stellar evolution,

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 dwarfbrown dwarf,
in astronomy, celestial body that is larger than a planet but does not have sufficient mass to convert hydrogen into helium via nuclear fusion as stars do. Also called "failed stars," brown dwarfs form in the same way as true stars (by the contraction of a swirling
..... Click the link for more information. , or "failed star." In these thermonuclear reactions, loosely called "hydrogen burning," four hydrogen nuclei are fused to form a helium nucleus (see nucleosynthesisnucleosynthesis
or nucleogenesis,
in astronomy, production of all the chemical elements from the simplest element, hydrogen, by thermonuclear reactions within stars, supernovas, and in the big bang at the beginning of the universe (see nucleus; nuclear energy).
..... Click the link for more information. ). 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 diagramHertzsprung-Russell diagram
[for Ejnar Hertzsprung and H. N. Russell], graph showing the luminosity of a star as a function of its surface temperature. The luminosity, or absolute magnitude, increases upwards on the vertical axis; the temperature (or some temperature-dependent
..... Click the link for more information. , 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 giantred giant,
star that is relatively cool but very luminous because of its great size. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution.
..... Click the link for more information. . 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 supernovasupernova,
a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
..... Click the link for more information.  star; at the very least, it will become a pulsating variable starvariable star,
star that varies, either periodically or irregularly, in the intensity of the light it emits. Other physical changes are usually correlated with the fluctuations in brightness, such as pulsations in size, ejection of matter, and changes in spectral type, color, or
..... Click the link for more information. , possibly a Cepheid variableCepheid variables
, class of variable stars that brighten and dim in an extremely regular fashion. The periods of the fluctuations (the time to complete one cycle from bright to dim and back to bright) last several days, although they range from 1 to 50 days.
..... Click the link for more information. .

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 dwarfwhite dwarf,
in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be easily
..... Click the link for more information. , 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 isotopesisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
..... Click the link for more information.  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 starneutron star,
extremely small, extremely dense star, with as much as double the sun's mass but only a few miles in radius, in the final stage of stellar evolution. Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933.
..... Click the link for more information. , of which pulsarspulsar,
in astronomy, a neutron star that emits brief, sharp pulses of energy instead of the steady radiation associated with other natural sources. The study of pulsars began when Antony Hewish and his students at Cambridge built a primitive radio telescope to study a
..... Click the link for more information.  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 holeblack hole,
in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping.
..... Click the link for more information.  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 supernovasupernova,
a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
..... Click the link for more information.  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 cosmologycosmology,
area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe. Modern Cosmological Theories
..... Click the link for more information. ; star clustersstar cluster,
a group of stars near each other in space and resembling each other in certain characteristics that suggest a common origin for the group. Stars in the same cluster move at the same rate and in the same direction.
..... Click the link for more information. .

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).

stellar evolution

Further contraction of the core increases the temperature to 10⁸ kelvin, at which stage the star can convert its helium core into carbon through the triple alpha process. The sudden onset of helium burning – the helium flash – may disturb the equilibrium of a low-mass star. The triple alpha reaction gives out less energy than hydrogen fusion and soon the star finds itself once again without a nuclear energy source. Further contraction may result in helium-shell burning but it is doubtful if low-mass stars have sufficient gravitational energy to go further than this stage. The star becomes a red giant again, pulsating and varying in its brightness because of the vast extent of its atmosphere (see pulsating variables). Eventually the atmosphere gently drifts away from the compact core of the star with velocities of only a few km s^–1, resulting in the formation of a planetary nebula. The collapsed core forms a white dwarf star, which continues to radiate its heat away into space for several millions of years.

Stars more than twice as massive as the Sun convert hydrogen to helium through the carbon cycle, which makes their cores fully convective and therefore less dense than solar-mass stars. Their main-sequence lifetime decreases with increasing mass; it is only about 40 million years for a star of 8 solar masses and a few million years for the most massive stars. When they exhaust their hydrogen, the onset of helium burning occurs only gradually: this is because the core is nondegenerate, and so no instabilities arise. Having consumed its helium, a massive star has the potential energy to contract further so that carbon – formed by the triple alpha process – can burn to oxygen, neon, and magnesium (see nucleosynthesis). Should the star be sufficiently massive, it will build elements up to iron in its interior. But iron is at the limit of nuclear-fusion reactions and further contraction of a massive star's core can only result in catastrophic collapse, leading to a supernova explosion. The core collapses to become a neutron star or black hole.

stellar evolution