White dwarf

In Astronomy the term white dwarf refers to very dense stellar mass objects, that have exhausted all the elements they had available for nuclear fusion, and then had gravity compress them to roughly the size of Planet Earth.

Very massive stars end their lives as black holes, or pulsars. So, all white dwarfs were less massive than those. On the other hand, since our Universe is believed to be just over 13 billion years old, stars that have turned into white dwarfs have to have been massive enough to use up all their fuel in that period. Nearby Tau Ceti is a star almost as bright as Sol, our sun, with an expected lifetime of 15 billion years of nuclear fusion. So, no stars of that mass will have lived long enough to turn into a white dwarf.

The gravitational collapse that follows the exhaustion of a star's nuclear fuel leave a white dwarf extremely hot. Since they have a relatively small surface area they remain hot for trillions of years.

There are multiple kinds of supernova. A type 2 supernova occurs when a star more than 8 times the mass of Sol exhausts the fuel for nuclear fusion. Type 1 supernova occur when a white dwarf star, a star that wasn't massive enough to go supernova when it ran out of fuel for nuclear fusion, starts to steal material from a nearby companion, when the companion leaves the main sequence, and become a red giant star. A white dwarf will go supernova when its mass gats to 1.44 times the mass of Sol.