

Assessment Criteria

Think about what you have beenasked to do. Scan ahead through thechapter activities to see how theymight help you to meet the challenge.Work with your classmates and yourteachers to define the criteria forassessing your work. Record all ofthis information. Make sure that youunderstand the criteria as well as youcan before you begin. Your teachermay provide you with a samplerubric to help you get started.

have raised concern in yourcommunity about the possibility ofa comet or asteroid hitting theEarth. Your class will be studyingouter space and the effects that theSun and other objects in the solarsystem can have on the Earth. Canyou share your knowledge withfellow citizens and publish a bookletthat will discuss some of thepossible hazards from outer space?

Chapter Challenge

In your publication, you will needto do the following:

• Describe Earth and its place in theuniverse. Include informationabout the formation and evolutionof the solar system, and about theEarth’s distance from and orbitaround the Sun. Be sure to mentionEarth’s place in the galaxy, and thegalaxy’s place in the universe.

• Describe the kinds of solar activitiesthat influence the Earth. Explainthe hazardous and beneficial effectsthat solar activity (sunspots andradiation, for example) have on theplanet. Discuss briefly the Sun’scomposition and structure, and thatof other stars.

• Discuss the Earth’s orbital andgravitational relationships withthe Sun and the Moon.

• Explain what comets andasteroids are, how they behave,

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GoalsIn this activity you will:

• Produce a scale model of the solarsystem.

• Identify some strengths andlimitations of scale models.

• Calculate distances to objects in thesolar system in astronomical units(AU), light- years, and parsecs.

• Explain, in your own words, thenebular theory of the formation ofthe solar system.

• Explain the formation of theuniverse.

Think about It

Earth is part of a large number of objects that orbit around a starcalled the Sun.

• What objects make up the solar system? Where are they locatedin relation to Earth?

What do you think? Record your ideas in the form of a diagramof the solar system in your EarthComm notebook. Withoutlooking ahead in this book, draw the Sun and the planets, and thedistances from the Sun to the planets, as nearly to scale as youcan. Be prepared to discuss your diagram with your small groupand the class.

Activity 1 The History and Scale of the Solar System

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Earth System Evolution Astronomy


Investigate

1. Use the data in Table 1 to make a scalemodel of the solar system. Try usingthe scale 1 m = 150,000,000 km.

a) Divide all the distances in the firstcolumn by 150,000,000 (onehundred and fifty million). Writeyour scaled-down distances inyour notebook, in meters.

b) Divide all the diameters in thesecond column by 150,000,000.Write your scaled-down diametersin your notebook, in meters.

c) Looking at your numbers, whatmajor drawback is there to usingthe scale 1 m = 150,000,000 km?

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Table 1 Diameters of the Sun and Planets, and Distances from the Sun

Object Distance from Sun (km) Diameter (km)

Sun 0 1,391,400

Mercury 57,900,000 4878

Venus 108,209,000 12,104

Earth 149,598,770 12,756

Mars 227,900,000 6794

Jupiter 778,200,000 142,984

Saturn 1,429,200,000 120,536

Uranus 2,875,000,000 51,118

Neptune 4,504,400,000 49,528

Pluto 5,915,800,000 2302

2. Now try another scale: 1 m = 3,000,000 km (three millionkilometers).

a) Divide all the distances in the firstcolumn by 3,000,000. Write yourscaled-down distances in yournotebook in meters.

b) Divide all the diameters in thesecond column by 3,000,000.Write your scaled-down diametersin your notebook in meters.

c) Looking at your numbers, whatmajor drawback is there to usingthe scale 1 m = 3,000,000 km?

3. Using what you have learned aboutscaling distances and diameters in thesolar system, make models of the Sunand the planets. Each of the planetscan be drawn on a different sheet ofpaper using a ruler to lay out thecorrect sizes for the different planetsand the Sun.

4. To represent the distances from theSun to the planets you will need touse a tape measure. You may want tomeasure the size of your stride anduse this as a simple measuring tool.


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To do this, stand behind a line and takefive steps in as normal a way as possibleand note where your last step ended.Now measure the distance from whereyou started to the end. Divide by five todetermine how far you walk with eachstep. Knowing the length of your strideis an easy way to determine distances.

a) Explain the scale(s) you decided touse and your reasons for yourchoices.

b) Is it possible to make a model ofthe solar system on your schoolcampus in which both thedistances between bodies and thediameters of the bodies are to thesame scale? Why or why not?

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Reflecting on the Activity and the Challenge

In this activity you used ratios to makea scale model of the solar system. Youfound out that scale models help youappreciate the vastness of distances inthe solar system. You also found out

that there are some drawbacks to theuse of scale models. Think about howyou might use the model you made aspart of your Chapter Challenge.

Digging Deeper

OUR PLACE IN THE UNIVERSE

Distances in the Universe

Astronomers often study objects far from Earth. It is cumbersome to use unitslike kilometers (or even a million kilometers) to describe the distances to thestars and planets. For example, the star nearest to the Sun is called ProximaCentauri. It is 39,826,600,000,000 km away. (How would you say this distance?)

Astronomers get around the problem by using larger units to measuredistances.When discussing distances inside the solar system, they often use theastronomical unit (abbreviated as AU). One AU is the average distance ofthe Earth from the Sun. It is equal to 149,598,770 km (about 93 million miles).

Stars are so far away that using astronomical units quickly becomes difficult,too! For example, Proxima Centauri is 266,221 AU away.This number is easierto use than kilometers, but it is still too cumbersome for most purposes. Fordistances to stars and galaxies, astronomers use a unit called a light-year.Alight-year sounds as though it is a unit of time, because a year is a unit of time,but it is really the distance that light travels in a year. Because light travels

Geo Wordsastronomical unit: a unit ofmeasurement equal to theaverage distance between theSun and Earth, i.e., about149,600,000 (1.496 x 108) km.

light-year: a unit ofmeasurement equal to thedistance light travels in oneyear, i.e., 9.46 x 1012 km.


Geo Wordsparsec: a unit used inastronomy to describe largedistances. One parsec equals3.26 light-years.

nebula: general term used forany “fuzzy” patch on the sky,either light or dark; a cloud ofinterstellar gas and dust.

constellation: a grouping ofstars in the night sky into arecognizable pattern. Most ofthe constellations get theirname from the Latintranslation of one of theancient Greek star patternsthat lies within it. In morerecent times, more modernastronomers introduced anumber of additional groups,and there are now 88standard configurationsrecognized.

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extremely fast, a light-year is a very large distance. For example, the Sun is only8 light minutes away from Earth, and the nearest stars are several light-yearsaway. Light travels at a speed of 300,000 km/s.This makes a light-year 9.46 x 1012 (9,460,000,000,000) km. Light from Proxima Centauri takes 4.21 years to reach Earth, so this star is 4.21 light-years from Earth.

Astronomers also use a unit called the parsec (symbol pc) to describe largedistances. One parsec equals 3.26 light-years.Thus, Proxima Centauri is 1.29 pc away.The kiloparsec (1000 pc) and megaparsec (1,000,000 pc) areused for objects that are extremely far away.The nearest spiral galaxy to theMilky Way galaxy is the Andromeda galaxy. It is about 2.5 million light-years,or about 767 kpc (kiloparsecs), away.

The Nebular Theory

As you created a scale model of the solar system, you probably noticed howlarge the Sun is in comparison to most of the planets. In fact, the Suncontains over 99% of all of the mass of the solar system.Where did all thismass come from? According to current thinking, the birthplace of our solarsystem was a nebula.A nebula is a cloud of gas and dust probably cast offfrom other stars that used to live in this region of our galaxy. More than 4.5 billion years ago this nebula started down the long road to the formationof a star and planets.The idea that the solar system evolved from such aswirling cloud of dust is called the nebular theory.

You can see one such nebula in thewinter constellation Orion (seeFigure 1), just below the three starsthat make up the Belt of Orion.Through a pair of binoculars or asmall, backyard-type telescope, theOrion Nebula looks like a faint green,hazy patch of light. If you were ableto view this starbirth region througha much higher-power telescope, youwould be able to see amazing detailsin the gas and dust clouds.The OrionNebula is very much like the onethat formed our star, the Sun.Thereare many star nurseries like this onescattered around our galaxy. On adark night, with binoculars or a smalltelescope, you can see many gasclouds that are forming stars.

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Figure 1 Orion is a prominent constellation in the night sky.


In the nebula that gave birth to our solarsystem, gravity caused the gases and dustto be drawn together into a densercloud.At the same time, the rate ofrotation (swirling) of the entire nebulagradually increased.The effect is the sameas when a rotating ice skater draws hisor her arms in, causing the rate ofrotation to speed up.As the nebularcloud began to collapse and spin faster, itflattened out to resemble a disk, withmost of the mass collapsing into thecenter. Matter in the rest of the diskclumped together into small massescalled planetesimals, which thengradually collided together to form largerbodies called protoplanetary bodies.

At the center of the developing solar system, material kept collapsing undergravitational force.As the moving gases became more concentrated, thetemperature and pressure of the center of the cloud started to rise.Thesame kind of thing happens when you pump up a bicycle tire with a tirepump: the pump gets warmer as the air is compressed.When you let the airout of a tire, the opposite occurs and the air gets colder as it expandsrapidly.When the temperature in the center of the gas cloud reached about15 million degrees Celsius, hydrogen atoms in the gas combined or fused tocreate helium atoms.This process, called nuclear fusion, is the source ofthe energy from the Sun.A star—the Sun—was born!

Fusion reactions inside the Sun create very high pressure, and like a bomb,threaten to blow the Sun apart.The Sun doesn’t fly apart under all thisoutward pressure, however.The Sun is in a state of equilibrium.The gravityof the Sun is pulling on each part of it and keeps the Sun together as itradiates energy out in all directions, providing solar energy to the Earthcommunity.

The Birth of the Planets

The rest of the solar system formed in the swirling disk of materialsurrounding the newborn Sun. Nine planets, 67 satellites (with new ones stillbeing discovered!), and a large number of comets and asteroids formed.Thelarger objects were formed mostly in the flat disk surrounding where theSun was forming.

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Figure 2 The Keyhole Nebula. Imaged bythe Hubble Space Telescope.

Geo Wordsplanetesimal: one of thesmall bodies (usuallymicrometers to kilometers indiameter) that formed fromthe solar nebula andeventually grew into proto-planets.

protoplanetary body: aclump of material, formed inthe early stages of solarsystem formation, which wasthe forerunner of the planetswe see today.

nuclear fusion: a nuclearprocess that releases energywhen lightweight nucleicombine to form heaviernuclei.


Geo Wordsterrestrial planets: any of theplanets Mercury, Venus, Earth,or Mars, or a planet similar insize, composition, and densityto the Earth. A planet thatconsists mainly of rockymaterial.

gas giant planets: the outersolar system planets: Jupiter,Saturn, Uranus, and Neptune,composed mostly of hydrogen,helium, and methane, andhaving a density of less than 2 gm/cm2.

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Four of these planets, shown in Figure 3—Mercury,Venus, Earth, and Mars—arecalled the terrestrial (“Earth-like”) planets.They formed in the inner part ofour solar system, where temperatures in the original nebula were high.They arerelatively small, rocky bodies. Some have molten centers, with a layer of rockcalled a mantle outside their centers, and a surface called a crust.The Earth’scrust is its outer layer. Even the deepest oil wells do not penetrate the crust.

The larger planets shown in Figure 3—Jupiter, Saturn, Uranus, and Neptune—consist mostly of dense fluids like liquid hydrogen.These gas giants formed inthe colder, outer parts of the early solar nebula.They have solid rocky coresabout the size of Earth, covered with layers of hydrogen in both gas and liquidform.They lie far from the Sun and their surfaces are extremely cold.

Pluto is the most distant planetfrom the Sun. Some astronomersdo not even classify it as a planet,and there is controversy aboutwhether Pluto should be includedamong the "official" planets. Pluto is very different from the terrestrialor the gaseous planets. Somescientists think that it may not have been part of the original SolarSystem but instead was capturedlater by the Sun’s gravity. Othersthink that it was once a moon of an outer planet because itresembles the icy moons of the gas giants.

All life, as we know it on Earth, isbased on carbon and water.Although there is evidence thatwater may once have been plentifulon Mars, liquid water has not been found on any of the planets in the SolarSystem.Without water, these planets cannot support life as we know it.There is evidence, however, that Jupiter's moon Europa may be covered withice, and that an ocean may lie beneath the ice. Scientists have found that lifeon Earth can thrive in even some of the most extreme environments.Powerful telescopes have discovered organic molecules in interstellar gasand have discovered planets around many nearby stars.Although life may notexist on the other planets in the solar system, the possibility of life onEuropa and in other places in the universe exists!

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Figure 3 Composite image of the planets inthe solar system, plus the Moon.


There are trillions of comets and asteroids scattered throughout the solarsystem. Earth and other solar-system bodies are scarred by impact cratersformed when comets and asteroids collided with them. On Earth, erosion hasremoved obvious signs of many of these craters.Astronomers see these cometsand asteroids as the leftovers from the formation of the solar system.Asteroidsare dark, rocky bodies that orbit the Sun at different distances. Many are foundbetween the orbits of Mars and Jupiter, making up what is called the asteroidbelt. Many others have orbits outside of the asteroid belt. Comets are mixturesof ice and dust grains.They exist mainly in the outer solar system, but whentheir looping orbits bring them close to the Sun, their ices begin to melt.That iswhen you can see tails streaming out from them in the direction away from theSun. Some comets come unexpectedly into the inner solar system. Others haveorbits that bring them close to the Sun at regular intervals. For example, theorbit of Halley’s comet brings it into the inner solar system every 76 years.

Where is the Solar System in Our Galaxy?

Have you ever seen the Milky Way? It is a swath of light, formed by the glowof billions of stars, which stretches across the dark night sky. From Earth, thisband of celestial light is best seen from dark-sky viewing sites. Binoculars andbackyard-type telescopes magnify the view and reveal individual stars andnebulae. Unfortunately, for those who like to view the night sky, lightpollution in densely populated areas makes it impossible to see the MilkyWay even on nights when the atmosphere is clear and cloudless.

Galaxies are classified according to their shape: elliptical, spiral, or irregular. Ourhome galaxy is a flat spiral, a pinwheel-shaped collection of stars held togetherby their mutual gravitational attraction. Our galaxy shown in Figure 5 is called theMilky Way Galaxy, or just the galaxy. Our solar system is located in one of thespiral arms about two-thirds of the way out from the center of the galaxy.Whatis called the Milky Way is the view along the flat part of our galaxy.When youlook at the Milky Way, you are looking out through the galaxy parallel to the

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Geo Wordscomet: a chunk of frozengases, ice, and rocky debristhat orbits the Sun.

asteroid: a small planetarybody in orbit around the Sun,larger than a meteoroid (a particle in space less than afew meters in diameter) butsmaller than a planet. Manyasteroids can be found in abelt between the orbits ofMars and Jupiter.

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Saturn

Uranus Neptune

Pluto

VenusMercury

Earth

Mars

Jupiter

Jupiter

Figure 4 Two diagrams are required to show the orbits of the planets to scale.


Geo Wordscosmologist: a scientist whostudies the origin anddynamics of the universe.

1. What are the distancesrepresented by a light-year, an astronomicalunit, and a parsec?

2. Which of the units inQuestion 1 would youuse to describe each ofthe following? Justifyyour choice.

a) Distances to variousstars (but not ourSun)?

b) Distances to variousplanets within oursolar system?

c) Widths of galaxies?

3. In your own words,explain the nebulartheory for thebeginning of our solarsystem.

4. Briefly describe theorigin of the universe.

Check YourUnderstanding

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plane of its disk.The individual stars you see dotting the night sky are just theones nearest to Earth in the galaxy.When you view the Milky Way, you are“looking through” those nearest stars to see the more distant parts of thegalaxy. In a sense, you are looking at our galaxy from the inside. In otherdirections, you look through the nearest stars to see out into intergalactic space!

Our Milky Way Galaxy formed about 10 billion years ago and is one ofbillions of galaxies in the universe.The universe itself formed somewherebetween 12 to 14 billion years ago in an event called the Big Bang.Thissounds like the universe began in an explosion, but it did not. In thebeginning, at what a scientist would call “time zero,” the universe consistedalmost entirely of energy, concentrated into a volume smaller than a grain ofsand.The temperatures were unimaginably high.Then the universe expanded,extremely rapidly, and as it expanded the temperature dropped and matterwas formed from some of the original energy. Cosmologists (scientistswho study the origin and dynamics of the universe) think that most of thematter in the universe was formed within minutes of time zero! Theexpansion and cooling that started with the Big Bang continues to this day.

The galaxies and stars are the visible evidence of the Big Bang, but there isother, unseen evidence that it happened. It’s called the cosmic backgroundradiation, which is radiation that is left over from the initial moments of theBig Bang.Astronomers using special instruments sensitive to low-energyradio waves have detected it coming in from all directions from the universe.The existence of the cosmic background radiation is generally considered tobe solid evidence of how the Big Bang happened.

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Sun

arm

nucleus

100,000 light-years

sun

nucleus

rotation

nucleus

plan view

100,000 light-years

edge-on view

100,000 light-years

Sun

Figure 5 The Milky Way Galaxy. Our solar system is located in a spiral band about two-thirds ofthe way from the nucleus of the galaxy.


Preparing for the Chapter Challenge

Understanding and Applying What You Have Learned

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1. Using the second scale (1 m =3,000,000 km) you used for distancein your model of the solar system:

a) How far away would ProximaCentauri be from Earth?

b) How far away would theAndromeda galaxy be on yourscale, given that Andromeda is767 kiloparsecs or 2.5 millionlight-years away?

2. The Moon is 384,000 km fromEarth and has a diameter of 3476 km. Calculate the diameterof the Moon and its distance fromthe Earth using the scale of themodel you developed in theInvestigate section.

3. Refer again to Table 1. If the SpaceShuttle could travel at 100,000 km/hr, how long would ittake to go from Earth to each ofthe following objects? Assume thateach object is as close to the Earthas it can be in its orbit.

a) The Moon? b) Mars? c) Pluto?

4. What is the largest distancepossible between any two planetsin the solar system?

5. Use your understanding of a light-year and the distances from theSun shown in Table 1. Calculatehow many minutes it takes forsunlight to reach each of the nineplanets in the solar system. Thenuse the unit “light-minutes” (howfar light travels in one minute) todescribe the distances to eachobject.

6. Write down your school address inthe following ways:

a) As you would normally addressan envelope.

b) To receive a letter from anothercountry.

c) To receive a letter from a friendwho lives at the center of ourgalaxy.

d) To receive a letter from a friendwho lives in a distant galaxy.

Begin to develop your brochure forthe Chapter Challenge. In your ownwords, explain your community’sposition relative to the Earth, Sun, theother planets in our solar system, and

the entire universe. Include a fewparagraphs explaining what yourscale model represents and how youchose the scale or scales you used.

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Inquiring Further

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1. Solar-system walk

Create a “solar-system walk” onyour school grounds or yourneighborhood. Draw the Sun andthe planets to scale on thesidewalk in chalk. Pace off thedistances between the Sun and thenine planets at a scale that isappropriate for the site.

2. Scaling the nearest stars

Look up the distances to the fivestars nearest to the Sun. Wherewould they be in your scale model?To show their location, would youneed a map of your state?Country? Continent? The world?

3. Nuclear fusion

Find out more about the process ofnuclear fusion. Explain how and

why energy is released in theprocess by which hydrogen atomsare converted into helium atomswithin the Sun. Be sure to includeAlbert Einstein’s famous equation,E = mc2, in your explanation, andexplain what it means.

4. Star formation

Write a newspaper story about starformation. Visit the EarthCommweb site to find informationavailable on the web sites of theHubble Space Telescope and theEuropean Southern Observatory tofind examples of star-formingnebulae in the galaxy. How arethey similar? How are theydifferent? What instruments doastronomers use to study thesenebulae?

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Ch 1 Astronomy 9/20/04 2:41 PM Page 2 1 Astronomy …and .... 1 The... · The booklet should have a model of the solar system that will help citizens understand the relative sizes