Subdwarf star
Encyclopedia
A subdwarf star, sometimes denoted by "sd", is luminosity class VI under the Yerkes spectral classification system. They are defined as star
s 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 Kuiper
in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs."
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:
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 cluster
s and elliptical galaxies
.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s with luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
1.5 to 2 magnitudes
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
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
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
.
The term "subdwarf" was coined by Gerard Kuiper
Gerard Kuiper
Gerard Peter Kuiper , Netherlands – December 24, 1973, Mexico City) was a Dutch-American astronomer after whom the Kuiper belt was named.-Early life:...
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 sequenceMain sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
stars, cool subdwarfs (of spectral types G to M) produce their energy from hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
. The explanation of their underluminosity lies in their low metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
: these stars are unenriched in elements heavier than helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
. The lower metallicity decreases the opacity
Opacity (optics)
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
of their outer layers and decreases the radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
, resulting in a smaller, hotter star for a given mass. This lower opacity also allows them to emit a higher percentage of ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
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
Galactic halo
The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...
, they frequently have high space velocities relative to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
Subclasses of cool subdwarfs are as following:
- cool subdwarf: Example: SSSPM J1930-4311 (sdM7)
- extreme subdwarf: Example: APMPM J0559-2903 (esdM7)
Hot subdwarfs
Hot subdwarfs, of spectral types O and B, also termed "extreme horizontal branchHorizontal branch
The horizontal branch is a stage of stellar evolution which immediately follows the red giant branch in stars whose masses are similar to the Sun's...
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
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
star loses its outer hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
layers before the core begins to fuse helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
. The reasons why this premature mass loss occurs are unclear, but the interaction of stars in a binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
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 cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
s and elliptical galaxies
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
.
Notable subdwarfs
- Kapteyn's StarKapteyn's StarKapteyn's Star is a class M1 red dwarf star about 13 light years from Earth in the southern constellation of Pictor. With a magnitude of nearly 9 it is visible through binoculars or a telescope.-History:...
- Groombridge 1830Groombridge 1830Groombridge 1830 is a star in the constellation Ursa Major.-Description:It is a yellowish class G8 subdwarf catalogued by Stephen Groombridge with the Groombridge Transit Circle between 1806 and the 1830s and published posthumously in his star catalog, Catalogue of Circumpolar Stars...
- Mu CassiopeiaeMu CassiopeiaeMu Cassiopeiae is a binary star system in the constellation Cassiopeia. This system shares the name Marfak with Theta Cassiopeiae, and the name was from Al Marfik or Al Mirfaq , meaning "the elbow"....
- 2MASS J05325346+8246465, a possible haloGalactic haloThe term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...
brown dwarfBrown dwarfBrown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
and the first substellar subdwarf. - SSSPM J1549-3544