Spectral Type: B

Abstract

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

Keywords: stars, spectral type, spectral type B

Introduction

In this class we have "an effective combination of number and intrinsic brightness" so that type B stars seem to dominate the naked eye sky, comprising a full third of the hundred brightest (m_v) stars (Kaler 1997, p. 183).

Type B stars are often found in spatial proximity to type O stars; O+B binaries and OB associations are common.

Characteristics

Spectra

Type B stars encompass a very large mass and temperature range. Consequently, although these stars are all hot (with T_eff ranging from ~10,000K at B9 to nearly 30,000K at B0) and blue, with a high proportion of radiation in the ultraviolet, spectral line details vary widely.

B spectra are characterised by the first appearance of neutral He lines at about type B9. HeI strengthens up to about B2, then fades. HeII first appears at about B0. Most metallic lines are absent or weak, except some absorption lines for the higher ionisation states of silicon, oxygen and carbon.

(After Kaler 1997, pp. 188-189.)

Occurrence

Usually confined to the galactic plane. Type B stars are often found in spatial proximity to type O stars; O+B binaries and OB associations are common.

Despite their strong representation among the naked eye stars, in absolute terms B stars are still relatively rare, comprising only 0.1% of dwarf stars.

Mass and Luminosity

Mass ranges roughly from 3 M_¤ to 20 M_¤.

For dwarfs, L ranges from M_v ~1 to ~-5; giants and supergiants range from roughly -5 to -7.

Variability/Mass Loss

Summary

Interpretation

Type O and B stars are short-lived, therefore they are never found too far from their birthplaces in the galactic plane.

Examples

In the type B stars we have "an effective combination of number and intrinsic brightness" so that type B stars seem to dominate the naked eye sky, comprising a full third of the hundred (apparently) brightest stars (Kaler 1997, p. 183). Some of the best known and most distinctive stars in the sky - such as all seven of the brighter Pleiades - belong to this group. Two are to be found in that most definitive of southern constellations, Crux: a¹ Crucis and b Crucis are the 8th and 5th brightest type B stars, respectively.

Some of the brightest and best known type B stars are:

• Rigel (b Orionis) - The brightest star in Orion; a blue-white supergiant; approximately 291 pc (949 light years) distant; has a 7th magnitude companion which is a possible though difficult object for small telescopes on account of glare from the primary. Rigel, together with most of the bright Orion stars except Betelgeuse and Bellatrix (h, k, l, s Orionis, as well as the stars of the belt and sword) are members of the Orion OB1 association - a young grouping of stars something like a very loose open cluster.

• Achenar (a Eridani) - A blue white star approximately 45 pc (148 light years) distant. Achenar is located at the southern end of the constellation Eridanus, the 'River', and its name is derived from an Arabic phrase meaning "the end of the river."

• Hadar (b Centauri) - A blue giant approximately 167 pc (543 light years) distant. As noted above, Hadar is a double, but the components are separated by only ~1.3" which, together with glare from the primary, makes the companion a difficult object for small (less than ~20 cm) telescopes. A deep neutral filter may be useful. A small H II nebula surrounding the system has been observed in photographs.

• Spica (a Virginis) - An eclipsing binary varying by about 0.1 mag. every 4 days; approximately 83 pc (271 light years) distant.

• b Crucis - A variable star of the b Cephei type, fluctuating by less than 0.1 mag. every ~6 hours; approximately 111 pc (362 light years) distant. In the same field (2'.4 in pa 260º) lies the mag. 8.6, crimson-red carbon star, EsB 365, providing a striking contrast.

(After Ridpath & Tirion 1984; Ochsenbein & Halbwachs 1987.)

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.

Ridpath, Ian; Tirion, Wil (1984): Collins Guide to Stars and Planets. Collins. 384 pp.