Dwarf stars refers to several types of star. The term was originally used in 1906 by the Danish astronomer Ejnar Hertzsprung. He noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups. They are either much brighter than the Sun, or much fainter. To distinguish these groups, he called them "giant" and "dwarf" stars. The dwarf stars were fainter and the giants brighter than the sun.
However, the term "dwarf" was later expanded to include:
- Dwarf star: this usually refers to any main-sequence star, a star of luminosity class V.
- A white dwarf is a star composed of electron-degenerate matter. This may be the final stage in the evolution of stars not massive enough to collapse into a neutron star or undergo a Type II supernova—stars less massive than about 9 solar masses.
- A black dwarf is a white dwarf which has cooled enough that it no longer emits any visible light.
- A brown dwarf is a "substellar object". It is not massive enough to ever fuse hydrogen into helium, but still massive enough to fuse deuterium—less than about 0.08 solar masses and more than about 13 Jupiter masses.