Stars are believed to be born in groups from the collapse of large, cold clouds of interstellar material composed chiefly of hydrogen gas. Whenever the mass of such a cloud exceeds the Jeans mass (named after the British astrophysicist James Jeans), the gravitational force within it is greater than any outward thermal pressure and causes the cloud to collapse.
Protostars
Observations suggest that if the Jeans mass of a cloud is equal to many solar masses, the collapse of the cloud leads to the formation of the same number of stars as there are solar masses, each star having approximately one solar mass. These stars are then part of a star cluster. As the whole cloud collapses, regions within it undergo their own localized contractions in a process called fragmentation. The temperature of these regions starts to rise, because their density is so high that heat cannot escape. Eventually, the temperature rises far enough for outward thermal pressure to halt the collapse of the localized regions, and the fragmentation ends. These now stable, noncollapsing regions of high density and temperature are called protostars.
Main-sequence stars and supergiants
When the hydrogen-burning reactions begin in a newly born star, the star is at the Zero-Age Main Sequence (ZAMS) stage. A star of one solar mass burns its hydrogen for about 10 billion years and for this time remains on the main sequence. According to this analysis, the sun, which is about 4.6 billion years old, is a middle-aged star.
White dwarfs
A star may remain a giant or super-giant for several million years before all nuclear reactions cease. Gravitational collapse then occurs with no outward pressure to stop it, and the final result may be a white dwarf. Such a star is small (about the same size as the earth) but has about 1 million times the density of water. The temperature at the surface is a few tens of thousands of degrees, yet the luminosity is quite low—about one thousandth of our sun.
Neutron stars and pulsars
If the mass of a star’s core is
between about 1.5 and 3 solar masses, the star’s collapse is thought to
continue until very high densities are reached. At such densities,
electrons collide with protons and produce neutrons. Eventually, so many
neutrons are created (relative to the number of protons) that the
nuclei of atoms begin to break up, and virtually nothing but neutrons
remain, forming a neutron star.
Neutron stars have some bizarre
properties. Each has the mass of several suns, yet is only about 12
miles (19 kilometers) across. The outer layer of a neutron star is
solid, although the star that gave birth to it was gaseous.
Read More Stellar evolution
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