It is the mass of the Sun compressed into a sphere the size of the earth. Eventually, the white dwarf cools, venting its energy into space, until it no longer shines. This dead star is called a brown dwarf, or an exhausted star. Such a fate eventually awaits our Sun. More massive stars, however, do not have such a kindly death.
Supergiant stars are extremely luminous, and have tremendous surface areas.
Supergiant stars, occasionally, undergo tremendous change. Their cores, with no fuel left to sustain reactions, collapse violently. This collapse triggers a tremendous explosion, called a supernova. Supernovae are so brilliant that they can outshine an entire galaxy . If the star was only moderately more massive than our Sun, then electron pressure in its core is not strong enough to halt the collapse.