Be stars - an introduction.

Be stars are a major subclass of the B stars. In spectroscopic convention, Be means a star of spectral class B which shows emission lines. B stars are characterised by the presence of absorption lines of neutral helium (HeI in astrophysical notation) in their optical spectrum. This means that the surface temperature of these stars is high enough to allow the presence of the excited levels of neutral helium that originate those lines. When the temperature is so high that some of the helium atoms lose one electron (become ionised), we see absorption lines corresponding to ionised helium (HeII). The stars which show HeII lines are called O stars and they are the hottest of all stars. B stars are also very hot and big. O and B stars are blue in colour.

When the temperature is not high enough to see HeI lines, the spectrum is dominated by neutral hydrogen (HI) lines. These stars are called A stars and they are white-bluish.

Normal stars show only absorption lines and therefore the presence of emission lines needs to be explained. In the case of Be stars, emission is caused by the presence of a circumstellar envelope, a cloud of material around the star. This material is thought to have been ejected from the star, though the mechanisms leading to this are not clear. The envelope is hot, though colder than the star. The very hot luminous star is radiating away vast amounts of energy in the form of light photons. The electrons in the atoms in the envelope absorb the most energetic photons coming from the star, passing to excited levels. Then they de-excite emitting lower energy photons. This is how the emission lines are formed.


A Be star surrounded by its circumstellar envelope. The arrows indicate the rotation of the star and possible directions for mass ejection. The temperature slowly decreases as we move outwards. There is a hot high-density perturbation slowly progressing on the right side. It is not known with certainty how big the discs are, but most people believe that their radii are between five and twenty times those of the stars.

The presence of this envelope is also sustained by another observational facts. Be stars emit more light in the infrared than normal B stars. This infrared excess matches very well theoretical models calculating the emission from such a hot envelope. The radiation originates in two different ways: thermal emission from the hot material forming the envelope and scattering of photons coming out from the star by the free electrons that have jumped from the atoms in the envelope.

There is strong evidence pointing to a disc-like geometry for the circumstellar material, rather than a ring or an spherical shell. However, there is much argument about the size of the discs. B stars have radii between three and twelve times larger than the Sun. Different models propose disc radii ranging from two times the radius of the central star to more than twenty times. Most experts think that the central stars are basically normal B stars and that the envelope is the only responsible for the abnormal behaviour. Some authors believe that all B stars come become Be stars at some moment.

Even though this phenomenon is mostly present in B-type stars, some A and O stars also exhibit this behaviour. In correct spectroscopic notation, they are called Oe and Ae stars, but it is customary to refer to the whole group of hot stars with emission lines coming from circumstellar envelopes as Be stars.