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An A-type main-sequence star (AV) or A dwarf star is a main-sequence (hydrogen burning) star of spectral type A and luminosity class V (five). These stars have spectra defined by strong hydrogen Balmer absorption lines.^[1] They measure between 1.4 and 2.1 solar masses (M_☉), 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 (A7), Sirius A (A1), and Vega (A0). A-type stars do not have convective zones and thus are not expected to harbor magnetic dynamos. As a consequence, because they do not have strong stellar winds, they lack a means to generate X-ray emissions.

Spectral standard stars

Properties of typical A-type main-sequence stars^[2]^[3]^[4]^[5]^[6]
Spectral type	Mass (`M`_☉)	Radius (`R`_☉)	Luminosity (`L`_☉)	Effective temperature (K)	Color index (B − V)
A0V	2.18	2.193	38.02	9,700	0.00
A1V	2.05	2.136	30.90	9,300	0.04
A2V	1.98	2.117	23.99	8,800	0.07
A3V	1.93	1.861	16.98	8,600	0.10
A4V	1.88	1.794	13.49	8,250	0.14
A5V	1.86	1.785	12.30	8,100	0.16
A6V	1.83	1.775	11.22	7,910	0.19
A7V	1.81	1.750	10.00	7,760	0.21
A8V	1.77	1.748	9.12	7,590	0.25
A9V	1.75	1.747	8.32	7,400	0.27

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 remained unchanged over years and can be considered to define the system, are Vega (A0 V), Phecda (A0 V), and Fomalhaut (A3 V).^[8]^[9] The seminal review of MK classification by Morgan & Keenan (1973)^[9] didn't 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 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 (IR) 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 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.^[14] 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.^[14] As of early 2011 about 30 Jupiter class planets have been found around evolved K-giant stars including Pollux, 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 are orbited by one or more Jupiter-sized planets, compared to about 1 in 16 for Sun-like stars. ^[15]

A-type star systems known to feature planets include HD 15082, Beta Pictoris, HR 8799 and HD 95086.^[16]

Examples

Within 40 light years:

Name	Spectral type	Constellation	vis Mag	Mass (M_☉)	Radius (R_☉)	Luminosity (L_☉)	Distance (ly)
Sirius	A0mA1 Va	Canis Major	−1.47	2.063	1.711	25.4	8.60 ± 0.04
Altair	A7 V	Aquila	0.76	1.79	1.63–2.03	10.6	16.73
Vega	A0 Va	Lyra	0.026	2.135	2.362 × 2.818	40.12	25.04
Fomalhaut	A3 V	Piscis Austrinus	1.17	1.91	1.84	16	25.1
Denebola	A3 V	Leo	2.14	1.78	1.73	15	35.8
Delta Capricorni	A5 IV	Capricornus	2.83	2.0	1.91	11	38.6

Delta Capricorni is likely a subgiant or giant star, and Altair is a disputed subgiant. In addition, Sirius is the brightest star in the night sky.

References

^ "Stellar Spectral Types". hyperphysics.phy-astr.gsu.edu. Retrieved June 19, 2007.
^ Pecaut, Mark J.; Mamajek, Eric E. (1 September 2013). "Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-main-sequence Stars". The Astrophysical Journal Supplement Series. 208 (1): 9. arXiv:1307.2657. Bibcode:2013ApJS..208....9P. doi:10.1088/0067-0049/208/1/9. ISSN 0067-0049. S2CID 119308564.
^ Mamajek, Eric (2 March 2021). "A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence". University of Rochester, Department of Physics and Astronomy. Retrieved 5 July 2021.
^ Dale A. Ostlie; Bradley W. Carroll (2007). An Introduction to Modern Stellar Astrophysics. Pearson Addison-Wesley. ISBN 978-0-8053-0348-3.
^ Habets, G. M. H. J.; Heintze, J. R. W. (1981). "Empirical bolometric corrections for the main-sequence". Astronomy and Astrophysics Supplement Series. 46: 193. Bibcode:1981A&AS...46..193H.Tables VII, VIII
^ 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.
^ Johnson, H. L.; Morgan, W. W. (1953). "Fundamental stellar photometry for standards of spectral type on the Revised System of the Yerkes Spectral Atlas". The Astrophysical Journal. 117: 313. Bibcode:1953ApJ...117..313J. doi:10.1086/145697.
^ Robert F. Garrison. "MK ANCHOR POINTS". Archived from the original on 2019-06-25. Retrieved 2022-01-05.
^ ^a ^b Morgan, W. W.; Keenan, P. C. (1973). "Spectral Classification". Annual Review of Astronomy and Astrophysics. 11: 29. Bibcode:1973ARA&A..11...29M. doi:10.1146/annurev.aa.11.090173.000333.
^ Morgan, W. W.; Abt, Helmut A.; Tapscott, J. W. (1978). Revised MK Spectral Atlas for stars earlier than the sun. Bibcode:1978rmsa.book.....M.
^ Gray, R. O.; Garrison, R. F. (1987). "The Early A-Type Stars: Refined MK Classification, Confrontation with Stroemgren Photometry, and the Effects of Rotation". The Astrophysical Journal Supplement Series. 65: 581. Bibcode:1987ApJS...65..581G. doi:10.1086/191237.
^ Gray, R. O.; Garrison, R. F. (1989). "The Late A-Type Stars: Refined MK Classification, Confrontation with Stroemgren Photometry, and the Effects of Rotation". The Astrophysical Journal Supplement Series. 70: 623. Bibcode:1989ApJS...70..623G. doi:10.1086/191349.
^ 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. S2CID 3450920.
^ ^a ^b Johnson, John Asher; Fischer, Debra A.; Marcy, Geoffrey W.; Wright, Jason T.; Driscoll, Peter; Butler, R. Paul; Hekker, Saskia; Reffert, Sabine; Vogt, Steven S. (2007). "Retired a Stars and Their Companions: Exoplanets Orbiting Three Intermediate‐Mass Subgiants". The Astrophysical Journal. 665: 785–793. arXiv:0704.2455. doi:10.1086/519677. S2CID 15076579.
^ Johnson, J. A. (2011). "The Stars that Host Planets". Sky & Telescope (April): 22–27.
^ Smalley, J. B. (2014). "Eclipsing Am binary systems in the SuperWASP survey". Astronomy and Astrophysics (April): 20.