An A-type main-sequence star (A) or A dwarf star is a

main-sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or dwarf stars, and positions of star ...

(

hydrogen burning In astrophysics, stellar nucleosynthesis is the creation of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a ...

)

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

A and

luminosity class In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

(five). These stars have spectra defined by strong hydrogen Balmer absorption lines. They measure between 1.7 and 2.1

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es (), have surface temperatures between 7,600 and 10,000 K, and live for about a quarter of the lifetime of our Sun. Bright and nearby examples are

Altair Altair is the brightest star in the constellation of Aquila (constellation), Aquila and the list of brightest stars, twelfth-brightest star in the night sky. It has the Bayer designation Alpha Aquilae, which is Latinisation of name ...

(A7),

Sirius A Sirius is the brightest star in the night sky. Its name is derived from the Greek word (Latin script: ), meaning 'glowing' or 'scorching'. The star is designated Canis Majoris, Latinized to Alpha Canis Majoris, and abbre ...

(A1), and

Vega Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, and ...

(A0). A-type stars do not have

convective zone A convection zone, convective zone or convective region of a star is a layer which is unstable due to convection. Energy is primarily or partially transported by convection in such a region. In a radiation zone, energy is transported by radiation ...

s and thus are not expected to harbor magnetic dynamos. As a consequence, because they do not have strong

stellar wind A stellar wind is a flow of gas ejected from the stellar atmosphere, upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spheri ...

s, they lack a means to generate X-ray emissions.

Spectral standard stars

The revised Yerkes Atlas system listed a dense grid of A-type dwarf spectral standard stars, but not all of these have survived to this day as standards. The "anchor points" and "dagger standards" of the

MK spectral classification In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

system among the A-type main-sequence dwarf stars, i.e. those standard stars that have remained unchanged over years and can be considered to define the system, are

(A0 V),

Phecda Phecda , also called Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa), is a star in the constellation of Ursa Major. Since 1943, the spectrum of this star has served as one of the stable anchor points ...

(A0 V), and

Fomalhaut Fomalhaut (, ) is the brightest star in the southern constellation of Piscis Austrinus, the Southern Fish, and one of the brightest stars in the night sky. It has the Bayer designation Alpha Piscis Austrini, which is an alternative ...

(A3 V). The seminal review of MK classification by Morgan & Keenan (1973) didn't provide any dagger standards between types A3 V and F2 V.

HD 23886 HD may refer to: Business * H-D or Harley-Davidson, a motorcycle manufacturer * The Home Depot, NYSE stock symbol: HD Chemistry * Hydrogen deuteride, a diatomic compound of hydrogen and deuterium * Mustard gas Codes * Air Do, formerly Ho ...

was suggested as an A5 V standard in 1978. Richard Gray & Robert Garrison provided the most recent contributions to the A dwarf spectral sequence in a pair of papers in 1987 and 1989. They list an assortment of fast- and slow-rotating A-type dwarf spectral standards, including

HD 45320 HD may refer to: Business * H-D or Harley-Davidson, a motorcycle manufacturer * The Home Depot, NYSE stock symbol: HD Chemistry * Hydrogen deuteride, a diatomic compound of hydrogen and deuterium * Mustard gas Codes * Air Do, formerly Ho ...

(A1 V),

HD 88955 HD 88955 is a single, white-hued star in the southern constellation of Vela. It can be viewed with the naked eye, having an apparent visual magnitude of 3.85. The distance to HD 88955 can be determined from its annual parallax shift of ...

(A2 V), 2 Hydri (A7 V), 21 Leonis Minoris (A7 V), and 44 Ceti (A9 V). Besides the MK standards provided in Morgan's papers and the Gray & Garrison papers, one also occasionally sees Zosma (A4 V) listed as a standard. There are no published A6 V and A8 V standard stars.

Planets

A-type stars are young (typically few hundred million years old) and many emit

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

(IR) radiation beyond what would be expected from the star alone. This IR excess is attributable to dust emission from a

debris disk A debris disk (American English), or debris disc ( Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris ...

where planets form. Surveys indicate massive

planets A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the te ...

commonly form around A-type stars although these planets are difficult to detect using the

Doppler spectroscopy Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in ...

method. This is because A-type stars typically rotate very quickly, which makes it difficult to measure the small Doppler shifts induced by orbiting planets since the spectral lines are very broad. However, this type of massive star eventually evolves into a cooler

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...

which rotates more slowly and thus can be measured using the radial velocity method. As of early 2011 about 30 Jupiter class planets have been found around evolved K-giant stars including Pollux,

Gamma Cephei Gamma Cephei (γ Cephei, abbreviated Gamma Cep, γ Cep) is a binary star system approximately 45 light-years away in the northern constellation of Cepheus (constellation), Cepheus. The primary (designated Gamma Cephei A, officially name ...

and

Iota Draconis Iota Draconis (ι Draconis, abbreviated Iota Dra, ι Dra), also named Edasich , is a star in the northern circumpolar constellation of Draco. A visually unremarkable star of apparent magnitude 3.3, in 2002 it was discovered to have a ...

. Doppler surveys around a wide variety of stars indicate about 1 in 6 stars having twice the mass of the Sun are orbited by one or more Jupiter-sized planets, compared to about 1 in 16 for Sun-like stars. A-type star systems known to feature planets include

HD 15082 HD 15082 (also known as WASP-33) is a star located 397 light years away in the northern constellation of Andromeda. The star is a Delta Scuti variable and a planetary transit variable. A hot Jupiter type extrasolar planet, named WASP-33 ...

Beta Pictoris Beta Pictoris (abbreviated β Pictoris or β Pic) is the second brightest star in the constellation Pictor. It is located from the Solar System, and is 1.75 times as massive and 8.7 times as luminous as the Sun. The Beta Pictoris s ...

HR 8799 HR 8799 is a roughly 30 million-year-old main sequence, main-sequence star located away from Earth in the constellation of Pegasus (constellation), Pegasus. It has roughly 1.5 times the Sun's mass and 4.9 times its lumino ...

and HD 95086.

Examples

Within 40 light years: Delta Capricorni is likely a

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

giant star A giant star has a substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsp ...

, and Altair is a disputed subgiant. In addition, Sirius is the brightest star in the night sky.

References

{{DEFAULTSORT:A-type main-sequence star Star types

Spectral standard stars

Planets

Examples

See also

References