BETELGEUSE, the brightest star in the constellation Orion (a Orionis). It is yellowish-red, and is thus easily distinguished from the other important stars in the constellation, which are white. Betelgeuse is a multiple star, being an irregular variable, sometimes above and sometimes below the first magnitude. It was the first star whose apparent diameter was measured, in 1920, by Michelson's interferometer method. (See INTERFEROMETER.) It is of spectral type M, its spectrum showing the flutings due to titanium oxide. (See ORION ; SPECTROSCOPY and STAR.)