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 .[1][2] They have masses from 1.4 to 2.1 times the mass of the Sun and surface temperatures between 7,600 and 10,000 K.[3] Bright and nearby examples are Altair (A7 V), Sirius A (A1 V), and Vega (A0 V).[4]
The revised Yerkes Atlas system (Johnson & Morgan 1953)[5] listed a dense grid of A-type dwarf spectral standard stars, however 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 (F3 V)[6][7]. The seminal review of MK classification by Morgan & Keenan (1973)[7] unfortunately did not provide any dagger standards between types A3 V and F2 V. The spectral atlas of Morgan, Abt, & Tapscott (1978)[8] contributed a A5 V standard (HD 23886). In the late 1980's, Richard Gray & Robert Garrison provided the most recent contributions to the A dwarf spectral sequence in a pair of papers in 1987[9] and 1989[10]. 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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