Be stars 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. 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. 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. 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.