A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system.
[a] The term "subdwarf" was coined by Gerard Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs".
Like ordinary main-sequence stars, cool subdwarfs (of spectral types G to M) produce their energy from hydrogen fusion.
The early universe had a low content of elements heavier than helium and formed stars and brown dwarfs with lower metallicity.
The low metallicity increase the collision induced absorption of hydrogen, causing this suppressed near-infrared spectrum.
This leads to a bluer W1-W2 (WISE) or ch1-ch2 (Spitzer) color, compared to objects with similar temperature, but with solar metallicity.
The closest directly imaged exoplanet, COCONUTS-2b, was first classified as a subdwarf of type T due to its color, while not showing a high tangential velocity.
[12] The first Y-type subdwarf candidate was discovered in 2021, the brown dwarf WISE 1534–1043, which shows a moderate red Spitzer Space Telescope color (ch1-ch2 = 0.925±0.039 mag).
The subdwarf VVV 1256−62B (sdL3) was discovered as a companion to a halo white dwarf, allowing the age to be measured at 8.4 to 13.8 billion years.
B-type subdwarfs, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters and elliptical galaxies.