To us, stars appear permanent and unchanging, but like many other things, they
are born, grow old, and die. Stars are not alive, of course. They are merely
immense balls of gas, but astronomers nevertheless refer to the changes that
stars undergo as stellar evolution. These changes are many, and they are driven
by the physical laws that govern a star's structure. For example, gravity holds
a star together, and the pressure of its gas supports it against gravity's inward
pull. A star generates its supporting pressure from heat energy created by gravity
compressing the material in its core. That heat energy steadily escapes from
the star as the starlight we see, and it must be replenished, or the star will
collapse. A star replenishes its energy by nuclear fusion in its core. In the
early stages of its evolution, a star fuses hydrogen into helium. It eventually
consumes the hydrogen in its core, however, and changes its structure, swelling
into a red giant. Stars like the Sun consume their core's hydrogen in about
10 billion years, but more massive stars use up their hydrogen much faster.
They therefore evolve much faster than the Sun and burn out in only a few million
years. To understand why massive stars evolve faster, recall that according
to the mass-luminosity relation, massive stars are far more luminous than stars
like the Sun. Thus, to supply their greater luminosity, massive stars must burn
their fuel faster than less-massive stars. When a star runs out of fuel, its evolution is nearly finished. A star
like the Sun dies slowly but quietly and becomes a white dwarf. A more massive
star explodes and leaves a neutron star or black hole as a remnant. Such stellar
explosions, though marking the death of one star, may trigger the birth of another.
Moreover, the explosion of a dying star blasts into space elements vital to
human life such as carbon and iron, elements that eventually become incorporated
into new stars, planets, and, ultimately, people. We therefore owe our very
existence to stellar evolution. The goal of this chapter is to illustrate how
such evolution occurs. |