White Dwarf

A White Dwarf  is a tiny, but incredibly massive stellar remnant composed mostly of electron-degenerate matter. Along with Neutron Stars and Black Holes, it is one of three possible stellar remnants.

Evolution
White dwarfs are formed from main sequence stars which stop fusing hydrogen at their cores and thus enter their post-main sequence stage. From there, stars of less than 8 M☉ will eventually become white dwarfs in the following ways:

Low Mass Progenitor Star
Very low mass stars (0.08 M☉to 0.5 M☉) will never be hot enough to fuse helium at their cores, and thus decompose into helium-4 white dwarfs. This process takes considerably longer than the current age of the universe.

Low-Medium Mass Progenitor Star
Low to medium mass stars (0.5 M☉ to 8 M☉) will end their fusion processes at carbon and oxygen, and thus, once most of the mass of the star has been expelled into space as a planetary nebula, a carbon-oxygen white dwarf remains.

A white dwarf will remain stable and will eventually cool into a black dwarf. As this process takes longer than the current age of the universe, no black dwarfs have been observed.

Around 9.6% of stars in the Milky Way are white dwarfs

Physical Properties
White Dwarfs have a mass range of 0.08 M☉ to 1.44 M☉. The upper limit, known as the Chandrasekhar limit, is absolutely the maximum mass of a white dwarf. Any more mass added to the dwarf would cause it to explode in a Type Ia supernova.

The radius of a white dwarf can be calculated as:

Note that the radius is expressed not in solar radii, but in earth radii. This puts the lower limit of a white dwarf radius as 0.89 R⊕, and the upper limit as 1.81 R⊕.

White dwarfs have effective temperatures ranging from 5,600 K to 24,000 K.

Habitability
It is incredibly unlikely that a white dwarf could host a life-bearing planet, as no habitable zone that is also free of immense tidal forces could exist. However, colonies, particularly space stations, could quite easily be maintained around a white dwarf.

Worldbuilding in Practice
"One of the many white dwarfs in the Armstrong sector is actually a companion to a massive O class star OKE4-19a. The white dwarf in question, OKE4-19b, orbits its much larger companion rather closely, heated to an unnaturally hot 34,600 K. It is fairly massive, at 1.37 M☉. Evidence suggests it is slowly acreting matter from its companion, suggesting that, within the next million years, it could acrete enough matter to surpass the Chandrasekhar limit and explode in a type Ia supernova."