B Stars: Definition

The spectral characteristics that define the B type stars are the considerable strength of neutral helium absorption lines in the presence of hydrogen lines (although not as strong as in the A star spectra. On the main sequence the temperatures of B stars increase from 10,500 K at B to 28,000 K at B0 and their masses and radii increase from 3.2 solar masses and 2.5 solar radii to 17 and 10 times solar respectively. At their hottest, the B stars radiate 20,000 times the luminosity of the Sun. Usually confined to the galactic plane.

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.

B Stars: Description

Radiating at such high rates the B stars use up their nuclear fuel relatively quickly: a B9 star stays on the main sequence about 500 million years but a B0 star remains only five million years. The B stars that we see today must therefore been formed comparatively recently in the life of the Galaxy – they are young stars and rare compared with cooler stars because of their shorter lifetimes. Because of their recent formation, the B stars are associates with regions of gas and dust which signify active areas of star formation in the Galaxy. The hottest P stars (and even the hotter O stars) from loose groupings in the sky known as OB Associations.

The high surface temperatures of B stars results in most of their radiation being emitted in the far ultraviolet part of the spectrum. This energetic radiation is able to ionize gas in the vicinity of a B star, forming an HII region. The B stars and HII regions occur where the dust and gas as a result ac as tracers of the spiral arms in our Galaxy and in distant galaxies. This and their great luminosities, which enable them to be seen at great distances, make B stars valuable aids in the study of galactic structure.

O and B stars evolving from the main sequence become supergiants with radii up to 75 times that of the Sun. With luminosities up to 100,000 times solar, radiation pressure in these supergiants is sufficient to cause mass loss of about 1 millionth a solar mass per year. This is sufficient to reduce the mass of B stars significantly during their lifetime.

B stars are frequently found near (and may even be gravitationally bound) to even higher mass, spectral type "O" stars in or near stellar nurseries as OB associations. Usually found in the galactic plane where there are dusty clouds of gas, B stars are very rare (comprising around only 0.1 percent of dwarf stars). However, B stars have visual luminosities of around 100 to 40,000 times that of Sol for B supergiants like Rigel, and so these stars are abundantly visible in Earth's night sky despite their far distances.

Most B stars rotate rapidly. The fastest have rotational periods of a few hours and are surrounded by disks of gas which extend to several stellar radii out from their equators. Such a disk, excited by radiation from the central star, emits an emission line spectrum which gives the star a Be classification. The disks are larger and cooler than the stars and produce an observable excess of infrared radiation in Be stars relative to that in B stars.

The sub-class of the Be stars are the shell stars, which are Be stars oriented so that we see a circumstellar disk edge-on. The shell stars have spectra that show Balmer emission with sharp absorption cores, narrow absorption lines of ionized metals, and broad HeI absorption. Be stars are common, click here for a short catalog of selected Be and shell stars.

"Be" stars (such as R Coronae Australis) display at least one of the hydrogen Balmer lines. These stars rotate rapidly within a circumstellar shell and/or disk of gas that may have been thrown off by the star or accreted from an evolved companion star. Such stars vary in both brightness and spectra.

Only four stars of spectral type "B" (not including white dwarf stellar remnants) are currently believed to be located within 100 light-years or (or 30.7 parsecs) of Sol. All named, they are Algol, Algorab, Alperatz, and Regulus Aa. These relatively bright, large, massive, and rare stars are visually prominent objects in Earth's night sky.

B class main sequence stars are found, along with O class stars, in star forming regions of galaxies.
They are relatively short lived. The heavier stars in the class have timelines like O class stars - they blast apart as
supernovae at the end of their lives, leaving a neutron star remnant behind. The cores of the lighter stars in the class
collapse to white dwarfs and puff out planetary nebulae in the process.

B Stars: Variability/Peculiarity

Three types of pulsational variability are known among the B stars: the Beta Cephei Stars, the 53 Persei spectrum variables, and the Be variables. All arise from a mixture of radial and non-radial oscillations.

There are several kinds of peculiar spectra among the B stars. The Bp stars are a higher, temperature extension of the peculiar A stars (Ap) into the B8 and B9 spectral types. Among the hotter B stars a few have exceptionally strong helium lines and are generally slow rotators. The Ap, Bp and Helium-strong stars all have magnetic fields and probably all have similar compositions, caused by diffusion in their atmospheres. Other anomalous spectra occur in the Mercury-Manganese (Hg-Mn) Stars, which are slow rotators and constitute a substantial fraction of the B dwarfs with temperatures between 11,000K and 16,000K, nd the Helium-weak stars, Helium stars and OB Sub-dwarfs also have spectral peculiarities.