Dwarf Star

The term dwarf star refers to a variety of distinct classes of stars. The term was originally coined in 1906 when the Danish astronomer Ejnar Hertzsprung 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 scope of the term "dwarf" was later expanded to include the following:

• Dwarf star alone generally refers to any main-sequence star, a star of luminosity class V.
  • Red dwarfs are low-mass main-sequence stars.
  • Yellow dwarfs are main-sequence (dwarf) stars with masses comparable to that of the Sun. The Sun is a yellow dwarf.
• A blue dwarf is a hypothesized class of very low-mass stars that increase in temperature as they near the end of their main-sequence lifetime.
• A white dwarf is a star composed of electron-degenerate matter, thought to 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 roughly 9 solar masses.
  • A black dwarf is a white dwarf that has cooled sufficiently such that it no longer emits any visible light.
• A brown dwarf is a sub-stellar object 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.