A star forms from interstellar gas drawn together by gravity, which compresses and heats the gas to form a protostar. Further heating causes the core of the protostar to fuse hydrogen into helium. The energy released keeps the core hot and thereby maintains an outward pressure that stops the protostar’s collapse and changes it into a main-sequence star. When the hydrogen is exhausted in a star’s core, the core shrinks and heats, making the star swell into a red giant.

A low-mass star like the Sun continues to burn hydrogen in a shell during its first red-giant stage but eventually compresses the helium in its core enough to ignite the gas in a helium flash. The energy released expands the core, and the outer layers shrink, turning the star into a yellow giant. As the star burns helium, its core again shrinks, and it once more becomes a red giant. It is so luminous now that its radiation drives off its outer layers to form a planetary nebula shell. The core remains as a small, dense star: a white dwarf.

Because of the tremendous heating from gravitational compression of their cores, high-mass stars more easily ignite fuels heavier than hydrogen and undergo less dramatic changes initially than low-mass stars do. They build in their cores layers of heavy elements: carbon, neon, silicon, and eventually iron. The iron core cannot burn and will collapse, triggering a supernova explosion. The heavy elements produced during the star’s life and in the supernova that destroys it are blasted into space, enriching the interstellar medium. The core remains as a ball of neutrons—a neutron star—or may collapse entirely and become a black hole.

The lifetime of a star is dependent on its mass. High-mass stars burn their fuel quickly to supply the energy they need to support their great weight. These stars are blue because they are so hot. Thus blue stars burn out quickly and must be young.

Both high- and low-mass stars pass through a stage late in their lives when they pulsate as yellow giants. Low-mass stars become RR Lyrae variables, whereas high-mass stars become Cepheids. Table 13.2 summarizes these stages.

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