A white dwarf is a star that has burned out its nuclear fuel and cooled down to the point where it is no longer hot enough to produce energy. White dwarfs are extremely dense, with a mass similar to that of the Sun but compressed into a body about the size of Earth. The first white dwarf to be discovered was in 1774 by Sir William Herschel.
White dwarfs are thought to be the end product of stars like our Sun. When a star like our Sun runs out of nuclear fuel, it expands into a red giant and eventually sheds its outer layers, leaving behind only its core. This core continues to collapse until it becomes very hot and dense, producing energy through gravitational contraction. Eventually, though, even this process comes to an end as the star cools and can no longer produce energy.
The vast majority of known white dwarfs are members of binary star systems in which one star (the primary) is orbited by a smaller companion (the secondary). The secondary component in these systems often transfers material onto the surface of the primary via an accretion disk, causing the primary to increase in mass over time. If the accreting star grows too massive, it may reach a point at which nuclear fusion can occur in its core once again; this event is known as a Type Ia supernova.