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Beyond Our Solar System 707

25.2 Stellar Evolution

Reading StrategySequencing Copy the flowchart below. Asyou read, complete it to show how the sunevolves. Expand the chart to show theevolution of low-mass and high-mass stars.

Key ConceptsWhat stage marks thebirth of a star?

Why do all stars eventuallydie?

What stages make up thesun’s life cycle?

Vocabulary◆ protostar◆ supernova◆ white dwarf◆ neutron star◆ pulsar◆ black hole

Determining how stars are born, age, and then die was difficultbecause the life of a star can span billions of years. However, by study-ing stars of different ages, astronomers have been able to piece togetherthe evolution of a star. Imagine that an alien from outer space lands onEarth. This alien wants to study the stages of human life. By examininga large number of humans, the alien observes the birth of babies, theactivities of children and adults, and the death of elderly people. Fromthis information, the alien then attempts to put the stages of humandevelopment into proper sequence. Based on the number of humans ineach stage of development, the alien would conclude that humansspend more of their lives as adults than as children. In a similar way,astronomers have pieced together the story of stars.

Star BirthThe birthplaces of stars are dark, cool inter-stellar clouds, such as the one in Figure 8.These nebulae are made up of dust and gases.In the Milky Way, nebulae consist of 92 per-cent hydrogen, 7 percent helium, and less than1 percent of the remaining heavier elements.For some reason not yet fully understood,some nebulae become dense enough to beginto contract. A shock wave from an explosionof a nearby star may trigger the contraction.Once the process begins, gravity squeezes par-ticles in the nebula, pulling every particletoward the center. As the nebula shrinks, grav-itational energy is converted into heat energy.

Figure 8 Nebula Dark, coolclouds full of interstellar matterare the birthplace of stars.

a. ? b. ?Evolutionof Sun

FOCUS

Section Objectives25.5 Identify which stage marks the

birth of a star.25.6 Explain why all stars eventually

die.25.7 List the stages of the sun’s life

cycle.

Build VocabularyConcept Map As students read thesection, have them make a concept mapshowing how the vocabulary terms arerelated. The concept map should showhow a star changes during its lifetime.

Reading Strategya. cloud of dust and gases (nebulastage)b. protostar stage

INSTRUCT

Star BirthBuild Science SkillsUsing AnalogiesShow students a seriesof photographs ofpeople of differentages. Ask them to put the photos inorder by age. Then, show students aseries of photographs of the life cycle ofan insect, such as a butterfly, thatundergoes complete metamorphosis.Ask them to put the photos in order byage. Ask: Why was the second serieswas harder to sequence than the firstone? (You can’t easily tell by looking atthe photos which stage comes in whatorder.) Point out that astronomers havethe same problem with stars. It’s notobvious just from looking at variousstages of stars how old they are.Logical, Visual

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Reading Focus

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Section 25.2

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Protostar Stage The initial contractionspans a million years or so. As time passes, thetemperature of this gaseous body slowly risesuntil it is hot enough to radiate energy from itssurface in the form of long-wavelength red light.This large red object is called a protostar. Aprotostar is a developing star not yet hotenough to engage in nuclear fusion.

During the protostar stage, gravitationalcontraction continues—slowly at first, thenmuch more rapidly. This collapse causes the coreof the protostar to heat much more intenselythan the outer layer. When the core of aprotostar has reached about 10 million K, pres-sure within is so great that nuclear fusion ofhydrogen begins, and a star is born.

Heat from hydrogen fusion causes the gasesto increase their motion. This in turn causes anincrease in the outward gas pressure. At some

point, this outward pressure exactly balances the inward force of grav-ity, as shown in Figure 9. When this balance is reached, the star becomesa stable main-sequence star. Stated another way, a stable main-sequencestar is balanced between two forces: gravity, which is trying to squeezeit into a smaller sphere, and gas pressure, which is trying to expand it.

Main-Sequence Stage From this point in the evolution of amain-sequence star until its death, the internal gas pressure strugglesto offset the unyielding force of gravity. Typically, hydrogen fusion con-tinues for a few billion years and provides the outward pressurerequired to support the star from gravitational collapse.

Different stars age at different rates. Hot, massive blue stars radi-ate energy at such an enormous rate that they deplete their hydrogenfuel in only a few million years. By contrast, the least massive main-sequence stars may remain stable for hundreds of billions of years.A yellow star, such as the sun, remains a main-sequence star for about10 billion years.

An average star spends 90 percent of its life as a hydrogen-burning,main-sequence star. Once the hydrogen fuel in the star’s core isdepleted, it evolves rapidly and dies. However, with the exception ofthe least-massive red stars, a star can delay its death by fusing heavierelements and becoming a giant.

Figure 9 Balanced Forces Amain-sequence star is balancedbetween gravity, which is tryingto squeeze it, and gas pressure,which is trying to expand it.

Gaspressure

Gravity

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Integrate PhysicsFission and Fusion Review theconcepts of fission and fusion withstudents. During fission, atomic nucleiare split apart to make smaller nuclei.This is the process used in nuclear powerplants. During fusion, atomic nucleicombine to make larger nuclei.Sustained fusion occurs only withinstars. Main-sequence stars fuse fourhydrogen nuclei to form a heliumnucleus. The helium nucleus has slightlyless mass than the hydrogen nuclei.The remaining mass is converted intoenergy. Hotter stars can produce carbonand other elements by fusion.Logical, Verbal

Build Reading LiteracyRefer to p. 392D in Chapter 14, whichprovides guidelines for previewing.

Preview Before they read the section,have students skim the headings, visuals,and boldfaced words to preview howthe text is organized.Verbal, Visual

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Section 25.2 (continued)

Customize for Inclusion Students

Gifted Have students create a computergraphic presentation that compares thelife cycles of different masses of stars. Thepresentations should include scale diagrams

of each stage along with labels and a briefcaption. Invite students to share theirpresentations with the class.



Red-Giant Stage The red-giant stage occurs because the zone ofhydrogen fusion continually moves outward, leaving behind a heliumcore. Eventually, all the hydrogen in the star’s core is consumed. Whilehydrogen fusion is still progressing in the star’s outer shell, no fusionis taking place in the core. Without a source of energy, the core nolonger has enough pressure to support itself against the inward forceof gravity. As a result, the core begins to contract.

As the core contracts, it grows hotter by converting gravitationalenergy into heat energy. Some of this energy is radiated outward, increas-ing hydrogen fusion in the star’s outer shell. This energy in turn heatsand expands the star’s outer layer. The result is a giant body hundreds tothousands of times its main-sequence size, as shown in Figure 10.

As the star expands, its surface cools, which explains the star’s red-dish appearance. During expansion, the core continues to collapse andheat until it reaches 100 million K. At this temperature, it is hot enoughto convert helium to carbon. So, a red giant consumes both hydrogenand helium to produce energy.

Eventually, all the usable nuclear fuel in these giants will be con-sumed. The sun, for example, will spend less than a billion years as agiant. More massive stars will pass through this stage even more rapidly.The force of gravity will again control the star’s destiny as it squeezes thestar into the smallest, most dense piece of matter possible.

Why do red giants have a reddish appearance?

Figure 10 Life Cycle of aSunlike Star A medium-massstar, similar to the sun, will evolvealong the path shown here.Interpreting Diagrams What isthe first stage in the formation ofthe star? What is the last stage?

Ab

solu

te m

agni

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e–5

0

+5

+10

+1525,000 10,000 7000 5000 3000

Surface temperature (K)

PLANETARYNEBULASTAGE

GIANTSTAGE

VARIABLESTAGE

WHITEDWARFSTAGE

Dust andgases

Protostar

Main-sequence star

To black dwarf stage

Use VisualsFigure 10 Use this figure to explainhow a star like the sun evolves. Emphasizethat the drawing shows changes in thecharacteristics of the star, not movementin the sky. Also explain that the mainsequence is shown for reference. Ask:What happens to the star after itleaves the main sequence? (It becomesa red giant.) How do the characteristicsof the star change as it changes froma red giant to a planetary nebula? (Itbecomes hotter but less bright.) Wherewould the black dwarf stage be onthe drawing? Why? (It would be in thebottom right corner or past it. A blackdwarf is cold and dark.)Visual, Logical

Build Reading LiteracyRefer to p. 446D in Chapter 16, whichprovides guidelines for this sequencestrategy.

Sequence Some students may beconfused by the lifecycle of a sunlike staras pictured in Figure 10. To bypass thecomplexity of criteria such as surfacetemperature and absolute magnitude,have students write each step or phaseon a separate flashcard. Then have themmix up the cards and practice placingthem in the proper sequence.Verbal, Kinesthetic

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Answer to . . .

Figure 10 The first stage is a nebula,or cloud of dust and gases. The laststage is a black dwarf.

As they expand, theirsurfaces cool, which

explains the red appearance.


Burnout andDeathMost of the events of stellarevolution discussed so far arewell documented. What hap-pens next is based more ontheory. We do know that allstars, regardless of their size,eventually run out of fuel andcollapse due to gravity. Withthis in mind, let’s consider thefinal stages of stars of differentmasses.

Death of Low-MassStars As shown in Figure11A, stars less than one half themass of the sun consume theirfuel at a fairly slow rate.

Consequently, these small, cool red stars may remain on the mainsequence for up to 100 billion years. Because the interior of a low-massstar never reaches high enough temperatures and pressures to fusehelium, its only energy source is hydrogen. So, low-mass stars neverevolve into red giants. Instead, they remain as stable main-sequencestars until they consume their hydrogen fuel and collapse into a whitedwarf, which you will learn more about later.

Death of Medium-Mass Stars Asshown in Figure 11B, stars with masses similarto the sun evolve in essentially the same way.During their giant phase, sunlike stars fusehydrogen and helium fuel at a fast rate. Oncethis fuel is exhausted, these stars also collapseinto white dwarfs.

During their collapse from red giants towhite dwarfs, medium-mass stars are thoughtto cast off their bloated outer layer, creating anexpanding round cloud of gas. The remaininghot, central white dwarf heats the gas cloud,causing it to glow. These often beautiful, gleam-ing spherical clouds are called planetarynebulae. An example of a planetary nebula isshown in Figure 12.

Birth Stellar Stage Death

Nebula Protostar Main-sequence

starWhitedwarf

Low mass stars

Nebula Protostar Mainsequence

starRed giant Planetary

nebulaWhitedwarf

Medium mass stars

Nebula Protostar Red supergiant Supernovaexplosion

Neutron star

or

Black hole

Massive stars

Blackdwarf

Blackdwarf

Mainsequence

star

Figure 11 Stellar Evolution A A low-mass star uses fuel at alow rate and has a long life span.B Like a low-mass star, a medium-mass star ends as a black dwarf. C Massive stars end in hugeexplosions, then become eitherneutron stars or black holes.

A

B

C

Figure 12 Planetary NebulaDuring its collapse from a redgiant to a white dwarf, a medium-mass star ejects its outer layer,forming a round cloud of gas.

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Burnout and DeathUse VisualsFigure 11 Use this figure to explainhow the evolution of stars depends ontheir mass. Ask: What stages do allstars go through? (nebula, protostar,main-sequence star) Which stars becomered giants? (medium-mass and massivestars) Which stars become whitedwarfs? (low-mass and medium-massstars) What are two possible results ofthe death of a massive star? (It canbecome a neutron star or a black hole.)Visual, Logical

Students may be confused by the termplanetary nebula and think that planetsform from planetary nebulae. Explainthat a planetary nebula is a stage in theevolution of a star and has nothing todo with planets. The name is a result ofhistorical accident. When planetarynebulae were discovered 200 years ago,they looked like small greenish diskssimilar to the planet Uranus. Eventhough they are not related to planets,the name stuck.Verbal

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In a few billion years, the sun’s core will runout of hydrogen fuel, triggering hydrogenfusion in the surrounding shell. As a result, thesun’s outer envelope will expand, producinga red giant hundreds of times larger andbrighter. Intense solar radiation will boilEarth’s oceans, and solar winds will drive

away Earth’s atmosphere. Another billion yearslater, the sun will expel its outermost layer,producing a planetary nebula. The interiorwill collapse to produce a small, densewhite dwarf. Gradually the sun will emitits remaining energy and become a coldblack dwarf.

Facts and Figures

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Death of Massive StarsIn contrast to sunlike stars, whichdie gracefully, stars with massesthree times that of the sun haverelatively short life spans, asshown in Figure 11C. These starsend their lives in a brilliant explo-sion called a supernova. Duringa supernova, a star becomes mil-lions of times brighter than itsprenova stage. If one of the near-est stars to Earth produced suchan outburst, it would be brighterthan the sun. Supernovae arerare. None have been observed inour galaxy since the invention ofthe telescope, although TychoBrahe and Galileo each recordedone about 30 years apart. An evenlarger supernova was recorded in1054 by the Chinese. Today, theremnant of this great outburst isthe Crab Nebula, shown inFigure 13.

A supernova event is thoughtto be triggered when a massivestar consumes most of its nuclearfuel. Without a heat engine togenerate the gas pressurerequired to balance its immensegravitational field, the star collapses. This implosion, or burstinginward, is huge, resulting in a shock wave that moves out from the star’sinterior. This energetic shock wave destroys the star and blasts theouter shell into space, generating the supernova event.

H-R Diagrams and Stellar Evolution Hertzsprung-Russelldiagrams have been helpful in formulating and testing models of stel-lar evolution. They are also useful for illustrating the changes that takeplace in an individual star during its life span. Refer back to Figure 10,which shows the evolution of a star about the size of the sun. Keep inmind that the star does not physically move along this path. Its posi-tion on the H-R diagram represents the color and absolute magnitudeof the star at various stages in its evolution.

What is a supernova?

Figure 13 Crab NebulaThis nebula, found in theconstellation Taurus, is theremains of a supernova that took place in 1054.

Integrate ChemistryFormation of Heavy Elements Tellstudents that all of the heavy (highatomic mass) atoms on Earth wereformed inside stars. Initially the universecontained almost entirely hydrogen andhelium. Biologically important elementssuch as carbon, nitrogen, and oxygenform inside many stars. However, veryheavy elements such as lead and goldform only in the incredible explosions ofsupernovas. When the supernova fades,the elements are scattered throughoutthe area and may become part of a newsolar system such as our own. Havestudents research the chain reactionsinside stars and supernovas that produceheavy elements.Logical

Use CommunityResourcesInvite an astronomer or astrophysicistfrom a local college or university to theclassroom to discuss astronomy andresearch. Ask students to preparequestions in advance to ask the visitor.Interpersonal

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Answer to . . .

A supernova is thebrilliant explosion that

marks the end of a massive star.


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Stellar RemnantsEventually, all stars consume their nuclear fuel and collapse into one ofthree documented states—white dwarf, neutron star, or black hole.Although different in some ways, these small, compact objects are allcomposed of incomprehensibly dense material and all have extreme sur-face gravity.

White Dwarfs White dwarfs are the remains of low-mass andmedium-mass stars. They are extremely small stars with densitiesgreater than any known material on Earth. Although some whitedwarfs are no larger than Earth, the mass of such a dwarf can equal1.4 times that of the sun. So, their densities may be a million timesgreater than water. A spoonful of such matter would weigh severaltons. Densities this great are possible only when electrons are displacedinward from their regular orbits, around an atom’s nucleus, allowingthe atoms to take up less than the “normal” amount of space. Materialin this state is called degenerate matter.

In degenerate matter, the atoms have been squeezed together sotightly that the electrons are displaced much nearer to the nucleus.Degenerate matter uses electrical repulsion instead of molecularmotion to support itself from total collapse. Although atomic parti-cles in degenerate matter are much closer together than in normalEarth matter, they still are not packed as tightly as possible. Stars madeof matter that has an even greater density are thought to exist.

As a star contracts into a white dwarf, its surface becomes very hot,sometimes exceeding 25,000 K. Even so, without a source of energy, itcan only become cooler and dimmer. Although none have beenobserved, the last stage of a white dwarf must be a small, cold bodycalled a black dwarf. Table 2 summarizes the evolution of stars of vari-ous masses. As you can see, the sun begins as a nebula, spendsmuch of its life as a main-sequence star, becomes a red giant, plane-tary nebula, white dwarf, and finally, black dwarf.

Initial Mass of Main-Sequence Giant Phase Evolution Final StageInterstellar Cloud Stage After

(Sun � 1) Giant Phase

1–3 Yellow Yes Planetary nebula White dwarf

6 White Yes Supernova Neutron star

20 Blue Yes (Supergiant) Supernova Black hole

Table 2 Summary of Evolution for Stars of Various Masses

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Stellar RemnantsBuild Science SkillsUsing Analogies Use an analogy of astar to a charcoal briquette in a barbecuegrill to describe the life of a low-massstar. When the briquette is first lit, itbegins to consume its fuel and glowsred-hot (main sequence). As it runs outof fuel, it becomes cooler and dimmer(white dwarf). Eventually it runs out offuel and becomes a cold cinder (blackdwarf). Ask students how the briquetteis different from an actual star. (Thebriquette burns because of chemicalreactions, not fusion. It doesn’t gethotter in the “white dwarf” stage.)Logical, Visual

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Neutron Stars After studying whitedwarfs, scientists made what might at firstappear to be a surprising conclusion. Thesmallest white dwarfs are the most mas-sive, and the largest are the least massive.The explanation for this is that a moremassive star, because of its greater gravita-tional force, is able to squeeze itself into asmaller, more densely packed object thancan a less massive star. So, the smallerwhite dwarfs were produced from the col-lapse of larger, more massive stars thanwere the larger white dwarfs.

This conclusion led to the predictionthat stars smaller and more massive thanwhite dwarfs must exist. These objects,called neutron stars, are thought to be theremnants of supernova events. In a whitedwarf, the electrons are pushed close to thenucleus, while in a neutron star, the elec-trons are forced to combine with protonsto produce neutrons. If Earth were to collapse to the density of a neu-tron star, it would have a diameter equal to the length of a footballfield. A pea-size sample of this matter would weigh 100 million tons.This is approximately the density of an atomic nucleus. Neutron starscan be thought of as large atomic nuclei.

Supernovae During a supernova, the outer layer of the star isejected, while the core collapses into a very hot neutron star about 20kilometers in diameter. Although neutron stars have high surface tem-peratures, their small size would greatly limit their brightness. Findingone with a telescope would be extremely difficult.

However, astronomers think that a neutron star would have a verystrong magnetic field. Further, as a star collapses, it will rotate faster, forthe same reason ice skaters rotate faster as they pull in their arms.Radio waves generated by these rotating stars would be concentratedinto two narrow zones that would align with the star’s magnetic poles.Consequently, these stars would resemble a rapidly rotating beaconemitting strong radio waves. If Earth happened to be in the path ofthese beacons, the star would appear to blink on and off, or pulsate, asthe waves swept past.

In the early 1970s, a source that radiates short bursts or pulses ofradio energy, called a pulsar, was discovered in the Crab Nebula.Studies of this radio source revealed it to be a small spinning starcentered in the nebula. The pulsar found in the Crab Nebula isundoubtedly the remains of the supernova of 1054.

Figure 14 Veil Nebula Locatedin the constellation Cygnus, thisnebula is the remnant of anancient supernova.

Modeling a PulsarPurpose Students observe a model ofhow a pulsar appears from Earth.

Materials string; long, thin flashlight

Procedure Tie a string to the middle ofa long, thin flashlight. Twist the string afew times. Turn on the flashlight, turnoff the room lights, and let the flashlightspin around. Ask: What does theflashlight look like from yourperspective? (It looks as if it is flashingon and off.) Is the flashlight actuallyflashing? If not, ask what it is doing.(No, it is producing a continuous beam.)Why does the flashlight appears to beflashing? (The beam is only visible whenit points toward the observer.) Twist thestring more tightly and let the flashlightspin again. Ask: What is different thistime, and why? (The flashing is morefrequent because the flashlight is rotatingmore rapidly.)

Expected Outcome The flashlight willappear to be flashing and will appear toflash more rapidly as it rotates faster.Visual, Logical

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Pulsars were discovered by Jocelyn Bell Burnellin 1967. Burnell was a graduate studentworking on her advisor Anthony Hewish’sproject to study rapid fluctuations in radiowaves received from stars. One day she noticedsome strong, rapid, and regular pulses comingfrom fixed points on the sky. Some people

thought the pulses might by signals sent by analien civilization, or “little green men,” so thesources were at first called LGM 1, LGM 2,LGM 3, and LGM 4. However, astronomerssoon concluded that the signals had naturalsources, which were named pulsars.

Facts and Figures


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Section 25.2 Assessment

Reviewing Concepts1. What is a protostar?

2. At what point is a star born?

3. What causes a star to die?

4. Describe the life cycle of the sun.

Critical Thinking5. Inferring Why are less massive stars thought

to age more slowly than more massive stars,even though less massive stars have much less“fuel”?

6. Relating Cause and Effect Why isinterstellar matter important to stellarevolution?

Black Holes Are neutron stars made of the most dense materialspossible? No. During a supernova event, remnants of stars three timesmore massive than the sun apparently collapse into objects even

smaller and denser than neutron stars.Even though these objects, called blackholes, are very hot, their gravity is sostrong that not even light can escapetheir surface. So they disappear fromsight. Anything that moves too near ablack hole would be swept in by itsgravity and lost forever.

How can astronomers find anobject whose gravitational field pre-vents the escape of all matter andenergy? One strategy is to find evi-dence of matter being rapidly sweptinto a region of apparent nothingness.Scientists think that as matter is pulledinto a black hole, it should become

very hot and emit a flood of X-rays before being pulled in. Becauseisolated black holes would not have a source of matter to swallow up,astronomers first looked at binary-star systems.

A likely candidate for a black hole is Cygnus X-1, a strong X-raysource in the constellation Cygnus. In this case, the X-ray source canbe observed orbiting a supergiant companion with a period of 5.6 days.It appears that gases are pulled from this companion and spiral into thedisk-shaped structure around the black hole, as shown in Figure 15.

Red giant

Orbiting disk aroundblack hole

Figure 15 Black Hole Gasesfrom the red giant spiral into theblack hole.

Supernova If a supernova explosion wereto occur near our solar system, what mightbe some possible consequences of theintense X-ray radiation that would reachEarth?

For: Links on black holes

Visit: www.SciLinks.org

Web Code: cjn-7252

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Download a worksheet on blackholes for students to complete, andfind additional teacher supportfrom NSTA SciLinks.

Use VisualsFigure 15 Use this figure to explainhow black holes can be detected. Ask:Can a black hole be detected directly?Why or why not? (no, because its gravityis so strong that not even light can escape)What may happen if a black hole hasa companion star? (The black hole maypull material out of the star and intoitself.) What happens to material as itfalls into a black hole? (It becomes veryhot and emits X-rays.) What would begood evidence for a black hole? (stellarmaterial being heated and then apparentlydisappearing into nothingness)Visual, Logical

ASSESSEvaluateUnderstandingCall on students to describe the stagesthat stars of different masses go throughduring their life cycles.

ReteachUse Figure 10 to review the life cycle ofa sunlike star and to emphasize that theH-R diagram is a graph, not a star chart.

Sample answer: The effect on Earthwould likely be devastating. Dependingon the intensity of the radiation, livingorganisms would either be destroyed orgenetically and physically damaged.Because the biosphere interacts with theremaining Earth systems, the entireEarth would be altered eventually.

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4. The sun began as a nebula, became aprotostar and then a main-sequence star. Itwill become a red giant, planetary nebula,white dwarf, and finally a black dwarf.5. A less massive star will live longer becauseit consumes fuel at a slower rate than domore massive stars.6. Stars are born out of clouds of interstellarmatter.

Section 25.2 Assessment

1. A protostar is a developing star not yet hotenough to engage in nuclear fusion.2. When the core of a protostar has reachedabout 10 million K, pressure within it is sogreat that nuclear fusion of hydrogen begins,and a star is born.3. A star runs out of fuel and collapses due togravity.


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