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Stellar Evolution

Stellar evolution

In astrophysics and cosmology , stellar evolution refers to the life history of stars that is driven by the interplay of internal pressure and gravity .

Essentially, throughout the life of a star a tension exists between the compressing force of the star's own gravity and the expanding pressures generated by the nuclear reactions taking place in its core. After cycles of swelling and contraction associated with the burning of progressively heavier nuclear fuels , the star eventually expends its useable nuclear fuel and resumes contraction under the force of its own gravity. There are three possible fates for such a collapsing star. The particular fate for any star is determined by the mass of the star left after blowing away its outer layers.

A star less than 1.44 times the mass of the Sun (termed the Chandrasekhar limit) collapses until the pressure compacted electron clouds exerts enough pressure to balance the compressing force of gravity. Such stars become white dwarfs that are contracted to a radius the size of a planet. This is the fate of most stars.

If a star retains between 1.4 and roughly three times the mass of the Sun, the pressure of the electron clouds is insufficient to stop the gravitational collapse and stars of this mass continue their collapse to become neutron stars. Although neutron stars are only a few miles across, they have enormous density. Within a neutron star the nuclear forces and the repulsion of the compressed atomic nuclei balance the crushing force of gravity.

With the most massive stars, however, there is no known force in the Universe that can stop the final gravitational collapse and such stars collapse to form a singularity a geometrical point of infinite density. As such a star collapses, its gravitational field warps spacetime and a black hole forms around the singularity.

Gravitational collapse is the process which provides the energy required for star formation, which starts with hydrogen fusion in a protostar at a heat of over 15 million K. Gravity, always directed inwards, decreases the radius of interstellar gas clouds, causing them to collapse and form a protostar, the immediate precursor of a star. Interstellar gas is initially cold, but it is heated by the gravitational energy released by the cloud contraction process. The radius of the protostar will continue to shrink under the influence of gravity until enough internal gas pressure, always directed outwards, builds up to stabilize the collapse. At this stage, the protostar is still too cold for hydrogen fusion to be initiated. Protostars can be detected by infrared spectroscopy because the initial warming event releases infrared electromagnetic radiation. If the mass of the protostar is less than 0.08 solar masses, the temperature of its core never reaches the range required for nuclear fusion and the failed star becomes a brown dwarf.

If, however, the mass of the protostar exceeds 0.08 solar masses, hydrogen fusion can proceed and the protostar becomes a main sequence star, with average surface temperatures of 10,800°F (6,000°C) (the internal and coronal temperatures measure in the millions of degrees). Most stars in the Universe are main sequence stars and are found on the diagonal of a Hertzsprung-Russell diagram. The main sequence stage of star evolution is the most stable state a medium-sized star can reach, and it can last for billions of years as such stars undergo very gradual and slow changes in luminosity and temperature. This is because pressure and gravitational forces are in equilibrium and the core has reached the temperature required for the fusion of hydrogen to helium to proceed smoothly. The time spent by a star in the main sequence is a function of its mass. The more massive the star, the less time spent on the main sequence. Although massive stars have large amounts of fuel, hydrogen fusion proceeds so quickly that it is completed within a few hundred thousand years. The fate of such massive stars is to explode violently. Smaller stars fuse their hydrogen at a slower rate. The lightest stars created in the early history of the Universe, for example, are still on the main sequence. The Sun is approximately midway through its main sequence life.

A post-main sequence star has two distinct regions, consisting of a core of helium nuclei and electrons surrounded by an envelope of hydrogen. With two protons in its nucleus, helium requires a higher fusion temperature than the one at which hydrogen fusion is proceeding. Without a source of energy to increase its temperature, the core cannot counter the effect of gravitational collapse and it starts to collapse, heating up as it does. This heat is transferred to the fusing hydrogen layer, which increases the luminosity of the shell and causes it to expand. As it expands, the outer layers cool off. At this point, the star is characterized by expansion and cooling of its shell, which causes it to become redder with increased luminosity. This is termed the red giant phase. When the Sun reaches this stage, it will be large enough to include Mercury in its sphere and hot enough to evaporate Earth's oceans . The core temperature of a red giant is on the order of 100 million K, the threshold temperature for the fusion of helium into carbon . A red giant, however, is initially stable, as pressure and gravity reach equilibrium.

If helium continues to accumulate in the core as the outer portions of the hydrogen envelope continue to fuse, eventually the helium in the core starts fusing into carbon in a violent event referred to as a helium flash, lasting as little as a few seconds. During this phase, the star gradually blows away its outer atmosphere into an expanding shell of gas known as a planetary nebula.

A star takes thousands of years to go through the red giant phase, after which it evolves into a white dwarf. It is then a small, hot star with a surface temperature as high as 100,000 K that makes it glow white. Because of its small size, a white dwarf has a very high density. A white dwarf consists of those elements that were created in its previous evolutionary phases via nucelosynthesis. The original hydrogen was fused into helium then totally or partly fused into carbon. In addition, heavier elements fuse from the carbon. The temperature of a white dwarf is not high enough to initiate a new cycle of fusion. In time, it eventually becomes a black dwarf as it loses its residual heat over billions of years. The size of a white dwarf is limited by a process called electron degeneracy. Electron degeneracy is the stellar equivalent of the Pauli exclusion principle, as is neutron degeneracy. No two electrons can occupy identical states, even under the pressure of a collapsing star of several solar masses. For stellar masses smaller than about 1.44 solar masses, the energy from the gravitational collapse is not sufficient to produce the neutrons of a neutron star, so the collapse is effectively stopped. This maximum mass for a white dwarf is called the Chandrasekhar limit.

When a massive star has fused all of its hydrogen, gravitational collapse is capable of generating sufficient energy so that the core can begin to fuse helium nuclei to form carbon. If the process can go beyond the red giant stage, the star becomes a supergiant. Following fusion and disappearance of the helium, the core can successively burn carbon and other heavier elements until it acquires a core of iron , the heaviest element that can be formed by natural fusion. Another possible fate of white dwarfs is to evolve into novae or another type of supernovae. Novae occur in binary star systems in which one star is a white dwarf. If the companion star evolves into a red giant, it can expand far enough so that gas from its outer shell can be pulled onto the white dwarf. The white dwarf accumulates the additional gas until it reaches nuclear fusion temperatures, at which point the gas ignites explosively into a nova.

Alternatively, a white dwarf may accumulate enough material from its binary star to exceed the Chandrasekhar limit. This results in a sudden and total collapse of the white dwarf, with temperature increases in ranges capable of initiating rapid carbon fusion and subsequent explosion of the white dwarf into a spectacular supernova, that can shine with the brightness of 10 billion suns with a total energy output of 1044 joules, equivalent to the total energy output of the Sun during its entire lifetime.

See also Astronomy; Big Bang theory; Bohr model of the atom; Cosmology; Quantum electrodynamics (QED); Solar sunspot cycles; Solar system; Stellar life cycle

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stellar evolution

stellar evolution, life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration. Because a star's total energy reserve is finite, a star shining today cannot continue to produce its present luminosity steadily into the indefinite future, nor can it have done so from the indefinite past. Thus, stellar evolution is a necessary consequence of the physical theory of stellar structure, which requires that the luminosity, temperature, and size of a star must change as its chemical composition changes because of thermonuclear reactions.

Phases of Stellar Evolution

Contraction of the Protostar

The initial phase of stellar evolution is contraction of the protostar from the interstellar gas, which consists of mostly hydrogen, some helium, and traces of heavier elements. In this stage, which typically lasts millions of years, half the gravitational potential energy released by the collapsing protostar is radiated away and half goes into increasing the temperature of the forming star. Eventually the temperature becomes high enough for thermonuclear reactions to begin; if the mass of the protostar is too small to raise the temperature to the ignition point for the thermonuclear reaction, the result is a brown dwarf, or "failed star." In these thermonuclear reactions, loosely called "hydrogen burning," four hydrogen nuclei are fused to form a helium nucleus (see nucleosynthesis). This point in time is conventionally called age zero.

Many protostar contractions have been observed in isolated gas clouds; that is, where one cloud contracted to form one star. However, in 1995, the first example of a star-forming region was found in the Eagle Nebula, some 7,000 light-years from the earth. In this region, stars are being formed at the tips of long, fingerlike columns stretching from a huge cloud of interstellar gas and dust; the columns are being eroded by radiation (a process called photoevaporation) from stars in the vicinity, leaving scattered knots of matter that contract into stars.

Mature Stars and the Main Sequence

Once formed, a star settles into a long "middle age" during which it shines steadily as it converts its hydrogen supply into helium. For stars of a given chemical composition, the mass alone determines the luminosity, surface temperature, and size of the star. The luminosity increases very sharply with an increase in the mass; doubling the mass (which is proportional to the energy supply) increases the luminosity (which is proportional to the rate of using energy) more than 10 times. Hence the more massive and luminous a star is, the faster it depletes its hydrogen and the faster it evolves.

Because the middle age of a star is the longest period in stellar evolution, one would expect most of the observed stars to be at this stage and to show a strong correlation of luminosity with color (color is a measure of stellar temperature). This prediction is confirmed by plotting stars on a Hertzsprung-Russell diagram, in which the majority of stars fall along a diagonal line called the main sequence. The main sequence is most heavily populated at the low luminosity end; these are the stars that evolve most slowly and so remain longest on the main sequence.

As a star's hydrogen is converted into helium, its chemical composition becomes inhomogeneous: helium-rich in the core, where the nuclear reactions occur, and more nearly pure hydrogen in the surrounding envelope. The hydrogen near the center of the core is consumed first. As this is depleted, the site of the nuclear reactions moves out from the center of the core and fusion occurs in successive concentric shells. Finally fusion occurs only in a thin, outer shell of the core, the only place where both the hydrogen content and the temperature are high enough to sustain the reactions.

Old Stars and Death

As the helium content of the star's core builds up, the core contracts and releases gravitational energy, which heats up the core and actually increases the rates of the nuclear reactions. Thus the rate of hydrogen consumption rises as the hydrogen is used up. To accommodate the higher luminosity resulting from the increased reaction rates, the envelope must expand to allow an increased flow of energy to the surface of the star. As the outer regions of the star expand, they cool.

The star now consists of a dense, helium rich core surrounded by a huge, tenuous envelope of relatively cool gas; the star has become a red giant. Eventually, the contracting stellar core will reach temperatures in excess of 100 million degrees Kelvin. At this point, helium burning sets in. With the ignition of that process, the expansion of the envelope is halted and then reversed; the star retreats from the red giant phase, shrinking in size and luminosity, and reapproaches the main sequence. The exact course of evolution is uncertain, but as the star recrosses the main sequence, it will probably become unstable. The star may eject some of its mass or become an exploding nova or supernova star; at the very least, it will become a pulsating variable star, possibly a Cepheid variable.

In the later stages of evolution, further contraction and elevation of temperature open up new thermonuclear reactions. It is believed that the heavier elements in the universe, up to iron, were synthesized in the interiors of stars by a variety of intricate nuclear reactions, many involving neutron absorption. Elements heavier than iron are made in supernova explosions. As a result of the nuclear reactions, the chemical composition of the late-stage star becomes highly inhomogeneous; its structure is fractionated into a number of concentric shells consisting of different elements around an iron core.

The final outcome of stellar evolution depends critically on the remaining mass of the old star. The vast majority of stars do not develop iron cores. If the mass is not greater than the Chandrasekhar mass limit (1.5 times the sun's mass), the star will become a white dwarf, glowing feebly for billions of years by radiating away its remaining heat energy until it becomes a black dwarf, a totally dead star. If the star is too massive to become a stable white dwarf, contraction will continue until the temperature reaches about 5 billion degrees Kelvin. At this temperature the iron nuclei in the core begin to absorb electrons; this creates neutron-rich isotopes and simultaneously deprives the core of its pressure. With further collapse and increase in density, the core becomes a special kind of rigid solid. At still higher density, the solid "evaporates" as the nuclei break up into free neutrons. The resulting neutron fluid forms the core of a new astrophysical body, called a neutron star, of which pulsars are examples. If the stellar mass is too great to be stable even as a neutron star, complete gravitational collapse will ensue and a black hole will form.

Validating the Theory of Stellar Evolution

Because the computed lifetimes of stars range from millions to billions of years, one cannot follow an individual star through its life history observationally, or even observe significant changes in the whole span of human history, except from the violent events of nova and supernova explosions. However, new stars are continually being formed and hence stars of all ages exist at the present epoch; examples of the various stages of stellar evolution can be found in different stars. The age of a star is not a directly observable characteristic but must be inferred from the very evolutionary theory one is trying to validate. Confidence in this circular reasoning results from its self-consistency and its ability to draw together into a unified picture a wide variety of observational data on individual stars, clusters of stars, and galaxies.

See cosmology; star clusters.

Bibliography

See I. S. Shklovsky, Stars: Their Birth, Life, and Death (1978); D. A. Cooke, The Life and Death of Stars (1985); A. Harpaz, Stellar Evolution (1994); I. Asimov, Star Cycles: The Life and Death of Stars (1995).

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Stellar Evolution

Stellar Evolution

From atoms to stars

Stellar mechanics

Resources

The changes that occur during a stars life are called stellar evolution. The mass of a star determines the ultimate fate of a star. Stars that are more massive burn their fuel quicker and lead shorter lives. Because stars shine, they must change. The energy they lose by emitting light must come from the matter of which the star is made. This will lead to a change in its composition. Stars are formed from the material between stars, shine until they exhaust their fuel, and then die a predictable death based upon their initial mass.

From atoms to stars

Understanding of the processes of stellar evolution came as a result of twentieth century advances in both astronomy and atomic physics. Advances in quantum theory and improved models of atomic structure made it clear to astronomers that deeper understanding of the life cycle of stars and of cosmological theories explaining the vastness of space was to be forever tied to advances in understanding inner workings of the universe on an atomic scale. In addition, a complete understanding of the energies of mass conversion in stars was provided by German American physicist Albert Einsteins (18791955) special theory of relativity and his relation of mass to energy (E= mc2, or energy [E] equals mass [m] times the speed of light [c] squared).

Indianborn American astrophysicist Subrahmanyan Chandrasekhar (19101995) first articulated the evolution of stars into supernovae, white dwarfs, and neutron stars; and predicted the conditions required for the formation of black holes, which were subsequently confirmed by observation in the last years of the twentieth century.

Stellar mechanics

The material between stars occurs in clouds of varying mass. By processes that are still not completely clear, but involve cooling of the cloudcenter with the formation of molecules, and the squeezing of the cloud by outside starlight or perhaps a stellar explosion, the cloud begins to collapse under its own selfgravity. The collapse of the cloud results in the material becoming hotter simply from the squeezing of the collapse. At this point, the interior of the star churns. This churning process is called convection. Its rate of collapse is determined by the rate at which it can lose energy from its surface. Atomic processes keep the surface near a constant temperature so that a rapid collapse is slowed by the radiating surface area shrinking during the collapse. The star simply gets fainter while the interior gets progressively hotter.

Finally, the internal temperature rises to the point where atoms located at the center of the star, where the temperature is the hottest, are moving so fast from the heat generated that they begin to stick together. This process is called nuclear fusion, and it results in an additional production of energy. Thus, the star has a new source of heat. The subsequent evolution of the star will be largely determined by its mass.

If the mass of the star is about equal to that of the Sun or less, the nuclear fires that now provide the energy for the star to shine will determine its internal structure. A central radiative core is surrounded by a convective envelope. In the radiative core the material remains quiescent, while energy generated by nuclear fusion of hydrogen to helium simply diffuses through it like the light from automobile headlights shines through a fog. It is at the very center of this radiative core that the helium ash of the nuclear fires accumulates as the star ages. Beyond the radiative core lies the churning convective envelope through which the energy is carried by blobs of hot matter rising past returning cooler blobs. At the atmospheric surface, the energy again flows as it did in the core until it physically leaves the star as starlight.

The structure of stars more than twice the mass of the Sun is essentially the reverse of the lowmass stars. The cores of these stars are fully convective so that the energy produced by nuclear fusion is carried outward by the churning motion of the material in the core. The surrounding radiative envelope behaves much like the cores of lowermass stars except no new energy is produced there. The churning motion of the material in the convective core causes the nuclear ash of helium to be mixed with the surrounding hydrogen fuel. This motion ensures that virtually all the hydrogen will be available to the nuclear fires that heat the star.

Both high and lowmass stars respond to the depletion of hydrogen fuel in a similar manner. In order to supply the heat to oppose its own selfgravity, the stars core again responds by shrinking. In a sort of reflex reaction, the outer regions of the star expand, causing a great increase of its radiating surface area. Although the total energy output of the star increases during this phase, the greatly enhanced surface area results in a cooling of the surface and the star takes on a redder appearance. The size and color change lead to the name of red giant for these stars. If the star is very massive, it may become what is called a red supergiant.

For the lowmass stars, the expansion to the red giant phase will begin when about 90% of its hydrogen has been converted to helium. During the contraction of its core, a complicated sequence of events occurs. The shrinkage required to produce the energy radiated by the large giant causes the core to shrink to the dimensions of a white dwarf, while hydrogen continues to burn by nuclear fusion in a thin shell surrounding the core. This shell provides most of the energy that is radiated away by the star. However, the core material, having attained the dimensions of a white dwarf, behaves very differently than the highdensity gas that it was earlier in its life. No longer must it be heated to generate the pressure required to oppose the weight of the overlying material. When matter reaches this state, it is called degenerate matter. The degenerate core just sits there, becoming hotter from the energy released by the surrounding hydrogenburning shell and growing slowly from the helium ash generated by the shell. The hydrogenburning shell is required to produce increasing amounts of energy from decreasing amounts of hydrogen fuel to sustain the brightening red giant. This continuing increase in the energy output from the shell heats the core, which finally reaches a temperature where the helium begins to undergo nuclear reactions, producing carbon. In this fusion process, three helium nuclei collide yielding one carbon nucleus and additional energy for the support of the star.

The star now has a new energy source. However, the degenerate nature of the core does not allow it to expand and cool as would a core made of ordinary gas. Thus, the onset of helium burning leads to a rapid rise in core temperature, which is not balanced by a cooling expansion. The increased core temperature leads to a dramatic increase in helium burning. This sequence, known as the helium flash, continues until the degeneracy of the material making up the core is removed by the intense heat. The return of the material to its ordinary gaseous state leads to a rapid expansion, which cools the core and reduces the helium burning. An equilibrium is established with the star generating progressively more energy from helium fusion, while the energy from the hydrogen burning shell is reduced, and it ultimately goes out from lack of fuel. The star continues to shine through the red giant phase by converting helium into carbon through nuclear fusion.

As the helium becomes depleted, the outer layers of the star become unstable and rather gently lift off the star to be slowly blown away by the light from the star. (Such an image was captured in 1998 by the Hubble Space Telescope when a dying star known as NGC7027, located about 3,000 lightyears from the Sun in the direction of Cygnus the Swan, was observed.) Such shells of expanding gas are observed as greenish disks that eventually become greenish rings and the material becomes less dense. These greenish clouds are called planetary nebulae, even though they have nothing whatever to do with planets. Astronomers of two centuries ago gave them that name, for their telescopic appearance from the Earth was like that of the outer planets Uranus and Neptune.

The remaining core of the red giant, now exposed, cools rapidly, again becomes degenerate, and is known as a white dwarf. A white dwarf has reached a stalemate between its own selfgravity and the nature of the degenerate stuff of which it is now composed. It may now simply cool off to become a dark stellar cinder about the size of the Earth.

The evolution of a massive stars follow a somewhat different course. The churning of the convective core makes most of the hydrogen fuel available for consumption in the nuclear fires. Thus, these stars will not suffer the effects of core contraction until more than 99.9% of their hydrogen has been consumed. Even though they can consume more of their hydrogen (on a percentage basis), and they have more fuel to burn, they also shine much brighter than the lowmass stars. Thus, their overall lifetimes will be far less than the lowmass stars. While the lifetime of a star like the Sun may approach ten billion years, a star with ten times the mass of the Sun may last less than ten million years.

The exhaustion of the hydrogen convective core leads to its contraction and the expansion of the outer layers, as was the case with the lowmass stars. However, the fate of the core is rather different from that of the lowmass stars. The core is far too massive to reach equilibrium as a degenerate structure like a white dwarf, so that contraction continues heating the core until the ignition of helium fusion is achieved. Unlike the lowermass stars where the onset of helium burning occurs with a flash, the helium fusion in massive stars begins slowly and systematically takes over from the hydrogenburning shell surrounding the core. Throughout the red giant or supergiantphase the role of energy production steadily shifts from hydrogen burning to helium burning. Eventually, helium becomes exhausted around a growing carbon core. While helium continues to undergo fusion in a shell surrounding the core, carbon fusion is ignited. Just as a degenerate helium core gives rise to the unstable ignition of helium, called the helium flash in lowmass stars, so the degenerate carbon core of moderate mass stars can result in an unstable ignition of carbon. However, whereas the helium flash is quickly quelled in lowmass stars, the carbon ignites explosively in the cores of these moderatemass stars. This process is called carbon deflagration and may result in the destruction of the star.

In even more massive stars, the onset of carbon burning is a controlled process and the star develops multiple shells of energy sources involving carbon fusion, helium fusion, and even some hydrogen fusion in the outer regions of the star. The ignition by nuclear fusion of each new element yields less energy than the one before it. In addition, the increased temperature required for the nuclear fusion of these additional sources leads to an increase in the stellar luminosity. The result is an everincreasing rate of the formation of lessefficient energy sources. When nuclear fusion in the core of the star yields iron, further nuclear fusion will no longer yield energy. Instead, nuclear fusion of iron will use up energyrobbing thermal energy from the surrounding material. This sudden cooling of the core will bring about its collapse.

As the density increases in the collapsing core, there is less and less room for the free electrons that have been stripped from the atomic nuclei by the extreme temperature. These electrons must go somewhere, so they begin to be absorbed in the protons of the atomic nuclei, turning them into neutrons. The process is called neutronization. This reaction generates particles called neutrinos, which interact very weakly with ordinary matter, and so normally escape directly from the star. The energy robbed from the core by the neutrinos also adds to the energy crises in the core and contributes to the core collapse.

The production of elements with masses greater than iron also produces large quantities of neutrinos, so that whichever process dominates, a great deal of energy is lost directly from the star, resulting in a catastrophic gravitational collapse of the core. This action is followed promptly by the collapse of the entire star. The rapid increase in the density of the collapsing core finally reaches the point where the material becomes opaque to the energyrobbing neutrinos, and their continued escape is stopped. The sudden deposition of the neutrino energy in the collapsing core reverses the collapse, bringing about an explosion of unprecedented magnitude. The infalling matter and trapped photons are hurled into space, liberating as much energy in a few minutes as the star has radiated in its lifetime of millions of years.

The remains of this titanic explosion depend on the initial mass of the collapsing star. Verymassive stars may leave a black hole of completely collapsed matter behind. Should the collapse involve a star of less mass, the remainder may be something called a neutron star, similar to that formed by the collapse of a white dwarf. In some instances, the entire star may be involved in the explosion and there will be no remains at all. While there have been recent attempts to refine the classification of these explosions, astronomers still refer to the explosion of a massive star as a supernova of type II. Supernovae of type I are thought to result from the collapse of a white dwarf, which has exceeded its critical mass. Unlike the evolution of lowmass stars, in which an accommodation between the forces of gravity and degenerate structure of the star is achieved through the formation of a white dwarf, the evolution of a massive star must end in a violent stellar explosion. Gravity appears to win its struggle with nuclear physics, but at the last moment, the energy of collapse is turned to an explosion leaving either a collapsed corpse, or perhaps nothing at all.

The accession of the Hubble Space Telescope had given astronomers a valuable tool to study the evolution of stars in the universe, at the same time challenging their understanding. In 1997, Hubble detected rogue stars that do not belong to any galaxy, displaced long ago and now hanging in empty intergalactic space among star clusters like the Virgo Cluster, about 60 million lightyears from the Earth.

In 1996, astronomers found evidence of many isolated, dim brown dwarfs, lacking sufficient mass to start nuclear fusion. They detected light spectra from the element lithium, which quickly burns in true stars. These brown dwarfs, called L dwarfs, are typically smaller then the sun but much larger than even Jupiter, and some may resemble Saturns moon Titan.

On the opposite scale, in 1997, Hubble detected the then brightest star ever seen. Discovered at the core of the Milky Way galaxy and named the Pistol star, it has the energy of ten million Suns and would fill the distance of the Earths orbit around the sun. The Pistol Star is about 25,000 lightyears from the Earth; it is so turbulent that its eruptions create a gas cloud four lightyears across. It had been thought that a star so big could not have formed without blowing itself apart, and so the Pistol Star will require astronomers to reexamine their ideas about stellar formation, especially of supermassive stars near the centers of galaxies.

By 2003, other observations, including xray observations from the ROSAT (short for Rontgensatellit) Observatory and NASAs Chandra Xray Observatory (CXO), allowed the identification of high intensity ultrabright xray sources that many astronomers argued were evidence of black holes in starforming galaxies. Although there are other explanations for these phenomena, the fact that they provide additional confirmation of black holes is enhanced by Hubble observations of stars rotating around stellar cores of these galaxies.

In early 2003, the Chandra Xray Observatory, provided extended observations of Sagittarius A (or Sgr A), the supermassive black hole at the center of Earths own Milky Way galaxy. In 2004, astronomers found over 30 black holes. In fact, a black hole was discovered in June 2004 that scientists conjecture will help to confirm that gigantic black holes were

KEY TERMS

Convective core The central, or surrounding regions of a star where the energy is carried by envelope convection. Convective transport of energy is the same as that found in a pan of boiling water where hot material physically rises, carrying energy, and having deposited that energy at the top of the region, descends as cooler material.

Deflagration The explosive onset of nuclear fusion leading to the disruption of the reaction structure.

Degenerate gas A gas whose constituents are packed to such high densities that further packing would violate the Pauli Exclusion Principle. The pressure of such a gas exhibits almost no dependence on temperature.

Ideal gas Gas that obeys the ideal gas law, where the pressure is proportional to the product of the local temperature and density.

Neutrino A nuclear particle resulting from nuclear reactions. Neutrinos interact very weakly with ordinary matter, and can easily traverse a normal star without colliding with any of the stellar material.

Neutron Together with protons, neutrons comprise the basic building blocks of the nuclei of the elements. They have a mass just slightly greater than that of a proton, but lack its electric charge.

Neutron star A star with a mass similar to the sun, but composed almost entirely of neutrons. The neutrons are packed so tightly that they are degenerate, like the electrons of a white dwarfbut the resulting density is far greater. The typical size of such a star is about 6.2 mi (10 km).

Nuclear fusion The combining of the nuclei of two elements so as to produce a new, more massive element. The process is accompanied by the release of energy as long as the end product of the reaction is less massive than iron.

Planetary nebula An expanding shell of gas ejected by a lowmass red giant, which may take on the appearance of one of the outer planets of the solar system when seen in a small telescope.

Radiative core The central, or surrounding regions of a star where the energy is carried by envelope radiative diffusion. Radiative diffusion describes the flow of particles of light (photons) through a medium where there is little mechanical change to the medium.

Supernova The final collapse stage of a supergiant star.

White dwarf A star that has used up all of its thermonuclear energy sources and has collapsed gravitationally to the equilibrium against further collapse that is maintained by a degenerate electron gas.

created early in the formation of the universe. In 2005, a black hole was discovered to be traveling at twice the escape velocity of the galaxy as it exited the Milky Way. Scientists think that such a black hole may help to support the theory that a black hole exists in the center of the Milky Way galaxy.

See also Gravity and gravitation; Red giant star; Star formation.

Resources

BOOKS

Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGrawHill, 2006.

Aveni, Anthony F. Uncommon Sense: Understanding Natures Truths Across Time and Culture. Boulder, CO: University Press of Colorado, 2006.

Chaisson, Eric. Astronomy: A Beginners Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Dopita, Michael A. Astrophysics of the Diffuse Universe. Berlin, Germany, and New York: Springer, 2003.

Freeman, Kenneth C. In Search of Dark Matter. Berlin, Germany, and New York: Springer, 2006.

Krishna, K.S. Krishna. Dust in the Universe: Similarities and Differences. Singapore and London, UK: World Scientific, 2005.

Kundt, Wolfgang. Astrophysics: A New Approach. Berlin and New York: Springer, 2005.

Lequeux, James. The Interstellar Medium. Berlin, Germany, and New York: Springer, 2005.

Mallary, Michael. Our Improbable Universe: A Physicist Considers How We Got Here. New York: Thunders Mouth Press, 2004.

OTHER

University of Cambridge. Our Own Galaxy: The Milky Way. Cambridge Cosmology. <http://www.damtp.cam.ac.uk/user/gr/public/gal_milky.html> (accessed October 29, 2006).

George W. Collins, II

K. Lee Lerner

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Stellar Evolution

Stellar evolution

The mass of a star determines the ultimate fate of a star. Stars that are more massive burn their fuel quicker and lead shorter lives. Because stars shine, they must change. The energy they lose by emitting light must come from the matter of which the star is made. This will lead to a change in its composition. Stars are formed from the material between stars, shine until they exhaust their fuel, and then die a predictable death based upon their initial mass. The changes that occur during a star's life are called stellar evolution .


From atoms to stars

Understanding of the processes of stellar evolution came as a result of twentieth century advances in both astronomy and atomic physics . Advances in quantum theory and improved models of atomic structure made it clear to astronomers that deeper understanding of the life cycle of stars and of cosmological theories explaining the vastness of space was to be forever tied to advances in understanding inner workings of the universe on an atomic scale. In addiiton, a complete understanding of the energetics of mass conversion in stars was provided by Albert Einstein's (1879–1955) special theory of relativity and his relation of mass to energy (Energy = mass times the square of the speed of light squared).

Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910–1995) first articulated the evolution of stars into supernova , white dwarfs, neutron stars and for predicting the conditions required for the formation of black holes subsequently confirmed by observation in the last years of the twentieth century.


Stellar mechanics

The material between stars occurs in clouds of varying mass. By processes that are still not completely clear, but involve cooling of the cloud-center with the formation of molecules, and the squeezing of the cloud by outside star light or perhaps a stellar explosion, the cloud begins to collapse under its own self-gravity. The collapse of the cloud results in the material becoming hotter simply from the squeezing of the collapse. At this point, the interior of the star churns. This churning process is called convection . Its rate of collapse is determined by the rate at which it can lose energy from its surface. Atomic processes keep the surface near a constant temperature so that a rapid collapse is slowed by the radiating surface area shrinking during the collapse. The star simply gets fainter while the interior gets progressively hotter.

Finally, the internal temperature rises to the point where atoms located at the center of the star, where the temperature is the hottest, are moving so fast from the heat that they begin to stick together. This process is called nuclear fusion , and it results in an additional production of energy. Thus the star has a new source of heat. The subsequent evolution of the star will be largely determined by its mass.

If the mass of the star is about like that of the Sun or less, the nuclear "fires" which now provide the energy for the star to shine will determine its internal structure. A central radiative core is surrounded by a convective envelope. In the radiative core the material remains quiescent, while energy generated by nuclear fusion of hydrogen to helium simply diffuses through it like the light from auto headlights shines through a fog . It is at the very center of this radiative core that the helium "ash" of the nuclear "fires" accumulates as the star ages. Beyond the radiative core lies the churning convective envelope through which the energy is carried by blobs of hot matter rising past returning cooler blobs. At the atmospheric surface, the energy again flows as it did in the core until it physically leaves the star as starlight.

The structure of stars more than twice the mass of the sun is essentially the reverse of the low-mass stars. The cores of these stars are fully convective so that the energy produced by nuclear fusion is carried outward by the churning motion of the material in the core. The surrounding radiative envelope behaves much like the cores of lower-mass stars except no new energy is produced there. The churning motion of the material in the convective core causes the nuclear ash of helium to be well-mixed with the surrounding hydrogen fuel. This motion ensures that virtually all the hydrogen will be available to the nuclear fires which heat the star.

Both high- and low-mass stars respond to the depletion of hydrogen fuel in a similar manner. In order to supply the heat to oppose its own self-gravity, the star's core again responds by shrinking. In a sort of reflex reaction, the outer regions of the star expand, causing a great increase of its radiating surface area. Although the total energy output of the star increases during this phase, the greatly enhanced surface area results in a cooling of the surface and the star takes on a redder appearance. The size and color change lead to the name of red giant for these stars. If the star is very massive, it may become what is called a red supergiant.

For the low-mass stars, the expansion to the red giant phase will begin when about 90% of its hydrogen has been converted to helium. During the contraction of its core, a complicated sequence of events occurs. The shrinkage required to produce the energy radiated by the large giant causes the core to shrink to the dimensions of a white dwarf , while hydrogen continues to burn by nuclear fusion in a thin shell surrounding the core. It is this shell that provides most of the energy that is radiated away by the star. However, the core material, having attained the dimensions of a white dwarf, behaves very differently than the high-density gas that it was earlier in its life. No longer must it be heated to generate the pressure required to oppose the weight of the overlying material. When matter reaches this state it is called degenerate matter. The degenerate core just sits there, becoming hotter from the energy released by the surrounding hydrogen-burning shell and growing slowly from the helium ash generated by the shell. The hydrogen-burning shell is required to produce increasing amounts of energy from decreasing amounts of hydrogen fuel to sustain the brightening red giant. This continuing increase in the energy output from the shell heats the core, which finally reaches a temperature where the helium begins to undergo nuclear reactions, producing carbon . In this fusion process three helium nuclei collide yielding one carbon nucleus and additional energy for the support of the star.

The star now has a new energy source. However, the degenerate nature of the core doesn't allow it to expand and cool as would a core made of ordinary gas. Thus the onset of helium burning leads to a rapid rise in core temperature which is not balanced by a cooling expansion. The increased core temperature leads to a dramatic increase in helium burning. This sequence, known as the helium flash, continues until the degeneracy of the material making up the core is removed by the intense heat. The return of the material to its ordinary gaseous state leads to a rapid expansion which cools the core and reduces the helium burning. An equilibrium is established with the star generating progressively more energy from helium fusion, while the energy from the hydrogen burning shell is reduced, and it ultimately goes out from lack of fuel. The star continues to shine through the red giant phase by converting helium into carbon through nuclear fusion.

As the helium becomes depleted, the outer layers of the star become unstable and rather gently lift off the star to be slowly blown away by the light from the star. (Such an image was captured in 1998 by the Hubble Space Telescope , of a dying star known as NGC7027, located about 3,000 light-years from the sun in the direction of Cygnus the Swan.) Such shells of expanding gas are observed as greenish disks that eventually become greenish rings and the material becomes less dense. These greenish clouds are called planetary nebulae , even though they have nothing whatever to do with planets. Astronomers of two centuries ago gave them that name, for their telescopic appearance from Earth was like that of the outer planets Uranus and Neptune .

The remaining core of the red giant, now exposed, cools rapidly, again becomes degenerate, and is known as a white dwarf. A white dwarf has reached a stalemate between its own self-gravity and the nature of the degenerate stuff of which it is now composed. It may now simply cool off to become a dark stellar cinder about the size of the earth.

The evolution of a massive stars follow a somewhat different course. The churning of the convective core makes most of the hydrogen fuel available for consumption in the nuclear fires. Thus, these stars will not suffer the effects of core contraction until more than 99.9% of their hydrogen has been consumed. Even though they can consume more of their hydrogen (on a percentage basis), and they have more fuel to burn, they also shine much brighter than the low-mass stars. Thus their overall lifetimes will be far less than the low-mass stars. While the lifetime of a star like the sun may approach ten billion years, a star with ten times the mass of the sun may last less than ten million years.

The exhaustion of the hydrogen convective core leads to its contraction and the expansion of the outer layers, as was the case with the low-mass stars. However, the fate of the core is rather different from that of the low-mass stars. The core is far too massive to reach equilibrium as a degenerate structure like a white dwarf, so that contraction continues heating the core until the ignition of helium fusion is achieved. Unlike the lower-mass stars where the onset of helium burning occurs with a flash, the helium fusion in massive stars begins slowly and systematically takes over from the hydrogen-burning shell surrounding the core. Throughout the red giant or supergiant phase the role of energy production steadily shifts from hydrogen burning to helium burning. Eventually, helium becomes exhausted around a growing carbon core. While helium continues to undergo fusion in a shell surrounding the core, carbon fusion is ignited. Just as a degenerate helium core gives rise to the unstable ignition of helium, called the helium flash in low-mass stars, so the degenerate carbon core of moderate mass stars can result in an unstable ignition of carbon. However, whereas the helium flash is quickly quelled in low-mass stars, the carbon ignites explosively in the cores of these moderate-mass stars. This process is called carbon deflagration and may result in the destruction of the star.

In even more massive stars, the onset of carbon burning is a controlled process and the star develops multiple shells of energy sources involving carbon fusion, helium fusion, and even some hydrogen fusion in the outer regions of the star. The ignition by nuclear fusion of each new element yields less energy than the one before it. In addition, the increased temperature required for the nuclear fusion of these additional sources leads to an increase in the stellar luminosity. The result is an ever-increasing rate of the formation of less-efficient energy sources. When nuclear fusion in the core of the star yields iron , further nuclear fusion will no longer yield energy. Instead, nuclear fusion of iron will use up energy-robbing thermal energy from the surrounding material. This sudden cooling of the core will bring about its collapse.

As the density increases in the collapsing core, there is less and less room for the free electrons that have been stripped from the atomic nuclei by the extreme temperature. These electrons must go somewhere, so they will begin to be "absorbed" in the protons of the atomic nuclei, turning them into neutrons. The process is called neutronization. This reaction generates particles called neutrinos, which interact very weakly with ordinary matter, and so normally escape directly from the star. The energy robbed from the core by the neutrinos also adds to the energy crises in the core and contributes to the core collapse.

The production of elements with masses greater than iron also produces large quantities of neutrinos, so that whichever process dominates, a great deal of energy is lost directly from the star, resulting in a catastrophic gravitational collapse of the core. This is followed promptly by the collapse of the entire star. The rapid increase in the density of the collapsing core finally reaches the point where the material becomes opaque to the energy-robbing neutrinos, and their continued escape is stopped. The sudden deposition of the neutrino energy in the collapsing core reverses the collapse, bringing about an explosion of unprecedented magnitude. The infalling matter and trapped photons are hurled into space, liberating as much energy in a few minutes as the star has radiated in its lifetime of millions of years.

The remains of this titanic explosion depend on the initial mass of the collapsing star. Very-massive stars may leave a black hole of completely collapsed matter behind. Should the collapse involve a star of less mass, the remainder may be something called a neutron star , similar to that formed by the collapse of a white dwarf. In some instances, the entire star may be involved in the explosion and there will be no remains at all. While there have been recent attempts to refine the classification of these explosions, astronomers still refer to the explosion of a massive star as a supernova of type II. Supernovae of type I are thought to result from the collapse of a white dwarf which has exceeded its critical mass. Unlike the evolution of low-mass stars, in which an accommodation between the forces of gravity and degenerate structure of the star is achieved through the formation of a white dwarf, the evolution of a massive star must end in a violent stellar explosion. Gravity appears to win its struggle with nuclear physics, but at the last moment, the energy of collapse is turned to an explosion leaving either a collapsed corpse, or perhaps nothing at all.

The accession of the Hubble Space Telescope had given astronomers a valuable tool to study the evolution of stars in the universe, at the same time challenging their understanding. In 1997, Hubble detected rogue stars that belong to no galaxy , displaced long ago and now hanging in empty intergalactic space among star clusters like the Virgo Cluster, about 60 million light-years from Earth.

In 1996, astronomers found evidence of many isolated, dim brown dwarfs, lacking sufficient mass to start nuclear fusion. They detected light spectra from the element lithium , which quickly burns in true stars. These brown dwarfs, called "L dwarfs," are typically smaller then our sun but much larger than even Jupiter , and some may resemble Saturn's moon Titan.

On the opposite scale, in 1997 Hubble detected the then brightest star ever seen. Discovered at the core of our own galaxy and named the Pistol Star, it has the energy of ten million Suns and would fill the distance of the Earth's orbit around the Sun. The Pistol Star is about 25,000 light-years from Earth; it is so turbulent that its eruptions create a gas cloud four light-years across. It had been thought that a star so big could not have formed without blowing itself apart, and so the Pistol Star will require astronomers to reexamine their ideas about stellar formation, especially of supermassive stars near the centers of galaxies.

By 2003, other observations, including x-ray observations from the ROSAT Observatory and NASA's Chandra x-ray Observatory, allowed the identification of high intensity ultra-bright x-ray sources that many astronomers argued were evidence of black holes in star-forming galaxies. Although there are other explanations for these phenomena, the fact that they provide additional confirmation of black holes is enhanced by Hubble observations of stars rotating around stellar cores of these galaxies.

In early 2003, the Chandra x ray Observatory, provided extended observations of Sagittarius A (or Sgr A), the supermassive black hole at the center of Earth's own Milky Way galaxy.

See also Gravity and gravitation; Red giant star; Star formation.

Resources

books

Collins, G.W., II, Fundamentals of Stellar Astrophysics. New York: W.H. Freeman, 1989.

Prialnik, Dina. An Introduction to the Theory of Stellar Structure and Evolution. Cambridge University Press, 2000.


other

University of Cambridge. "Our Own Galaxy: The Milky Way." Cambridge Cosmology, May 16, 2002 [cited January 21 2003]. <http://www. damtp. cam.ac.uk/user/gr/public/gal_milky.htm>.


George W. Collins
K. Lee Lerner

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Convective core

—The central, or surrounding regions of a star where the energy is carried by envelope convection. Convective transport of energy is the same as that found in a pan of boiling water where hot material physically rises, carrying energy, and having deposited that energy at the top of the region, descends as cooler material.

Deflagration

—The explosive onset of nuclear fusion leading to the disruption of the reaction structure.

Degenerate gas

—A gas whose constituents are packed to such high densities that further packing would violate the Pauli Exclusion Principle. The pressure of such a gas exhibits almost no dependence on temperature.

Ideal gas

—Gas that obeys the Ideal Gas Law, where the pressure is proportional to the product of the local temperature and density.

Neutrino

—A nuclear particle resulting from nuclear reactions. Neutrinos interact very weakly with ordinary matter, and can easily traverse a normal star without colliding with any of the stellar material.

Neutron

—Together with protons, neutrons comprise the basic building blocks of the nuclei of the elements. They have a mass just slightly greater than that of a proton, but lack its electric charge.

Neutron star

—A star with a mass similar to the Sun, but composed almost entirely of neutrons. The neutrons are packed so tightly that they are degenerate, like the electrons of a white dwarf—but the resulting density is far greater. The typical size of such a star is about 6.2 mi (10 km).

Nuclear fusion

—The combining of the nuclei of two elements so as to produce a new, more massive element. The process is accompanied by the release of energy as long as the end product of the reaction is less massive than iron.

Planetary nebula

—An expanding shell of gas ejected by a low-mass red giant, which may take on the appearance of one of the outer planets of the solar system when seen in a small telescope.

Radiative core

—The central, or surrounding regions of a star where the energy is carried by envelope radiative diffusion. Radiative diffusion describes the flow of particles of light (photons) through a medium where there is little mechanical change to the medium.

Supernova

—The final collapse stage of a supergiant star.

White dwarf

—A star that has used up all of its thermonuclear energy sources and has collapsed gravitationally to the equilibrium against further collapse that is maintained by a degenerate electron gas.

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