A star of spectral type B, whose spectrum contains predominantly absorption lines of hydrogen and neutral helium. B-type stars are hot and appear blue in colour, emitting strongly in the ultraviolet. Temperatures for B-type stars on the main sequence are 10 500–30 000 K. Main-sequence luminosities are 100–52 000 times the Sun’s, and masses are 3–18 solar masses. B-type supergiants are stars evolving from the top of the main sequence. As such, they have masses up to 25 solar masses and luminosities as high as 260 000 times the Sun’s. Regulus and Spica are B-type dwarfs, and Rigel is a B-type supergiant. Together with O stars they are the most prominent constituents of OB associations, which delineate the spiral arms of galaxies. Being massive, they have lifetimes of only a few tens of millions of years. See also Be Star; Bp Star; Bw Star.