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A-type main-sequence star
An 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 .〔(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〕 They have masses from 1.4 to 2.1 times the mass of the Sun and surface temperatures between 7600 and 11 500 K.〔(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.〕
Bright and nearby examples are Altair (A7 V), Sirius A (A1 V), and Vega (A0 V).〔SIMBAD, entries on (Sirius A ) and (Vega ), accessed June 19, 2007.〕
==Spectral Standard Stars==
The revised Yerkes Atlas system〔(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〕 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 (A0 V), gamma Ursae Majoris (A0 V), and Fomalhaut (A3 V).〔(MK ANCHOR POINTS ), Robert F. Garrison〕〔(Spectral Classification ), W.W. Morgan & P.C. Keenan, 1973, Annual Review of Astronomy and Astrophysics, vol. 11, p.29〕 The seminal review of MK classification by Morgan & Keenan (1973)〔
did not provide any dagger standards between types A3 V and F2 V. HD 23886 was suggested as an A5 V standard in 1978.〔(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〕 Richard Gray & Robert Garrison provided the most recent contributions to the A dwarf spectral sequence in a pair of papers in 1987〔(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〕 and 1989.〔(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〕 They list an assortment of fast- and slow-rotating A-type dwarf spectral standards, including HD 45320 (A1 V), HD 88955 (A2 V), 2 Hya (A7 V), 21 LMi (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 (A4 V) listed as a standard. There are no published A6 V and A8 V standard stars.
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