Gamma Orionis, or Bellatrix, is a blue-white star of magnitude 1.64. It is the third brightest star in the constellation Orion, near the left (northwest) shoulder of the Hunter. Along with Rigel and Betelgeuse, it is one of the most prominent stars in the constellation. The name Bellatrix translates from Latin as "the female warrior", and sometimes as the "Amazon Star."
Properties
Bellatrix is 240 light-years from earth. It is one of the hotter naked eye stars, of spectral class B2, with a temperature of 21,500 K. At 240 light years, it radiates 6400 times the luminosity than the Sun (accounting for ultraviolet radiation). If Bellatrix has not already ceased fusing hydrogen at its core, it will soon; it is now classified as a giant. Such hot giants, however, are nowhere near as large as the orange and red giants. Containing eight or nine solar masses, Bellatrix will become such an orange giant in a few million years, and then die quietly as a massive white dwarf. The star's great luminosity comes not so much from its diameter (six times the Sun's), but from its high temperature. Its small but measurable angular diameter gives the same physical dimensions as calculation from luminosity and temperature, indicating that all our measures are correct.
Bellatrix was once thought to belong to the physical association of stars that comprise much of Orion. But this is no longer believed to be the case, as it is much closer to us than the others. Historically, γ Ori was used as a luminosity standard against which astronomers could compare other stars to check for variability. But it was later discovered that Bellatrix itself is an eruptive variable, ranging in magnitude from 1.59 to 1.64.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]