Asked by: Tony Hersh, Newbury
The rate at which energy is radiated by a star (including white dwarfs) is proportional to the fourth power of its temperature as well as its total surface area. The more surface area it has the more energy it can dissipate in a given time for a given temperature. Since white dwarfs are very small (similar in size to the Earth), they are very inefficient radiators and consequently cool very slowly.
Furthermore, as a white dwarf cools its surface temperature decreases which further slows the rate of cooling. This means that white dwarfs will stay warm for many billions of years. In fact, they cool so slowly that the Universe isn’t old enough to contain any that have cooled off completely, becoming ‘black dwarfs’. By studying the rate of cooling and distribution of temperatures in white dwarfs astronomers have been able to determine the age of the Milky Way galaxy.