A subdwarf (sd) is a star with less luminosity than is typical for its spectrum. One criteria is that it indicates a star with an absolute magnitude of at least 1.5 higher than normal. The luminosity class VI indicates a subdwarf. An example is Kapteyn's Star. For early stars, the low luminosity is presumed to be due to mass loss, perhaps in some cases from binary-star interaction. For later stars, it occurs when metallicity is low, suggesting that Population II stars were smaller given their mass. Lower opacity decreases outward radiation pressure and they are less puffed up, with a hotter photosphere for their given radius. Some late subdwarfs have other characteristics associated with old stars.
Some B-type subdwarfs with about 0.5 Solar mass and radius on the order of 0.2 solar radii are presumed to be red giants that lost their outer layers by some mechanism, possibly stellar interactions.