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Stellar evolution
In astronomy, stellar evolution is the sequence of changes that a star undergoes during its "lifetime", the 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; rather, by observing numerous stars, each at a different point in its life cycle, and running computer models that simulate stellar structure.
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Birth
In the beginning, there is the Giant Molecular Cloud. Most of the empty space inside a galaxy actually contains around 0.1 to 1 particles per cm3; within a Giant Molecular Cloud, typical density is hundreds of particles per cm3 (compare with 100,000 in a good vacuum tube on Earth). Despite this sparsity, each Giant Molecular Cloud 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.
The cloud is stable, its constituent molecules too widely spaced for gravity to draw them closer, until a supernova explodes nearby, sending out a shockwave of successive compression and rarefaction analogous to a soundwave travelling through air, forming knots of matter, cores of greater density. When density exceeds 100,000 atoms / cm3, gravity takes over, and the region begins to collapse into a protostar (each dense core will produce anywhere from 1 protostar to tens of thousands). The atoms gain speed in their fall toward the center, providing the protostar with heat (heat is defined as particle motion), a weak infrared glow, and rotation -- think of an ice skater pulling in her arms as she goes into a spin. (Protostars can be detected in Bok Globules.)
In some protostars, contraction remains the only source of energy; these are brown dwarfs, and they die away slowly, over hundreds of billions of years. If a protostar is massive enough -- the threshold is around one tenth of a solar mass -- it is heated by gravitational contraction up to 15 MK (15 million degrees Kelvin) -- the electrons are stripped from their parent atoms, creating a plasma. Contraction continues, and eventually the speed of the atomic nuclei is great enough to overcome the electrical repulsion keeping them apart and nuclear fusion occurs: hydrogen nuclei fuse to form helium in the proton-proton chain or by the CNO cycle.
In doing so, they give off a tremendous amount of energy, which pours out from the core, setting up an outward pressure in the gas around it that balances the inward pull of gravity, stopping the protostar's contraction. When the energy reaches the outer layers, it continues into space in the form of electromagnetic radiation -- among other things, visible light.
Maturity
New stars come in a variety of sizes and colors. They range from blue to red, from less than half the size of our Sun to over 20 times its size. The brightness and color of a star depend on its surface temperature, which depends on its mass. (T Tauri stars, for example, are in the early stages of life.)
The remainder of the star's existence will be a tug of war between gravity, which wants to crush the star into a point, and the fusion going on inside, which wants to explode the star and send pieces of it hurtling through the universe.
A new star will fall at a specific point on the main sequence of the H-R diagram. It will rest there for a period of millions (for the biggest and hottest stars) to billions (for mid-sized stars like the Sun) to tens or hundreds of billions (for red dwarfs) of years, expending most of the hydrogen in its core. Eventually the supply of hydrogen runs out and the star enters a new phase of its life.
Beginning of the End
After millions to billions of years, depending on its initial mass, a star runs out of hydrogen, its main fuel. Once the core's ready supply of hydrogen is gone, nuclear processes there cease.
Without the outward pressure generated by these reactions to counteract the force of gravity, the outer layers of the star begin to collapse inward, toward the core. The temperature and pressure increase as during formation, but now to even higher levels, until helium fusion begins.
The newly generated heat temporarily counteracts the force of gravity, and the outer layers of the star are now pushed outward; the star becomes as much as 100 times larger than it ever was during its lifetime. It is now a red giant. The star's mass hasn't increased, so its density is much lower (except in the inner core, where the density is higher than during the hydrogen fusion phase).
What happens next depends, once more, on the star's mass.
The End
Geriatric Low Mass Stars: Anybody's Guess
Understanding what happens when a low mass star exhausts its fuel is impeded by no one ever having observed such a star: the universe is around 13.7 billion years old, less time (by several orders of magnitude, in some cases!) than it takes for the fuel to be exhausted. Current theory is based on computer modeling.
They may fuse helium in core hot-spots, an unstable and uneven reaction causing 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 with less than about half a solar mass will never be able to fuse helium, even after the core ceases hydrogen fusion. There simply isn't a stellar envelope massive enough to bear down enough pressure on the core. These are the red dwarf stars, such as Proxima Centauri, which live for hundreds of billions of years. When nuclear reactions eventually ceases in their cores, they will continue to glow weakly in the infrared and microwave part of the spectrum for many billions of years.
The Fate of Sun-Sized Stars: Black Dwarfs
Once a medium-size star (0.4 to 3.4 times the mass of our Sun) has reached the red giant phase, its outer layers continue to expand, the core contracts inward, and core-dwelling helium atoms 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.
The atomic structure of carbon is too strong to be further compressed by the mass of the surrounding material. No more fusion can happen. The core is stabilized and the end is near.
The star now begins to shed its outer layers as a diffuse cloud called a planetary nebula. Eventually, only about 20% of the star's initial mass remains and it spends the rest of its days cooling and shrinking until it is only a few thousand miles in diameter. The star has become a white dwarf.
White dwarfs are stable because the inward pull of gravity is balanced by the degeneracy pressure of the star's electrons. (This should not be confused with the electrical repulsion of electrons, but is a consequence of the Pauli exclusion principle.) With no fuel left to burn, the star radiates its remaining heat into icy space for many millions of years.
In the end, there is just a cold dark mass sometimes called a black dwarf. The universe is not old enough for any black dwarf stars to exist.
But the white dwarf may have another trick up its sleeve. No white dwarf more massive than 1.4 solar masses can exist; electron degeneracy pressure isn't strong enough. Consider what we know about novae: Matter is accreted around and onto a white dwarf until it gets hot enough to fuse, and fuses explosively. If the white dwarf's mass is tipped over the Chandrasekhar limit (1.4 solar masses; named for the physicist who discovered it) then electron degeneracy pressure fails and the star collapses. This causes the white dwarf to be blasted clean apart in a supernova event known as a type-I supernova. These supernovae may be many times more powerful than the death of a massive star (a type-II supernova).
The Fate of Massive Stars: Supernovae! and Then...
Shock wave expands.
Get closer!
Fate has something very different -- and very dramatic -- in store for stars more than 5 times as massive as our Sun. After the outer layers of the star have swollen into a red supergiant (a very big red giant), 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 begins to occur, creating and expending progressively heavier elements, temporarily halting the collapse of the core.
Eventually, several more stops down the periodic table, silicon fuses to iron-56. Until now, the star has been maintained by these energy-liberating fusion reactions, but iron cannot fuse. There is suddenly no energy outflow to counteract the enormous force of gravity, and the star collapses.
What happens next is not clearly understood. [1] But whatever it is can cause a supernova explosion in less than a fraction of a second, [2] one of the most spectacular displays of power in the Universe.
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, off into interstellar space. Then, streaming through space, the material from the supernova may collide with other cosmic debris, perhaps to form new stars, or planets and moons, or to serve as raw materials for a vast variety of living things.
So what, if anything, remains of the core of the original star?
Because we do not have a good understanding of the actual explosion mechanism, it's not entirely clear. But 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, a single atomic nucleus the size of Manhattan. This is a neutron star.
It is still an open question whether or not all supernovae do form neutron stars, however. It is believed that 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 and it becomes a black hole. However, our understanding of stellar collapse is not good enough to tell us 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.