Our Prodigal Sun

8/8/2019 Our Prodigal Sun

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Cover photo: Overlapping spectroheliograms

capture wavelengths of several solar elements onsingle strip of special film. Bright complete sphereis ionized helium. Ejected arch of helium, top,hangs suspended 800,000 km (500,000 miles)into corona.



ourprodigal sun

Not only our lives but also our wayof life depend upon the Sun. Nearly allenergy availableto us is or was created bysolar energy. Coal, oil, and gas that

have fueled the expansion of our civilizationwere made from plantsthat acquiredtheir energy by converting sunlight.Even thewinds that drive windmills originatefrom uneven heating of our atmosphere

by the Sun. Waterpoweris dependent uponrainfallwhich is possible only because theSun's heat eva porates water standingon Earth. Trees and other plants fromwhich man and animals obtain energycollect that energy from sunlight.

There are only three nonsolar sourcesof energy; geothermal (heat from Earth'sinterior); tidal(the rise and fal l of thet ides because of the Moon's gravity pull);and nuclea r. Nuclear energyis presentlyderived from atomic fission,the splittingof atom s. Nuclear fusion, which generatesthe hydrogen bomb's awesome destruc-tiveness, can potentially provide tremendousenergy for mankind if it can be controlled.Nuclear fusion is considered the sourceof the Sun's energy.

The world needs new, economicallyattractive,non-polluting ene rgy systemsasalternatives to dwindling reservesofoil, gas, and coal. A recent study by theNational Science Foundation and NASA hasconcluded that practicable solar energysystems could reach substantial levelsof public use within 10 years. NASAis contributing to a number of applicationsof solar power to meet man's needs onEarth.

The Sun will las t another fivebillion yearsin its current state as a normal, or mainsequence, star. So, solar powermay beconsidered inexhaustible. And no onecan cut off our imports of sunshine



The Sun may be considered prodigalin its generation of energy. Every second,it throws off into space more than man

has used since civilization began.Aboutone two-billionthsof this reachesEarth. In three days, this tiny fraction ofthe Sun's energy provides about as muchheat and light as is available from allour known reserves of coal, oil, and gas.

Man studies the Sun, therefore,because it is a major source of present andpotential future energyas well as thesource of life on Earth. Man also studiesthe Sun to learn more about the Earth'sweather and climate. How changes in solaractivity influence theseis not yet com-pletely understood. However, it hasbeen observed that a slight variation inthe Sun's output of energy could meltthe polar icecaps, submerging coastal a reas,or could bring on another ice age.

From time to time, enormous solareruptions hurl vast cloudsof matter andintense lethal radiation into space.The inhabitants of a craft traveling in inter-planetary space would be endangered ifin the path of such a cloud, but the

atmosphere and geomagnetic field protectlife on Earth. However,the power of thiscloud is revealed to mankind in many wa ys.

disruption inradio transmissbn



For exam ple, au roras light up thenight skies over the far northern andsouthern latitudes of Earth. Compass

needles on ships and airplanes may swingerratically. Electric power transformersmay trip out, and long distance radio,telegraph, and telephone communications,black out. Engineers want to know how

to predict such eruptions as a steptoward avoiding the problems they cause.

From the standpoint of physicists, theSun is a laboratory where temperatures,pressures, and magnetic field intensities can

be studied at magnitudes impossiblein laboratories on Earth. The Sun is alsothe astronomer's bridge to the studyof other stars. The more he knows aboutthe Sun, which is the only star close

enough for him tostudy in fine detail, themore he can use it as a reference point forunderstanding wha tis happening onother stars. Conversely, many other starsare like our Sun but in different stagesof their evolutions. Thesethen picture thepast and future of our Sun.

what makesthe sun shine?

Scientists generally agree that nuclearfus ion is the source of energy of the Sun

solar flare effectson earth environment





netic radiation. Electromagnetic radiationisradiation that is electrical and magnetic innature, as differentiated from nuclea r

particle radiation suchas protons andelectrons. From their highestto the lowestfrequencies, wide bands of electromagnticradiation have been designated gam marays, X-rays,ultravioletrays, visible light,

infrared rays, and radio w aves.Allelectrom ag netic rad ia tion trave ls

at the speed of light, about 300,000kilometers (186,000 miles) per second.But the path of the electromagnetic

radiation generated in the Sun's core is sotortuous that the radiation takes 20,000years or more to reach the surface.O n its way up, it continua lly collides w ithand bounces from closely packed protons,electrons, and other solar pa rticles .Inthese collisions,its energy is graduallydissipated, and its frequency steadily drops.As a result, by the time it speeds fromthe Sun, much of it has turned to infraredradiation (heat), visiblelight (sunshine),ultraviolet light (the part of solar radiationthat causes your skinto get a sunburn), radiowaves, and X-rays. So, the sunbeamsthat warm and brighten our world today"originated" 20,000 or more years ago,before the time of recorded history.But the sam e sunbeam s took only aboutminutes to streak from the Sun's fierysurface to Earth.

the solar activity cycle

1850

OSO-7: 1971ATM: 1973

1900 1950 2000



ourrhythmic sun

The number and size of sunspots(dark areas on the Sun) wax and wanethrough an 11-year rhythmic cycle. Solaractivity rises and falls in harmony with theappearance of sunspots. Sunspots appea r

to be composed of gases boiling upfrom the Sun's interior and a re frequentlyaccompanied by intensified emission ofall forms of radiation.

Sunspots are actually bright and hot.

Their temperatures, for example, aretypically4200 degrees Centigrade (about7600 deg rees Fahrenhe it) whichis hotterthan a blast furnace. However, theyare cool and look dark comparedto the

surrounding pho tosphe re (thebright visibledisk of the Sun) with 5700 degreesCentigrade (about 10,300 degrees Fahren-heit).

A small spot may be roughly the sizeof Earth. Larger onesmay be big enoughto hold hundreds or thousands of our planet.(The Sun itself contains aboutas muchmass as 330,000 Earths, and its volumeis about 1,300,000 times that of Earth.)

S trong loca l magnetic fields appearat the sites of sunspots. Most violen toutbursts of solar material such as solarflares seem to derive their energies fromthese magnetic fields.

strange rotation

The Sun is a churning everchanging

ball of hot gases. Because it is gaseous, itneed not rotate uniformly at all latitudes.Observations indicate its equatorialregions rotate around the Sun's axis every25 days w hile the area around either

pole may take as long as 33 days. Otherareas have periods of rotation betweenthose of the equator and pole. This unevenrotation distorts the Sun's magnetic field

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to such additional peculiarities as reversedpolarity (the North and South magneticpoles trading places) near the peak of

each solar cyc le. O ther va riab le solarfeatures probablyare related in someway to this contorted rotation.

solar polesand coronal holes

Comparatively cool polar areas werediscovered on the Sun in 1971 throughNASA's Orbiting Solar Observatory (OSO)7.

Electronic measurements indicated thatthe poles have a temperature of about1 milliondegrees Centigrade (about1.8million degrees Fahrenheit) comparedtoother parts of the Sun's corona, or outer

atmosphere, that generally register2 m illion degrees Centigrade (3.6milliondegrees Fahrenheit). Cyc lica l variationsin the sizes of the caps appear to correspondwith so lar ac tivity,with the caps smallest

at the peak of a solar cycle.In 1973, the sola r observa tory onNASA's

Skylabspace station confirmed that theso-called polar caps were not limited to polarareas. Similar conditions—cooler andmore rarified gases thanthe surroundingareas—occur elsewhereon the Sun.Astronomerscall them coronal holes. Theholes were found to extend into the upper-most part of the chromosphere (the layer

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X R AY PICTURE OF SUN



of several thousand kilometers betweenthe photosphere and the co rona ) wheretemperatures are only about 50,000 degrees

Centigrade (90,000 degrees Fahrenheit).Astronomersdon't know why the chrom o-sphere is different in these holes. Somebelieve that the holes may give new clues tothe solar interior.

Skylab data also showedthat magneticfields in the coronal holes stretchout-ward rather than arch backto the Sun.Scientists speculatethat the holes may bethe source of the solar wind,the hotelectrifiedgas particles (mostly protonsofhydrogen atoms) constantly rushing ou twardfrom the Sun to as far as the orbitofPluto. The so lar wind is considered bymany to be part of the Sun's corona , whichmeans the Sun's atmosphere engulfs thewhole solar system.

the wind from the sun

NASAExplorer, Mariner, O SO ,andPioneer spacecraft confirmedthe existenceof and have provided muchnew infor-mation about the solar wind. The spa cec raftalso indicated that wind tempreaturesarehundreds of thousands of degrees andwind velocities hundredsof thousands ofkilometers per hour. The wind's heatand speed vary with so lar ac tivity.Thewind more c lose ly resemblesa rocket blastthan any other thing we know aboutonEarth.

The solar wind drawsout parts of theSun's magnetic field. But the Sun's rotationtogether with the radial outward motionof the solar wind twistthe drawn-outmagnetic lines of forc e like strea ms of waterfrom a whirlinglawn sprinkler.

why isthe corona so hot?

Heat dissipates with distance fromitssource. Tem peratures also drop withdistance from the Sun's core where thermo-



nuclear reactionsare taking place,to itssurface. O ne would expectthe temperaturein the Sun's outer a tmosphere to be even

cooler. But instead, temperatures climbfrom severa l thousand degreesin thecoolest part of the atmosphere to severalm illion degrees in the corona . W hy thecorona heats up is not definitely known.

Some astrophysicists believe thattheseething bubbling granules at the Sun'ssurface break like oceanwaves, creating athundering roar. As this sound rushesupward into more rarified gas,it accelerates ,

eventually creating supersonic shock wavesthat heat the gases to high temperatures.

the mystery of thedisappearing granular structure

The granular structure poses mysteriestoo. In the photosphere, it is a finestructure with irregular cells typicallya fewthousand kilometers in diameter. In the

chromosphere, cells are about ten timeslarger in diam eter. Skylab show edthatthis granular structure persists throughthe ch rom os ph ere into the lower corona(300,000° Centigrade). Then, at the 1.5

m illion°C level, it has sudd enly becom esmeared and nearly d isappea rs. Thesignificanceof this is not understood, butit seems that the "super" granules aresomehow related to co rona l heating. This

will be studied with OSO-I (eye) in 1975.

prominencesSkylab da ta indica ted that prominences

(condensed streams of hot luminous gasrising from the Sun) erupt more frequentlythan astronomers expected. Prominencesappear to spring from sunspot groups.Their arched shapes indicate theyarecontrolled by strong magnetic fields.

A prominence, which is made up mainlyof ionized hydrogen atoms, is typicallyabout a hundred times as dense as thecorona. It sometimes rises at speeds of



rAstronaut EdwardG.Gibson at solarexperimentsconsoleduring Skylab mission.In solar eclipse, below,corona becomes visible

as glowing areasurrounding Sun. Close-up of Sun, bottom right,shows bright activeregions.

hundreds of kilometers a second butusually forms a vast bridge of glowing gashundreds of thousands of kilometersalong its curved length.

solarbright spots

Solar bright points have been describedas standing out on an X-ray image of theSun like stars against the night sky.

Sometimes, they have been called "mini-active regions". This term is scorned bysome astronomers but is more or lessjustified by the occa siona l fla ring observedon some bright points. These sm all flares

have been termed "miniflares". S kylabpictures show the bright points arescattered all over the Sun, even in thecoronal holes. Th is contrasts with theactive regions near sunspots which areconfined to a narrow belt extending40 degrees north and south of the solarequator, where most violent activities takeplace. (Observations in X-ray wavelengths



solar flares

andcoronal transientsA solar flare is the sudden release of

tremendous energy and material fromthe Sun, usually froma plage (the light area

bordering a sunspot). A flare may lastminutes or hours. It is marked by a suddenbrightening.

A flare can erupt with the force of abillion hydrogen bombs. If the Earth werenot protected by a strong magnetic fieldand the atmosphere, a flare's radiation coulddestroy all life. Its heat, if concentratedon Earth, could melt Earth's polarice caps.O ne solar flare may release as muchenergy as the whole world uses in 100,000years. Skylab showed that solar flaressometimes arch back to a different areaof the Sun and trigger new flares at these

areas. Scientists had suspected butnever observed this before Skylab.Skylab observations demonstrated that

the solar corona is much more dynamic(changeable) than many believed One of



the violent events tha tSkylab identifiedin the corona was a great explosion—calleda coronal transient for want of a better

name—that hurled outward enormousloops of material, hundreds of thousandsof tons in weight, at speeds of millionsof kilometers per hour. Such explos ions a lsoreshape magnetic fields and structuralfeatures in large parts of the corona.the birthand death of the sun

With all of the newsolar data pouring in

from spa cecra ft, the Sun is proving morecomplex than ever. But the answers tomany questions may be hidden in thedata now on hand.

O n one subject, however, most scien-tists seem to agree: how the Sun wasborn and how it w ill die. They have evencalculated how much longer it has to live:five billion yearsas a normal, or mainsequence, star.

Astronomers hold that the Sun andplanets formed from an enormouscontracting cloudof dust and gas. Allparts of this cloud did not move uniformly.Some parts formed local condensationsthat eventua lly becam eour planets, moons,comets , and asteroids.

Gradually, the main cloud tended tobecome spherical. Gravitationalcontractionincreased

itstemperature. Eventually

thecore temperature rose to a point whereits hydrogen nuclei beganto fuse. Nuclearenergy then produced enough outwardpressure of heated gas to balance the

inward force of gravity and maintain theSun as a glowing main sequence star.This is believed to have begun aboutfive billion years ago.

About five billion yea rs from now,the

Sun will have depleted the hydrogenfuel in its core. Its thermonuclear reactionswill then move ou twa rd whe re unusedhydrogen exists . As it does, the tremendous

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outward, expanding the Sun as much as60 t imes. As the Sun cools by expansion,its surface color will becomea deep red.

It will then be a Red Giant—nota mainsequence star. Looming across much ofour sky, it will boil off our water andair and incinerate any remnants of life.

When the Sun exhausts its hydrogenfuel , it will no longer be able to with-stand gravitational contraction. Eventually,it will shrinkto a white (hot) dwarf,nobigger than Earth, but so dense thata piece the size of a sugar cube wouldweigh thousands of kliog ram s. E ventually,after billions of yea rs more, our Sun willcool and dim to a black cinder. Onlythenwill eternal night fal l upon the solarsystem.

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