Stellar evolution

The birth of a star is not yet fully understood, but astronomers believe that protostars condense from interstellar clouds of gas and dust in a process of fragmentation. These fragments of cloud contract until the pressure and temperature at their centers rise and slow the rate of collapse. The central regions collapse faster than the outer layers. When the temperature of the contracting core is high enough to start the hydrogen-burning reactions, the contraction halts and the star joins the main-sequence band.

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.
The next stage in a star’s evolution depends on its mass. For a protostar of a mass similar to that of the sun, the collapse of the cloud leads to the formation of a hot, central region. This core contracts to form the nucleus of the future star. The outer regions of the protostar draw closer to the core, and the temperature at the center increases. About 60 million years after the interstellar cloud originally started its collapse, the temperature becomes high enough for nuclear fusion reactions to begin. The reactions keep the star stable for many millions of years, during which time it shines using energy derived from its conversion of hydrogen to helium.
For a star of more than one solar mass, the collapse is such that the initial nucleus expands quickly, and thermonuclear reactions begin much more rapidly. As a result, the nucleus becomes so bright that radiation pres sure prevents much of the outer parts of the protostar from moving inward to further increase its mass, and only about one-third of the protostar’s initial mass burns by hydrogen conversion. The corresponding stages of a star of 10 solar masses may last only 200,000 years.

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.
As the hydrogen fuel is used up in a star’s core, its energy production decreases, and the core slowly starts to collapse. The unburnt hydrogen in the shell outside the helium-filled core is gradually converted to helium, which becomes part of the core, and the resulting radiation halts the overall contraction. The collapse of the core itself continues, however, because it has to reach an even higher temperature to burn its helium and produce more energy-rich elements. This process continues until the helium core makes up about 10 to 15 per cent of the star’s entire mass, when it reaches the Schonberg-Chandrasekhar limit—which determines when the core must start contracting. Under its own weight, and the weight of the outer layers, the core contracts rapidly; the surrounding envelope expands and the star becomes a red giant. During this time, the contracting core becomes hot enough for the helium in it to “burn” and produce carbon, which stops the core from collapsing any further. Over a period of a few tens of millions of years, the star grows into an enormous, very luminous, yet relatively cool, red giant. Our sun will reach this stage about 5 billion years from now. It will expand about 30 to 40 million miles (48 to 64 million kilometers) to reach the planet Mercury. The earth’s temperature will increase so drastically that life will cease to exist there.

After depleting its hydrogen, the contracting core of a star with many solar masses becomes unstable and implodes; the outer layers explode as a supernova. The imploding core may become a neutron star and, eventually, a pulsar or black hole.

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.
If the core of a star has a final mass at this stage of less than about 1.4 solar masses (the Chandrasekhar limit), then the collapse stops at the white dwarf stage. With the passage of time, the remaining heat and light will be radiated away, so that the star eventually becomes a black dwarf. Even if a star begins its life with a much greater mass, it is possible for its final collapse to be stopped at the white dwarf phase if it can shed the extra mass at some stage. Stars of from 4 to 8 solar masses become red supergiants and eventually explode in a supernova explosion.

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.
In 1967, radio-astronomers discovered a radio source that gave a very brief burst of radio energy once every 1.34 seconds. Other such sources, known as pulsars, were soon tracked down, and one, in the Crab Nebula, was found to have a period of 33 thousandths of a second. To have such a rapid rate of spin, this object must be very small. The position of this particular pulsar corresponds with the site of a rare and spectacular phenomenon observed by Chinese astronomers in A.D. 1054— the violent end of a star’s life in a supernova explosion.