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Spectral Type: A

Abstract

The characteristic properties of type A stars – including spectra, mass, and luminosity – are briefly described, followed by notes about a few well-known examples.

Keywords: Spectral type A

Introduction

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Characteristics

Spectra

Dominated by H; neutral and singly ionised metals fade with increasing temperature; H absorptions much weakened in supergiant stars (conversely, strengthened in white dwarfs) – a function of gas pressure.

Occurrence

 
SAO# HD# Bayer R.A. Dec Type Mag(V) #Components
151881 48915 Alpha CMa 6:45:8.9 -16:42:58 A1Vm -1.46 4
67174 172167 Alpha Lyr 18:36:56.2 +38:47:1 A0Va 0.03 5
125122 187642 Alpha Aql 19:50:46.9 +8:52:6 A7V 0.76 3
191524 216956 Alpha PsA 22:57:39 -29:37:20 A3V 1.16  
49941 197345 Alpha Cyg 20:41:25.8 +45:16:49 A2Iae 1.25 2
  60179 Alpha Gem 7:34:35.9 +31:53:18 A1V 1.58 4
60198 60178 Alpha Gem 7:34:35.9 +31:53:18 A2Vm 1.58 4
250495 80007 Beta Car 9:13:12.1 -69:43:2 A2IV 1.69  
28553 112185 Epsilon UMa 12:54:1.7 +55:57:35 A0pCr 1.77  
40750 40183 Beta Aur 5:59:31.7 +44:56:51 A2IV 1.9 3
95912 47105 Gamma Gem 6:37:42.7 +16:23:57 A0IV 1.92 3
236232 74956 Delta Vel 8:44:42.2 -54:42:30 A1V 1.96 4
Table 1: A-type stars, magnitude 2.0 and greater.

SAO# = SAO catalogue number, HD# = Henry Draper catalogue number, Bayer = Bayer (or Flamsteed) reference, R.A. = right ascension, Dec = declination, Type = spectral type, Lum = luminosity class, Mag(V) = apparent visual magnitude, #Components = number of visual components in multiple systems (after Ochsenbein & Halbwachs 1987, except where otherwise noted).

Mass and Luminosity

"The giants of class A are only a magnitude or so brighter than the dwarfs, and only about 50% larger. The Balmer line widths are therefore not a very good discriminator among classes III, IV, and V, and we must rely on subtle positive luminosity effects exhibited by the spectra of Fe II and Si II" (Kaler 1997, p. 170).

Vega: L/L¤ ~55; M/M¤ ~3; R/R¤ ~3.

Variability/Mass Loss

 

Summary

 
  Cool Red Giants Sun A-Type Star LBV W-R
M dot (M¤ yr-1) 10-8 to 10-5 10-14   10-4 10-5 to 10-4
Lifetime (yr)   1010   105  
Teff (K)   5,800   20,000 30,000
Mbol          
Table 2: A comparison of some of the physical properties of type A with some other high mass stars.

Interpretation

 

Examples

Vega (Alpha Lyr), Sirius (Alpha CMa), Fomalhaut (Alpha PsA), Altair (Alpha Aql), Deneb (Alpha Cyg), Gemma/Alphecca (Alpha CrB), Procyon B (Alpha CMi B).

References

Kaler, James B. 1997: Stars and Their Spectra. Cambridge. (Corrected paperback ed.) 300 pp.

Ochsenbein F.; Halbwachs J.L. 1987: Le Catalogue des Etoiles les Plus Brillantes (Catalogue of the Brightest Stars). Bull. Inform. CDS 32, 83.  NASA Astronomical Data Centre, catalogue 5053A.

Smith, W.B. 1996: FK5 – SAO – HD – Common Name Cross Index.  NASA Astronomical Data Centre, catalogue 4022.
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