blue horizontal-branch star
A blue horizontal-branch star (BHB) is a kind of Population II star, found in the galactic halo and especially in globular clusters, that has evolved past the red giant stage and is now burning helium in its core. Blue horizontal-branch stars are typically of spectral type B3 to A0 (hence very blue in color), and have spectra characterized by strong, sharp hydrogen lines, a large Balmer jump (see Balmer series), and very weak lines of other elements.
BHB stars are found on the horizontal branch of the Hertzsprung-Russell diagram to the left, blue side of RR Lyrae variables. They are very good tracers because of their extreme brightness and the fact that most faint blue stars seen in the halo are usually BHB stars.