Subdwarf B star
Encyclopedia
The subdwarf B star is a kind of subdwarf star
with spectral type B. They differ from the typical subdwarf star by being much hotter and brighter. They are from the "extreme horizontal branch
stars" of the Hertzsprung–Russell diagram
. 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, spiral galaxy
bulge
s and elliptical galaxies
. They are prominent on ultraviolet images. The hot subdwarfs are proposed to be the cause of the UV-upturn in the light output of elliptical galaxies.
The sdB stars are expected to become white dwarfs without going through any more giant stages.
Masses of these stars are around 0.5 solar masses, and they contain only about 1% hydrogen, with the rest being helium. Their radius is from 0.15 to 0.25 solar radii, and their temperature is from 20000 to 40000K.
The hot subdwarf B stars form one group spectroscopically.
and Humason around 1947 when they found subluminous blue stars around the north galactic pole. In the tPalomar-Green survey they were discovered to be the commonest kind of faint blue star with a magnitude over 18. During the 1960s spectroscopy discovered that many of the sdB stars are deficient in helium, with abundances below that predicted by the big bang
theory. In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung–Russell diagram
.
s in this category:
Firstly there are the sdBV with periods from 90 to 600 seconds. They are also called EC14026 or V361 Hya stars. A proposed new nomenclature is sdBVr, with r standing for rapid. Yet another colloquial name for these is Betsy stars. The Charpinet theory of the oscillations of these stars is that the variations in brightness are due to acoustic mode oscillations
with low degree (l) and low order (n). It is driven by ionisation of iron group atoms causing opacity. The velocity curve is 90 degrees out of phase with the brightness curve. The gravity acceleration g has also been measured and is at a minimum when the brightness is a maximum. This is mostly due to the change in velocity rather than a change in gravity due to size differences.
Secondly there are the long period variables with periods from 45 to 180 minutes. A proposed new nomenclature is sdBVs, with s standing for slow. These only have a very small variation of 0.1%. They have also been called PG1716 or V1093 Her or abbreviated as LPsdBV. The long period variables have a narrower range of temperature from 29000K to 35000K
Stars that oscillate with both kinds of periods are called hybrid. with a standard nomenclature of sdBVrs. A prototype is DW Lyn also identified as HS 0702+6043.
*eclipsing binary star
was the first known to have a planet. HW Vir has two known planets.
Subdwarf star
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...
with spectral type B. They differ from the typical subdwarf star by being much hotter and brighter. They are from the "extreme horizontal branch
Horizontal 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" of the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...
. 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, spiral galaxy
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
bulge
Bulge
Bulge may refer to:*Slang metonym for penis, referring to a visible protrusion through clothing*Hernia, sometimes referred to as a bulge or bulge out-Military:*A localized discontinuity in an extended military line...
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...
. They are prominent on ultraviolet images. The hot subdwarfs are proposed to be the cause of the UV-upturn in the light output of elliptical galaxies.
The sdB stars are expected to become white dwarfs without going through any more giant stages.
Masses of these stars are around 0.5 solar masses, and they contain only about 1% hydrogen, with the rest being helium. Their radius is from 0.15 to 0.25 solar radii, and their temperature is from 20000 to 40000K.
The hot subdwarf B stars form one group spectroscopically.
History
Subdwarf B stars were discovered by ZwickyFritz Zwicky
Fritz Zwicky was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and observational astronomy.- Biography :Fritz Zwicky was born in Varna, Bulgaria to a Swiss father....
and Humason around 1947 when they found subluminous blue stars around the north galactic pole. In the tPalomar-Green survey they were discovered to be the commonest kind of faint blue star with a magnitude over 18. During the 1960s spectroscopy discovered that many of the sdB stars are deficient in helium, with abundances below that predicted by the big bang
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...
theory. In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...
.
Variables
There are three kinds of variable starVariable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
s in this category:
Firstly there are the sdBV with periods from 90 to 600 seconds. They are also called EC14026 or V361 Hya stars. A proposed new nomenclature is sdBVr, with r standing for rapid. Yet another colloquial name for these is Betsy stars. The Charpinet theory of the oscillations of these stars is that the variations in brightness are due to acoustic mode oscillations
Spherical harmonics
In mathematics, spherical harmonics are the angular portion of a set of solutions to Laplace's equation. Represented in a system of spherical coordinates, Laplace's spherical harmonics Y_\ell^m are a specific set of spherical harmonics that forms an orthogonal system, first introduced by Pierre...
with low degree (l) and low order (n). It is driven by ionisation of iron group atoms causing opacity. The velocity curve is 90 degrees out of phase with the brightness curve. The gravity acceleration g has also been measured and is at a minimum when the brightness is a maximum. This is mostly due to the change in velocity rather than a change in gravity due to size differences.
Secondly there are the long period variables with periods from 45 to 180 minutes. A proposed new nomenclature is sdBVs, with s standing for slow. These only have a very small variation of 0.1%. They have also been called PG1716 or V1093 Her or abbreviated as LPsdBV. The long period variables have a narrower range of temperature from 29000K to 35000K
Stars that oscillate with both kinds of periods are called hybrid. with a standard nomenclature of sdBVrs. A prototype is DW Lyn also identified as HS 0702+6043.
| variable star Variable star A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth... | |Constellation Constellation In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky.... | Distance (ly) | |
|---|---|---|---|
| V361 Hydrae | EC 14026-2647 | Hydra Hydra (constellation) Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees. It has a long history, having been included among the 48 constellations listed by the 2nd century astronomer Ptolemy. It is commonly represented as a water snake... |
? |
| V1093 Herculis | GSC 03081-00631 | Hercules Hercules (constellation) Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today... |
? |
| HW Virginis HW Virginis HW Virginis, abbreviated HW Vir, is an eclipsing binary system approximately 590 light-years away in the constellation of Virgo. The system comprises an eclipsing B-type subdwarf star and red dwarf star... * |
HIP 62157 | Virgo Virgo (constellation) Virgo is one of the constellations of the zodiac. Its name is Latin for virgin, and its symbol is . Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky... |
590 |
| NY Virginis* | GSC 04966-00491 | Virgo Virgo (constellation) Virgo is one of the constellations of the zodiac. Its name is Latin for virgin, and its symbol is . Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky... |
? |
| V391 Pegasi V391 Pegasi V391 Pegasi, also catalogued as HS 2201+2610, is a blue-white subdwarf star approximately 4,570 light-years away in the constellation of Pegasus. The star is clarified as an "extreme horizontal branch star." It is small with only half the mass and one-over-four-and-one-thirds the diameter of the... |
HS 2201+2610 | Pegasus Pegasus (constellation) Pegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:... |
4570 |
*eclipsing binary star
Companions
Two sdB stars are know to have planets. V391 PegasiV391 Pegasi
V391 Pegasi, also catalogued as HS 2201+2610, is a blue-white subdwarf star approximately 4,570 light-years away in the constellation of Pegasus. The star is clarified as an "extreme horizontal branch star." It is small with only half the mass and one-over-four-and-one-thirds the diameter of the...
was the first known to have a planet. HW Vir has two known planets.