Everything about Subdwarf totally explained
A
subdwarf star, sometimes denoted by "sd", is
luminosity class VI under the
Yerkes spectral classification system. They are defined as
stars with
luminosity 1.5 to 2
magnitudes lower than that of main-sequence stars of the same
spectral type. On an
Hertzsprung-Russell diagram subdwarfs appear to lie below the
main sequence.
The term "subdwarf" was coined by
Gerard Peter Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate
white dwarfs."
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. Often being members of the Milky Way's
halo, they frequently have high space velocities relative to the
Sun. They also emit a higher percentage of
ultraviolet light for the same
spectral type relative to a
Population I star; this
ultraviolet excess is a result of their low metallicity, which allows more of their ultraviolet light to escape. Thus, the relatively low
opacity of their outer layers lowers the
radiation pressure, resulting in a smaller, hotter star for a given mass.
Hot subdwarfs
Hot subdwarfs, of spectral types B and O, also termed "extreme
horizontal branch stars" are an entirely different class of objects to cool subdwarfs. These stars represent a late stage in the evolution of some stars, caused when a
red giant star loses its outer
hydrogen layers before the core begins to fuse
helium. The reasons why this premature mass loss occurs are unclear, but the interaction of stars in a
binary star system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two
white dwarf stars. Subdwarf B stars, 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.
Notable subdwarfs
Further Information
Get more info on 'Subdwarf'.
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