A white dwarf is a hot, dead, and dense star. This is the last stage in a star’s life before the black dwarf phase. The white light from the dead star comes from the thermal energy it emits.
At the end of a star’s life, when the outer shells have been been ejected, all that is left is the core. A white dwarf is a hot, dense core of a dead star. Due to its heat, it will glow white for millions or billions of years before becoming a black dwarf. A white dwarf is a dead star, which means there are no nuclear fusion reactions occurring. They do emit light, however they are a lot dimmer than the previous stages in a stars life cycle. The luminosity of white dwarfs comes from the thermal energy it emits.
The term "white dwarf" was first used by Dutch-American Scientist Willem Luyten and refers to their size and color. A white dwarf can typically have a mass half the of that of the sun, but be the size of the earth. This large mass, which is squeezed into small space, results in white dwarfs have a very high density--only neutron stars and black holes are denser.
White dwarves occur at the end of a star’s life when the star has a similar mass to our Sun. Stars that are much bigger than our Sun have a much more dramatic end to their lives. After a supernova, stars will either become a neutron star or a black hole (if they are very massive).
(a star with a similar mass to our Sun)