In astronomy, Stellar evolution is the sequence of stages that a star undergoes during its lifetime; the hundreds of thousands, millions or billions of years during which it emits light and heat. Over the course of that time, the star will change radically.
Stellar evolution is not studied by observing the life cycle of a single star—most stellar changes occur too slowly to be detected even over many centuries. Instead, astrophysicists come to understand how stars evolve through observing of numerous stars, each at a different point in its life cycle, and simulating stellar structure with computer models.
Stellar evolution begins with a giant molecular cloud (GMC), also known as a stellar nursery. Most of the 'empty' space inside a galaxy actually contains around 0.1 to 1 particle per cm³, but inside a GMC, the typical density is a few million particles per cm³. A GMC contains 100,000 to 10,000,000 times as much mass as our Sun by virtue of its size: 50 to 300 light years across.
As a GMC orbits the galaxy, one of several events might occur to cause its gravitational collapse. GMCs may collide with each other, or pass through dense regions of spiral arms. A nearby supernova explosion can also be a trigger, sending shocked matter into the GMC at very high speeds. Finally, galactic collisions can trigger massive bursts of star formation as the gas clouds in each galaxy are compressed and agitated by the collision.
A collapsing GMC fragments as it collapses, breaking into smaller and smaller chunks. Fragments with masses of less than about 50 solar masses are able to form into stars. In these fragments, the gas is heated as it collapses due to the release of gravitational potential energy, and the cloud becomes a protostar as it forms into a spherical rotating object.
This initial stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust. Often, these star-forming cocoons can be seen in silhouette against bright emission from surrounding gas, and are then known as Bok globules.
Very small protostars never reach temperatures high enough for nuclear fusion of hydrogen to begin; these are brown dwarfs of less than 0.1 solar mass. Brown dwarfs heavier than 13 Jupiter masses ($ M_J $) do fuse deuterium, and some astronomers prefer to call only these objects brown dwarfs, classifying anything larger than a planet but smaller than this a sub-stellar object. Both types, deuterium-burning or not, shine dimly and die away slowly, cooling gradually over hundreds of millions of years. The central temperature in more massive protostars, however, will eventually reach 10 megakelvins, at which point hydrogen begins to fuse by way of the proton-proton chain reaction to deuterium and then to helium. The onset of nuclear fusion leads over a relatively short time to a hydrostatic equilibrium in which energy released by the core prevents further gravitational collapse. The star thus evolves rapidly to a stable state.
The middle years of a star's life
New stars come in a variety of sizes and colors. They range in spectral type from hot and blue to cool and red, and in mass from less than 0.5 to more than 20 solar masses. The brightness and color of a star depend on its surface temperature, which in turn depends on its mass.
A new star will fall at a specific point on the main sequence of the Hertzsprung-Russell diagram. Small, cool red dwarfs burn hydrogen slowly and may remain on the main sequence for hundreds of billions of years, while massive hot supergiants will leave the main sequence after just a few million years. A mid-sized star like the Sun will remain on the main sequence for about 10 billion years. The Sun is thought to be in the middle of its lifespan; thus, it is on the main sequence. Once a star expends most of the hydrogen in its core, it moves off the main sequence.
After millions to billions of years, depending on its initial mass, the continuous fusion of hydrogen into helium will cause a build-up of helium in the core. Larger and hotter stars produce helium more rapidly than cooler and less massive ones. Once the concentration of helium in the core becomes great enough to interfere with the proton-proton process, nuclear processes there cease. This process is known as helium poisoning.
Without the outward pressure generated by these reactions to counteract the force of gravity, the outer layers of the star begin to collapse inward on the core. The temperature and pressure increase as during formation of the protostar, but now to even higher levels, until helium fusion begins at core temperatures of around 100 million kelvins.
The very hot core causes the outer layers of the star to expand enormously; the star becomes as much as 100 times larger than it was during its main sequence lifetime. It is now a red giant, and the helium burning phase lasts for a few million years. Almost all red giants are variable.
What happens next depends, once more, on the star's mass.
The later years and death of stars
What happens after a low-mass star ceases to produce energy through fusion is not directly known: the universe is thought to be around 13.7 billion years old, which is less time (by several orders of magnitude, in some cases) than it takes for the fusion to cease. Current theory is based on computer modelling.
Some stars may fuse helium in core hot-spots, causing an unstable and uneven reaction as well as a heavy solar wind. In this case, the star will form no planetary nebula but simply evaporate, leaving little more than a brown dwarf.
But a star of less than about 0.5 solar mass will never be able to fuse helium even after the core ceases hydrogen fusion. There simply is not a stellar envelope massive enough to bear down enough pressure on the core. These are the red dwarfs, such as Proxima Centauri, some of which will live thousands of times longer than the Sun. Recent astrophysical models suggest that red dwarfs of 0.1 solar masses may stay on the main sequence for almost six trillion years, and take several hundred billion more to slowly collapse into a white dwarf.(S&T, 22)
Once a medium-size star (between 0.4 and 3.4 solar masses) has reached the red giant phase, its outer layers continue to expand, the core contracts inward, and helium begins to fuse into carbon. The fusion releases energy, granting the star a temporary reprieve. In a Sun-sized star, this process will take approximately one billion years.
Changes in the energy output cause the star to go through instability periods involving changes in size, surface temperature and energy output, and the energy output itself is shifted to lower frequency emission. This is accompanied by increased mass loss through powerful stellar winds and violent pulsations. Stars in this phase of life are called Late type stars, OH-IR stars or Mira-type stars, depending on their exact characteristics. The expelled gas is relatively rich in heavy elements created within the star, and may be particularly oxygen or carbon enriched depending on the type of the star. The gas builds up in an expanding shell called a circumstellar envelope and cools as it moves away from the star, allowing dust particles and molecules to form. With the high infrared energy input from the central star ideal conditions are formed in these circumstellar envelopes for maser excitation.
Helium burning reactions are extremely sensitive to temperature, which causes great instability. Huge pulsations build up, which eventually give the outer layers of the star enough kinetic energy to be ejected as a planetary nebula. At the center of the nebula remains the core of the star, which cools down to become a small but dense white dwarf, typically weighing about 0.6 solar masses, but only the volume of the Earth.
- Main article: White dwarf
White dwarfs are stable because the inward pull of gravity is balanced by the degeneracy pressure of the star's electrons. (This is a consequence of the Pauli exclusion principle, not to be confused with the electrical repulsion of electrons which maintains the volume of normal matter.) With no fuel left to burn, the star radiates its remaining heat into space for thousands of millions of years.
In the end, all that remains is a cold dark mass sometimes called a black dwarf. However, the universe is not old enough for any black dwarf stars to exist.
If the white dwarf's mass increases above the Chandrasekhar limit of 1.4 solar masses, then electron degeneracy pressure fails and the star collapses. Hence, no white dwarf more massive than 1.4 solar masses can exist; electron degeneracy pressure isn't strong enough to support the star. Mass transfer in a binary system may cause such an increase in mass, which causes the white dwarf to be blasted apart in a type Ia supernova. These supernovae may be many times more powerful than the type II supernova marking the death of a massive star.
If a white dwarf forms a close binary system with another star, hydrogen from the larger companion may accrete around and onto a white dwarf until it gets hot enough to fuse in a runaway reaction, although the white dwarf remains below the Chandrasekhar limit. Such an explosion is termed a nova.
After the outer layers of a star greater than five solar masses have swollen into a gigantic red supergiant, the core begins to yield to gravity and starts to shrink. As it shrinks, it grows hotter and denser, and a new series of nuclear reactions begin to occur. These reactions fuse progressively heavier elements, temporarily halting the collapse of the core.
Eventually, as the star progresses through heavier elements on the periodic table, silicon fuses to iron-56. Until now, the star has been maintained by these energy-liberating fusion reactions, but iron cannot release energy through fusion; instead, iron fusion absorbs energy. Once this occurs, there is no further energy outflow to counteract the enormous force of gravity, and the interior of the star collapses nearly instantly.
The accompanying surge of neutrinos starts a shock wave while the continuing jets of neutrinos blast much of the star's accumulated material—the so-called seed elements, lighter than and including iron—into space. As some of the escaping mass is bombarded by the neutrinos, its atoms capture them, creating a spectrum of heavier-than-iron material including the radioactive elements up to uranium. Without supernovae, no elements heavier than iron would exist.
The shock wave and jets of neutrinos continue to propel the material away from the dying star and off into interstellar space. Then, streaming through space, the material from the supernova may collide with other cosmic debris, perhaps to form new stars, planets or moons, or to serve as raw materials for a vast variety of living things.
Modern science does not have a clear understanding of the actual supernova explosion mechanism, nor what exactly remains of the original star. There are, however, two possibilities:
- Main article: Neutron star
It is known that in some supernovae, the intense gravity inside the supergiant forces the electrons into the atomic nuclei, where they combine with the protons to form neutrons. The electromagnetic forces keeping separate nuclei apart are gone (proportionally, if nuclei were the size of dust motes, atoms would be as large as football stadiums), and the entire core of the star becomes nothing but a dense ball of contiguous neutrons or a single atomic nucleus.
These stars, known as neutron stars, are extremely small—on the order of 10km, no bigger than the size of a large city—and are phenomenally dense. Their period of revolution shortens dramatically as the star shrinks (due to conservation of angular momentum); some spin at over 600 revolutions per second. When these rapidly rotating stars' northern or southern magnetic poles are aligned with the Earth, a pulse of radiation is received each revolution. Such neutron stars are called pulsars, and were the first neutron stars to be discovered.
- Main article: Black hole
It is widely believed that not all supernovae form neutron stars. If the stellar mass is high enough, the neutrons themselves will be crushed and the star will collapse until its radius is smaller than the Schwarzschild radius. The star has then become a black hole.
Black holes are predicted by the theory of general relativity. According to classical general relativity, no matter or information can flow from the interior of a black hole to an outside observer, although quantum effects may allow deviations from this strict rule. The existence of black holes in the universe is well supported, both theoretically and by astronomical observation.
However, questions still remain. Current understanding of stellar collapse is not good enough to tell whether it is possible to collapse directly to a black hole without a supernova, if there are supernovae which then form black holes, or what the exact relationship is between the initial mass of the star and the final object that remains.
- "Why the Smallest Stars Stay Small." Sky & Telescope Nov. 1997: 22.