A White Dwarf is a star past its Main-Sequence thermonuclear stage that has expelled its exterior and only the core remains. Electron Degeneracy prevents their further collapse into Neutron Stars. For being the remains of a star, it is classified as a Stellar Remnant.
Most are Oxygen and Carbon but under some conditions can have neon, magnesium, or Helium. White dwarfs are no more than 1.4 solar masses (the Chandrasekhar Limit) and if they grow beyond that, e.g., due to mass transfer from a companion, a type Type Ia Supernova results. White dwarfs begin at the Temperature left over form the main sequence, then cool very slowly. When sufficiently cooled and no longer emitting appreciable Electromagnetic Radiation, the term Black Dwarf is used.
Some white dwarf spectral types (e.g., "DBV"):
Optional letter designating other features:
Black Widow Pulsar (B1957+20)
Calcium-rich Gap Transient
Cataclysmic Variable Star (CV)
H-R Diagram (HRD)
Low Mass Star
Main Sequence Star
Tolman-Oppenheimer-Volkoff Limit (TOV)
Type Ia Supernova
Villanova White Dwarf Catalog