Subluminous Stars

The term white dwarf was originally introduced, some 50 years ago, to describe intrinsically faint A-type stars. Spectroscopic studies and theoretical considerations confirm the suggestion that these are degenerate objects. Photographic colorimetry of faint proper motion stars can readily distinguish the blue objects and in this way several hundred white dwarf candidates have been discovered, mainly by Luyten. Accurate photometry of about 1000 of these stars has led to accurate estimates for the space density of 1.5 = 10^-3 pc^-3. However, subluminous late-type stars are more difficult to detect among the multitude of main sequence stars and some additional criteria are needed.