satellite data In the
Hertzsprung–Russell diagram, Herbig Ae/Be stars are located to the right of the
main sequence. They are named after the American astronomer
George Herbig, who first distinguished them from other
stars in 1960. The original
Herbig criteria were: •
Spectral type earlier than F0 (in order to exclude
T Tauri stars), •
Balmer emission lines in the stellar spectrum (in order to be similar to
T Tauri stars), • Projected location within the boundaries of a dark
interstellar cloud (in order to select really young stars near their birthplaces), • Illumination of a nearby bright
reflection nebula (in order to guarantee physical link with
star formation region). There are now several known isolated Herbig Ae/Be stars (i.e. not connected with dark clouds or nebulae). Thus the most reliable criteria now can be: •
Spectral type earlier than F0, •
Balmer emission lines in the stellar spectrum, •
Infrared radiation excess (in comparison with normal stars) due to
circumstellar dust (in order to distinguish from classical
Be stars, which have infrared excess due to free-free emission). Sometimes Herbig Ae/Be stars show significant brightness variability. They are believed to be due to clumps (
protoplanets and
planetesimals) in the circumstellar disk. In the lowest brightness stage the radiation from the star becomes bluer and
linearly polarized (when the clump obscures direct star light, scattered from disk light relatively increases – it is the same effect as the blue color of our sky). Analogs of Herbig Ae/Be stars in the smaller mass range (8 )
stars in pre-main-sequence stage are not observed, because they evolve very quickly: when they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are
main-sequence objects. ==Planets==