Overview

During its life, every star supports itself against gravity by burning nuclear fuel in its core. When its fuel is spent, the star collapses. This fate awaits the Sun 5 billion years from now. Gravity will crush it into a white dwarf--a star about 100 times smaller than the present Sun and roughly the size of the Earth. More massive stars surrender sooner and are more dramatically squeezed to even smaller dimensions, becoming either neutron stars or black holes.

Compact stars, as these three kinds of stellar remnants are known, are the end points of stellar evolution. Because nearly all stars must eventually reach this stage, the galaxy is littered with their shriveled bodies. But unlike stars at earlier stages of evolution, no nuclear fuel makes them shine. They shine if at all with heat inherited from their previous state. Their matter, too, is unusual. In crushing compact stars to their tiny dimensions, gravity squeezes them into exotic materials. For example, a piece of white dwarf material the size of an ice cube would weigh about 16 tons. Matter in a neutron star is so compressed that electrons have merged with protons, making the star resemble a giant atomic nucleus. So unmercifully has gravity squeezed the most massive of compact stars that they have collapsed completely, their immense gravity warping the space around them so that no light escapes, making them black holes in space.

Compact, however, does not mean inconspicuous: some of these crushed stars radiate intensely despite having no fuel of their own. Rather, a compact star may "parasitize" a companion star, capturing matter from it that may burn explosively and allow the crushed star to "rise from the dead" as a nova a "new" star. Moreover, because of a compact star's intense gravity, any material that falls on it releases such immense amounts of gravitational energy that this dead star may be thousands of times brighter than a "live" star like the Sun. But this brightness is bought at a dreadful price. The star's mass may increase to the point that gravity causes it to collapse even further or, for some stars, to explode as a type I supernova, blowing the star to atoms.