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An A-type main-sequence star
A-type main-sequence star
(A V) or A dwarf star is a main-sequence (hydrogen-burning) star of spectral type A and luminosity class V. These stars have spectra which are defined by strong hydrogen Balmer absorption lines.[1][2] They have masses from 1.4 to 2.1 times the mass of the Sun
Sun
and surface temperatures between 7600 and 11 500  K.[3] Bright and nearby examples are Altair
Altair
(A7 V), Sirius A
Sirius A
(A1 V), and Vega
Vega
(A0 V).[4] A-type stars are fully radiative and thus are not expected to harbor a magnetic dynamo. As a consequence, because they do not have strong stellar winds they lack a means to generate X-ray emission.[5]

Contents

1 Spectral Standard Stars 2 Planets 3 See also 4 References

Spectral Standard Stars[edit]

Typical characteristics[6]

Stellar Class Mass (M☉) Radius (R☉) Mv Teff (K)

A0V 2.40 1.87 0.7 11 500

A2V 2.19 1.78 1.3 10 530

A5V 1.86 1.69 2.0 8200

A6V 1.80 1.66 2.1 7672

A7V 1.74 1.63 2.3 7483

A8V 1.66 1.60 2.4 7305

A9V 1.62 1.55 2.5 7112

The revised Yerkes Atlas system[7] 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 system among the A-type main-sequence dwarf stars, i.e. those standard stars that have remain unchanged over years and can be considered to define the system, are Vega
Vega
(A0 V), Gamma Ursae Majoris (A0 V), and Fomalhaut
Fomalhaut
(A3 V).[8][9] The seminal review of MK classification by Morgan & Keenan (1973)[9] did not provide any dagger standards between types A3 V and F2 V. HD 23886 was suggested as an A5 V standard in 1978.[10] Richard Gray & Robert Garrison provided the most recent contributions to the A dwarf spectral sequence in a pair of papers in 1987[11] and 1989.[12] They list an assortment of fast- and slow-rotating A-type dwarf spectral standards, including HD 45320 (A1 V), HD 88955 (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 Delta Leonis
Delta Leonis
(A4 V) listed as a standard. There are no published A6 V and A8 V standard stars.

The Morgan-Keenan spectral classification

Planets[edit] A-type stars are young (typically few hundred million years) and many emit infrared radiation beyond what would be expected from the star alone. This IR excess is attributable to dust emission from a debris disk where planets form.[13] Surveys indicate massive planets commonly form around A-type stars although these planets are difficult to detect using the Doppler spectroscopy
Doppler spectroscopy
method. This is because A-type stars typically rotate very quickly, which makes it very 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 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
and Iota Draconis. Doppler surveys around a wide variety of stars indicate about 1 in 6 stars having twice the mass of the Sun
Sun
are orbited by one or more Jupiter-sized planets, vs. 1 in 16 for Sun-like stars. [14] A-type stars known to have planets include Fomalhaut, HD 15082, Beta Pictoris and HD 95086 b. See also[edit]

Stellar classification Star
Star
count, survey of stars

References[edit]

^ Stellar Spectral Types, entry at hyperphysics.phy-astr.gsu.edu, accessed on line June 19, 2007. ^ "An Introduction to Modern Astrophysics" by B.W Caroll and D.A Ostlie 1996 edition, chapter 8 ^ Empirical bolometric corrections for the main-sequence, G. M. H. J. Habets and J. R. W. Heintze, Astronomy and Astrophysics Supplement 46 (November 1981), pp. 193–237, Tables VII and VIII. ^ SIMBAD, entries on Sirius A
Sirius A
and Vega, accessed June 19, 2007. ^ Schröder, C.; Schmitt, J. H. M. M. (November 2007), "X-ray emission from A-type stars", Astronomy and Astrophysics, 475 (2): 677–684, Bibcode:2007A&A...475..677S, doi:10.1051/0004-6361:20077429.  ^ Adelman, S. J. (2005). "The physical properties of normal a stars". Proceedings of the International Astronomical Union. 2004. doi:10.1017/S1743921304004314.  ^ Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas H.L. Johnson & W.W. Morgan, 1953, Astrophysical Journal, 117, 313 ^ MK ANCHOR POINTS, Robert F. Garrison ^ a b Spectral Classification, W.W. Morgan & P.C. Keenan, 1973, Annual Review of Astronomy and Astrophysics, vol. 11, p.29 ^ Revised MK Spectral Atlas for stars earlier than the sun, W.W. Morgan, W. W., H.A. Abt, J.W. Tapscott, 1978, Williams Bay: Yerkes Observatory, and Tucson: Kitt Peak National Observatory ^ The early A type stars – Refined MK classification, confrontation with Stroemgren photometry, and the effects of rotation, R.O. Gray & R.F. Garrison, 1987, Astrophysical Journal Supplement Series, vol. 65, p. 581 ^ The late A-type stars – Refined MK classification, confrontation with Stromgren photometry, and the effects of rotation, R.O. Gray & R.F. Garrison, 1989, Astrophysical Journal Supplement Series, vol. 70, p. 623 ^ Song, Inseok; et al. (2002), "M-Type Vega-like Stars", The Astronomical Journal, 124 (1): 514–518, arXiv:astro-ph/0204255 , Bibcode:2002AJ....124..514S, doi:10.1086/341164  ^ Johnson, J. A. (2011). "The Stars that Host Planets". Sky & Telescope (April): 22–27. 

v t e

Star

Formation

Accretion Molecular cloud Bok globule Young stellar object Protostar Pre-main-sequence star

Herbig Ae/Be Orion

T Tauri FU Orionis

Herbig–Haro object Hayashi track Henyey track

Evolution

Main sequence Red giant
Red giant
branch Horizontal branch

Red clump

Asymptotic giant branch Protoplanetary nebula Planetary nebula PG1159 star Dredge-up Instability strip Luminous blue variable Blue straggler Stellar population Supernova Supernova
Supernova
impostor Hypernova Hertzsprung–Russell diagram Color–color diagram

Luminosity class

Subdwarf Dwarf

Blue Red White Yellow Brown

Subgiant Giant

Blue Red Yellow

Bright giant Supergiant

Blue Red Yellow

Hypergiant

Yellow

Spectral classification

O B A F G K M WR Be OB Subdwarf
Subdwarf
O Subdwarf
Subdwarf
B Late-type Chemically peculiar

Am Ap/Bp Barium Carbon CH CN Extreme helium Lambda Boötis Lead HgMn S Technetium

Shell

Remnants

White dwarf

Helium planet

Neutron star

Radio-quiet Pulsar

Binary X-ray

Magnetar

Stellar black hole X-ray binary

Burster

Theoretical stars

Black dwarf Exotic

Quark Strange Preon Planck Electroweak star

Dark-matter star Dark-energy star Black star Gravastar Frozen star Q star Quasi-star Thorne–Żytkow object Iron star Blitzar

Nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova

Symbiotic Remnant Luminous red nova

Structure

Core Convection zone

Microturbulence Oscillations

Radiation zone Atmosphere

Photosphere Starspot Chromosphere Corona

Stellar wind

Bubble Bipolar outflow

Accretion disk Asteroseismology

Helioseismology

Eddington luminosity Kelvin–Helmholtz mechanism

Properties

Designation Dynamics Effective temperature Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation UBV color Variable star

Mira variable

Star
Star
systems

Binary

Contact Common envelope Eclipsing Symbiotic

Multiple Star
Star
cluster

Open cluster Globular cluster Super star cluster

Planetary system Earth's Solar System

Earth-centric observation of

Pole star Circumpolar star Constellation Asterism Magnitude

Apparent Extinction Photographic

Radial velocity Proper motion Parallax Photometric-standard star

Lists

Star
Star
names

Arabic Chinese

Extremes Most massive Highest temperature Largest volume Smallest volume Brightest

Historical

Most luminous Nearest

Nearest bright

Stars with exoplanets Brown dwarfs White dwarfs Milky Way novae Notable supernovae

Supernova
Supernova
candidates

Supernova
Supernova
remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object

Brown dwarf Sub-brown dwarf Planet

Galactic year Galaxy Supercluster Guest star Gravity Icarus (most distant individual star) Intergalactic star Infrared
Infrared
dark cloud Starfield

Category:Stars · Star
Star
portal

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