Life Cycles of Stars

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

How Stars are Born

No one has ever observed stars go through their life cycle, but astronomers can observemany stars at different stages in their life cycles. In addition, astronomers cancalculate what would happen to stars under various conditions, and attempt to match thepredictions against actual stars. The overall outlines of stellar evolution are probablyaccurately known, but there are many unanswered questions, some of them big ones, and verylikely there are surprises waiting for astronomers as well.

Stars are believed to form when clouds of interstellar gas and dust start to contractunder the influence of gravity. In interstellar space, far from other stars, a cloud ofgas can be very thin and still be dense enough to begin contracting. Many astronomersbelieve violent explosions of older stars create shock waves that help start thecontraction process, at the same time enriching the gas in heavy chemical elements. In alllikelihood, a cloud will condense into many stars and form a star cluster.

Initially, a cloud destined to become a star will be spherical. The cloud will almostcertainly have a slight rotation, simply because of random gas motions in the cloud as itstarted contracting. As the cloud contracts, its rotation will speed up, causing the cloudto become disk-shaped. The cloud is still far larger than our solar system, and its outerreaches very thin. Most of the mass of the cloud falls to the center, adding energy to thecenter and heating it up. The center of the cloud becomes a protostar, emittingmostly infrared radiation. Finally the temperatures and pressures within the star reachthe point where nuclear reactions begin, and the star "turns on". Matter in thesurrounding disk may accrete to form planets, or, if the condensations are massive enough,companion stars.

Stars With Companions

Multiple Stars

Many, perhaps all stars, have companions. Many stars are binary stars or multiplestars, with several stars orbiting around one another. Some multiple stars may formwhen a rapidly rotating protostar becomes unstable and splits; others may form as separateprotostars. Our nearest neighbor, Alpha Centauri, is a triple star, with a primarystar much like our sun, a smaller and cooler secondary star orbiting about as faraway as Uranus orbits our sun, and a very faint companion many times farther away from theprimary than pluto is from the sun. Binary stars are of great value to astronomers becausethe orbital periods of binary stars depend on the masses of the stars, and binary starsenable astronomers to determine the masses of stars

Does the sun have a companion star? There has been speculation from time to time thatthe sun might have a companion, most recently the "nemesis" hypothesis. The"nemesis" hypothesis argued that an undiscovered companion of the sunperiodically caused comets to sweep through the solar system, triggering among otherthings the extinction of the dinosaurs. If any undiscovered companion exists, it must bevery faint, not very massive, and very far away. The odds are very much against such acompanion escaping detection this long.

Planetary Systems

The sun has planets. From what we know of the formation of the solar system, it seemsvery likely that many if not most solitary stars have planets, formed from the materialthat did not condense into the central protostar. Possibly widely-separated multiple starslike alpha centauri have planets as well. Whether exotic multiple stars with manycomponents, or very close binary systems can have planets is uncertain. Even if planetsform around such stars, close encounters with the stars may fling the planets into a staror out of the star system altogether. A number of stars have disks of solid matterorbiting them that are believed to be similar to the disk from which the planets of ourSolar System formed. Such disks have been termed proplyds (ProtoplanetaryDisks).

Locating other planetary systems is a very great challenge. One approach is to detectvery tiny, regular variations in the positions of stars. As a planet orbits a star, thestar and planet both orbit around their center of mass; if the star and planet were joinedby a beam, the point where the beam would balance is the center of mass. Tiny motions ofthe star can also be detected using the Doppler Effect. The motions are very tiny: itwould be hard to detect the effect of Jupiter on the sun from a nearby star. Anotherapproach is to block the light of the star and search for the reflected light fromplanets. All of these techniques are extreme challenges to existing technology but arebecoming more feasibile as instruments improve. Beginning in 1993 evidence for otherplanetary systems has rapidly accumulated. Some of these newly-discovered objects are somassive they may be so-called brown dwarfs, on the borderline between large planetsand tiny stars.

The newly-discovered objects around other stars are very unusual. None of the probableother solar systems look much like our own. Many of the planets are much more massive thanJupiter and some of these objects orbit surprisingly close to their suns. It appears thatour solar system may be unusual.

Are there objects midway between planets like Jupiter and small stars? The fainterstars are, the more numerous, so that we might expect such objects, nicknamed browndwarfs to be very common. None have been conclusively detected, yet, even thoughtheir infrared radiation would be easily detectable. It appears that there is a sharpdividing line between solitary and multiple stars, for reasons still unclear. Some of thenewly-discovered objects around other stars are so massive they may be brown dwarfs.

Evolution of Stars

Life on the Main Sequence

Once stars begin to shine, they assume a position on the main sequence and tend to staythere, shining steadily. Stars like the sun would brighten somewhat in their first coupleof billion years. The brightening of the sun poses a problem called the faint earlysun problem: geological evidence indicates that the earth has been warm enough tohave liquid water throughout its history, yet the early sun was perhaps 25% less brightthan the present sun, and the early earth should have been cold. Perhaps the early earthhad a denser atmosphere that trapped heat more than the atmosphere does now. Bothgeologists and astronomers are actively pursuing research on this question.

Almost every aspect of the life of a main sequence star is determined by one fact: itsmass. Very tiny stars emit light feebly and remain cool. Such stars are called reddwarfs. More massive stars are hotter and brighter. Massive stars have more fuel tosustain their output, but their energy output or luminosity is proportional to the fourthpower of their mass. A star twice as massive as the sun will emit energy 2 x 2 x 2 x 2 or16 times the rate of the sun. With only twice as much fuel to sustain it, the star willonly shine 2/16 or 1/8 as long as the sun before running out of fuel.

We might compare stars to people. Red dwarf stars spend their energy frugally, likesomeone with a very limited income, whereas bright, massive blue-white stars run throughtheir fuel quickly, like a lottery winner on a spending spree. Red dwarf stars can lasttens or hundreds of billions of years, doling out their fuel at a miserly rate. Our ownsun will shine for perhaps 10 billion years, but bright blue-white supergiants like denebor rigel might last only a few million years.

Leaving the Main Sequence

When main sequence stars run out of available fuel, the nuclear reactions in the centerof the star die out. Without the intense radiation pressure produced by these nuclearreactions, the star begins to contract under its own gravity. As matter falls in towardthe center of the star, it releases energy that heats the star until finally a newsequence of nuclear reactions begins. The renewed energy output heats the outer part ofthe star, causing it to expand. At this point the star leaves the main sequence. The outerlayers expand and cool, causing the star to redden, but the enormous size of the stargives it a vast surface area through which it emits a tremendous amount of energy. Thestar becones a red giant or supergiant.

The most massive stars leave the main sequence soonest. We can see the evolution ofstars clearly by plotting H-R diagrams for star clusters, whose members all formed at thesame time. Very young clusters may even have remnants of their parent gas clouds stillvisible and contain nothing but main sequence stars, and even stars that have not yetreached the main sequence. Older clusters have some of their brightest stars leaving themain sequence, and in very old clusters, most of the stars brighter than the sun have leftthe main sequence. The evolution of the H-R diagram of a star cluster is rather likepeeling a strip off a banana; as the star cluster ages, the giant star branch becomeslarger and the branching point moves farther down the main sequence.

The Fates of Stars

Gravity and the Collapse of Stars

All objects in the universe exist because of a balance between gravity and somecounteracting force. Four fundamental forces of nature (gravity, electromagnetism, weaknuclear force and strong nuclear force) govern the structure and bonding of atoms. It isfitting that we return to these same basic forces here when we examine matter on thelargest scale.

Left to itself, gravity would pull all masses together to a central point. In planets,the counteracting force is the atomic bonding between atoms, and the repulsion between thenegatively-charged electrons of neighboring atoms. In normal stars, the counteractingforce is the thermal motion of the atoms in the hot gas of the star, and the outwardpressure exerted by radiation.

When the radiation pressure in a star falters, gravity begins pulling the gas of thestar inward. As the gas falls inward, it gains energy and heats up the interior of thestar. Also, the gas becomes more tightly compressed and the pressure increases. Severalthings can happen, depending on the mass of the star. The temperature and pressure insidethe star can rise until a new generation of nuclear reactions begins, the star cancontinue to collapse until some new counteracting force stops the collapse, or the starcan collapse until gravity actually does pull all the mass of the star into a singlepoint: a black hole. The more massive a star is, the more dramatic its end.

The Fate of Very Small Stars

Small, faint, red dwarf stars probably never do anything very dramatic. They continueto fuse hydrogen to helium at a miserly rate. Even nearby red dwarf stars are very faint.If there were none within a few dozen light years of earth we would not know they exist atall, but judging from what we see in the space near the sun, red dwarfs are among the mostcommon stars. These stars can continue to shine, if one can use that word for such faintstars, for tens or hundreds of billions of years, gradually cooling as their fuel runsout. Their mass is so small they will never collapse enough to start a new cycle ofactivity. The atomic repulsion between atoms will counteract gravity. Even then, theirsurface area is so small they will remain warm for a very long time.

Average-Sized Stars, Red Giants, and White Dwarfs

Stars ranging from 10 per cent to several times the mass of the sun go through adifferent final history. As the star's hydrogen supply begins to run out and its energyoutput falters, the star begins to contract. As the matter of the star falls inward, it acquires energy and the star heats up. Eventually, the temperature and pressure inside thestar get high enough that the helium in the star can begin to fuse to make carbon. Thecore of the star is very dense, and its energy output heats the outer gases of the star,causing them to expand. The star swells enormously, becoming perhaps as large in diameteras the earth's orbit: 300 million kilometers (186 million miles). A red giant consists ofa dense core and a vast but very thin outer atmosphere. It has a huge surface area to radiate energy, so red giants are very luminous, but the energy is spread thinly. Thesurface temperature of the star is low, which is why red giants are red.

Many red giants are unstable. Instead of swelling to a given size and maintaining it,the stars pulsate and vary in brightness. Some red giants pulsate rhythmically, othersflare up in irregular bursts. Red giants include many varieties of variable stars.

Red giant stars also shed matter into space. Many giants shed matter steadily, othersviolently. In their later life cycles, some pulsating giants eject great shells of matterwhich form luminous envelopes around the star. Because these gas envelopes look disk-likein a telescope, they are called planetary nebulae.

Some giants are massive enough to start other cycles of nuclear reactions after theirhelium runs out; they fuse carbon and perhaps even heavier elements, but sooner or laterall red giants run out of nuclear fuel. Their thin outer envelopes are ejected into spaceor gathered up into their core. The core collapses until a new counteracting force comesinto play. The electron shells of the atoms are crushed out of existence and the electronswander between densely-packed atomic nuclei. The forces between these electrons preventthe star from collapsing further. By this time, the star may be only about as large indiameter as the earth, even though it is as massive as the sun. The matter of the star,called degenerate matter, is so dense that a teaspoonful would weigh many tons onearth. This final stage of the star is a white dwarf. White dwarfs are very hot,but their surface area is so small that they are very faint and lose their heat veryslowly.

The End of the Sun

In all likelihood, the sun will become a red giant. In about 10 billion years, all thehydrogen in the core of the sun will have been used and the sun will start to contractunder its own gravity. It will heat up and brighten as it does, probably making the earthtoo hot for life. When helium begins to fuse in the sun's core, the outer gases of the sunwill expand, probably enveloping the earth. The gas will be hot, but very thin, and forseveral thousand years the earth may actually orbit within the star, slowly heating up bycontact with the thin hot gas, and eventually being destroyed as friction with the gascauses the earth to spiral into the hotter interior of the star. Eventually the sun willeject its outer envelope, or absorb it, leaving only its core as a white dwarf. In theunlikely event the earth survives the red giant stage, the final white dwarf will emitonly a fraction of the present energy of the sun and the earth will be frozen solid.

Evolution of Multiple Stars

If binary or multiple stars are far apart, they will evolve independently of oneanother. However, astonishing things happen when multiple stars are close together. Whenone of the stars reaches the red giant phase, it can swell large enough to exchange gaswith its partner star. What happens depends on the ages of the two stars, their masses,and how rapidly they exchange matter.

If the partner star is a white dwarf, some very violent events can happen. White dwarfshave used up all their nuclear fuel. If a large amount of hydrogen falls onto a whitedwarf, it can undergo nuclear fusion right on the surface of the star. The resultingoutburst will cause the star to brighten briefly by hundreds of times, becoming a nova(Latin, new). Even more dramatic outbursts are possible: the white dwarf canaccumulate mass until its internal pressures become great enough for the next generationof nuclear reactions begin. When that happens, the renewed nuclear activity will blast offthe outer layers of the white dwarf, creating a type I supernova.

Massive Stars and Supernovae

Very massive stars have a more dramatic end yet: they become Type II Supernovae,stars that explode and briefly outshine all the other stars in the galaxy combined.Because old stars lose mass in the red giant stage, it is hard to predict exactly whichstars will meet this most violent of fates.

Red giants with 20 or so times the mass of the sun develop cores with a concentricshell structure. Each shell is hotter and denser than the one outside. In the outermostshell, hydrogen fuses to helium as in any normal star. In the next shell in, helium fusesto carbon. Succeeding shells within fuse carbon, oxygen, neon, and silicon until, at thecenter, there is a core of inert iron. Iron cannot yield energy by fusing to make heavieratoms, so this innermost core is the end product of fusion in the star.

At first glance, it looks as if the star can turn entirely to iron, because each shelluses up the residue from the shell outside it. But there is a catch: eventually the corebecomes so massive that it cannot withstand the pressure of gravity. The core collapsesuntil a new counteracting force halts the collapse. The only force capable of halting thecollapse is the strong nuclear force. The core of the star collapses until it becomes, ineffect, a gigantic atomic nucleus. The core becomes a neutron star. It may havemore mass than the sun but be only 15 kilometers (10 miles) across. A teaspoonful of thismatter would weigh thousands of tons on earth.

Where there was once the hot, dense core of the star, there is briefly a void. Theneutron star core, more massive than the sun but not much bigger than a mountain on earth,has an incredible gravitational pull. At a distance of 10,000 kilometers (6,000 miles) aneutron star as massive as the sun exerts a gravitational pull about 1,300 times strongerthan that on the surface of the earth. In this enormous gravitational field, the matter ofthe star falls inward, reaching perhaps a tenth of the speed of light.

The results are, to say the least, impressive. Several times the mass of our suncrashes into the surface of a neutron star at up to a tenth of the speed of light. Thisgas is heated and compressed beyond anything we can imagine. We can do the calculationsand write the numbers, but nobody can really comprehend the titanic amount of energyinvolved. Nuclear reactions run rampant; there is enough energy available and a highenough density of fast-moving particles to build nuclei as heavy as plutonium and probablyfar heavier. The neutron star core itself is almost incompressible even under theseconditions, and the impacting matter rebounds as a shock wave. This event, called the corebounce, tears the star apart. In a few hours, the shock wave reaches the surface,tearing off the outer layers and exposing the hot interior of the star. The star brightensto billions of times its normal brightness.

Historical Supernovae

Supernovae are commonly seen in distant galaxies. If our own galaxy has supernovae asoften as other galaxies, there is probably one every few years. Yet only a dozen or sosupernovae in our own galaxy have been witnessed from earth. The rest have been obscuredby gas and dust clouds in our galaxy.

The supernova of 1572 was brighter than Venus and could be seen in broad daylight, eventhough it was 5000 light years away. Another supernova occurred in 1604. These events cameat a critical time, just as astronomers were beginning to question the ancient notion thatthe heavens were perfect and unchanging. A supernova in 1006 rivalled the moon inbrightness. Most supernovae have absolute magnitudes of about -21: at a distance of 32.6light years it would shine at magnitude -21 and far outshine the moon. In 1885, asupernova in the andromeda galaxy reached magnitude 7. Across 2.2 million light years,that single star was almost bright enough to see with the unaided eye. All these events,however, happened before astronomers had the observing techniques to study supernovae indetail.

The Supernova of 1987

Almost four centuries of not-very-patient waiting ended in 1987 when the firstsupernova since 1604 visible to the unaided eye appeared. The supernova occurred not inour galaxy, but in the Small Magellanic Cloud, a small satellite galaxy of our own about180,000 light years away.

Almost from the beginning, supernova 1987 followed a script all its own. It onlyreached magnitude 3, not the magnitude 1 that astronomers expected, but it stayed at peakbrightness much longer than most supernovae. Most paradoxical of all, the star thatproduced the supernova was not an aging red supergiant, but a seemingly stable blue-whitesupergiant. It now appears that some red supergiants can lose their outer envelopesquietly, revealing their hot interiors more clearly.

Life (Briefly) Near a Supernova

What might it be like on a planet orbiting a star that went supernova? Imagine theplanet receives as much radiation as we get from the sun. It is hard enough to imagine theenergy output of the sun, let alone a supernova, so let us scale things down a bit firstby asking what it would take to match the sun's output at a distance of only onekilometer. The sun emits 77 megatons of energy every second, but it is 150 millionkilometers away, and the intensity of radiation drops off as the square of the distance.To find out what energy output we need for a distance of one kilometer, we must divide thesun's output by the square of its distance. The answer comes out to about .0000035megatons, or the equivalent of 3.5 tons of high explosive. It is not hard to picture anexplosion of 3.5 tons of high explosive (a truckload) a kilometer away giving off aone-second burst of heat and light that rivals the sun.

Now, if our hypothetical star were to go supernova, its brightness would increase 100billion times, or be equivalent to 350 billion tons of explosive a kilometer away. 350billion tons translates to 350,000 megatons, or much more than all the energy in all theearth's nuclear weapons. In other words, the planet would receive a blast of heat andlight equivalent to having every nuclear weapon on earth detonated at the same time akilometer away -- and this intensity would last for days. It is no exaggeration to saythat the planet would be vaporized.

Fortunately, the melancholy idea of a star going supernova and frying the life on itsplanets is unlikely. The very massive stars that produce supernovae do not shine longenough for life to evolve beyond the simplest forms, and many may not even last longenough for planets to finish the accretion process.

Supernova Remnants and Pulsars

For many thousands of years, a supernova can be recognized by the expanding shell ofgas blasted off the star. Such a shell is called a supernova remnant. The formerstar itself is often detectable as a pulsating radio source, or pulsar. Pulsarsemit tremendous bursts of energy in radio, visible light, and x-ray wavelengths. Thesebursts appear to originate in small regions of the neutron star, perhaps due to matterfalling onto the neutron star. These pulses are extremely regular, ranging in period from.001 second to a few seconds. The period of the pulses is the rotation period of theneutron star. As the parent star collapses, its rotation speeds up, just as a pirouettingskater speeds up by drawing in her arms. If the sun, with a diameter of 864,000 miles (1.3million km) were to shrink to a netron star 10 miles (16 km) in diameter, it would shrinkto 1/86400 of its present size, and its rotation would speed up 86400 times. Instead ofrotating every 28 days, the neutron sun would rotate in 28 seconds.

Supernovas and New Stars

Supernova remnants often appear to be associated with areas of new star formation, andit is very likely that a supernova triggers the formation of new stars. If the expandingblast wave from a supernova strikes a cloud of interstellar gas and dust, it can compressthe cloud enough that parts of the cloud start to contract gravitationally. The supernovadebris will also mingle with the cloud, enriching it in heavy elements.

In normal stars, it is not possible to form large amounts of elements much heavier thaniron. Elements like lead, gold, and uranium can form only in supernovae, as far as we cantell. The fact that we find these elements in the solar system is a sign that our sun isperhaps a third-generation star. The sun is only a third as old as the milky way galaxy,and there was time for many cycles of stellar birth and death before the sun formed. Theimplications of this idea are profound. Every atom in us formed in a star billions ofyears ago and many light-years away.

Black Holes

If the collapsed core of a supernova is more than 1.4 times as massive as the sun, itwill not form a neutron star. Instead, there is no known force that can halt thegravitational collapse. The star will contract until its gravity is so immense that noteven light can escape. As far as we know, the star will contract until it becomes a point,detectable only by its gravity. What happens to the matter in the star? We can onlyspeculate, but it is possible, according to some theories in physics, that the mattermight re-emerge somewhere else in space and time.

These bizarre objects, called black holes, are favorite topics of speculationamong science-fiction and popular magazine articles, but have any actually been detected?Possibly some have. A black hole orbiting another star might draw matter from thecompanion star. As the gas fell into the black hole, it would accelerate to enormousvelocities and emit intense x-rays. There are a few x-ray sources that are so massive thatthey appear likely to be black holes.

Stars and Origin of the Chemical Elements

Where did atoms come from, and how did there come to be so many different kinds? We cananswer this question in quite a bit of detail, thanks to what we know about nuclearreactions in particle accelerators, reactors, and nuclear bombs.

The sun gets its energy by fusing four hydrogen nuclei (protons) to make a heliumnucleus. This process is actually fairly complicated and proceeds in several steps. Thefinal nucleus contains four particles, or nucleons: two protons and two neutrons.

Beyond helium we encounter a bottleneck. If we try to add a neutron or proton to ahelium nucleus, the new nucleus disintegrates almost instantly. There is no nucleus withfive nucleons that lasts long enough to be a basis for heavier elements. Perhaps we canfuse two helium nuclei together? But it turns out that there are no long-lasting nucleiwith eight nucleons either.

However, there is a way around this bottleneck. When stars collapse to form red giants,the temperatures and pressures inside the star become high enough for three-way collisionsof helium nuclei to occur. These collisions are rare, to be sure, but they occur oftenenough to form heavier elements in the quantities we observe. The product of thesecollisions has 12 nucleons: 6 protons and 6 neutrons, and is a carbon nucleus.

What about lithium, beryllium, and boron, the elements between helium and carbon? Theseatoms form only when collisions knock nucleons off of heavier nuclei, a process called spallation,and they tend to be destroyed in the interiors of stars. Compared to carbon, they are rarein the universe. Actually, it is probably a good thing that it is so hard to form heavyatoms. If it were easier, stars might long ago have fused all their hydrogen to heavyatoms and there would be no available energy left in the universe.

Once carbon forms, it serves as a base for building heavier atoms. Nucleons can beadded one-by-one, or by fusing more helium nuclei to an existing nucleus. Buildingelements by fusing helium nuclei is a common process, and elements with even numbers ofprotons in their nuclei are more abundant than atoms with odd numbers. But each heaviernucleus takes more energy to make and gives less energy back, until at iron, with 26protons, the process ends. Beyond iron, it takes more energy to make a nucleus than thenuclear reaction gives back. Random collisions build some heavier nuclei, but theabundance of elements drops off sharply after iron.

Very heavy atoms like gold are extremely rare in the universe, and seem to form only inthe fierce environment in the core of a supernova. Much of the light given off by asupernova turns out to be due to the decay of radiocative nickel. In a supernova, there is so much energy available that particles can pile onto nuclei at a tremendous rate. Atoms at least as heavy as plutonium form this way, and probably atoms far heavier than that.These atoms require more energy to form than our most powerful particle accelerators can produce, but nuclear physics predicts that nuclei with about 110 protons might have very long lifetimes, possibly long enough to be left over from the formation of the earth. Some physicists have attempted to find such super-heavy elements in rocks, so far without success.

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Created 26 March 1998, Last Update 15 January 2020