Cool Subdwarfs
Like ordinary main-sequence stars, cool subdwarfs (of spectral types G to M) produce their energy from hydrogen fusion. The explanation of their underluminosity lies in their low metallicity: these stars are unenriched in elements heavier than helium. The lower metallicity decreases the opacity of their outer layers and decreases the radiation pressure, resulting in a smaller, hotter star for a given mass. This lower opacity also allows them to emit a higher percentage of ultraviolet light for the same spectral type relative to a Population I star, a feature known as the ultraviolet excess. Usually members of the Milky Way's halo, they frequently have high space velocities relative to the Sun.
Subclasses of cool subdwarfs are as following:
- cool subdwarf: Example: SSSPM J1930-4311 (sdM7)
- extreme subdwarf: Example: APMPM J0559-2903 (esdM7)
Read more about this topic: Subdwarf Star
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