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BELLATRIX
(Gamma Orionis).
If constellations could speak, they might well shout "unfair" at great Orion. One of only three constellations to have two first magnitude stars (the others Crux, the Southern Cross, and Centaurus, the Centaur), it also has the fourth and seventh-ranked SECOND magnitude stars. Bellatrix (magnitude 1.64) follows immediately behind Castor in Gemini, Gacrux (Gamma Crucis) in Crux, and Shaula in Scorpius. (Number 7 is Alnilam, Delta Orionis.) The name (oddly, originally applied to Capella) translates from Latin as "the female warrior" (a contrast with Orion as a giant warrior rather than as a hunter), and sometimes as the "Amazon Star." Blue-white luminous Bellatrix certainly lives up to its grand name. It is one of the hotter naked eye stars. With a temperature of 21,500 Kelvin (at the hot end of class B), it is in league with Spica, Adhara, and Shaula. Its measured distance of 240 light years shows it to radiate (accounting for a lot of ultraviolet radiation from the hot surface) 6400 times more light than does the Sun. If Bellatrix has not already ceased hydrogen fusion in its core, it will soon, and is even now classed as a "giant." Such hot giants however, are nowhere near as large as the classical orange and red giants. (Carrying eight or nine solar masses, Bellatrix will become such an orange giant in at most a few million years, and then die quietly as a massive white dwarf; it is not quite large enough to explode). The star's great luminosity comes not so much from its diameter (six times that of the Sun), but from its high temperature. The small but measurable angular diameter gives the same physical dimension as calculation from luminosity and temperature, telling us that all our measures are right on the mark. Bellatrix was long thought to be part of the physical association of stars that makes much of Orion, but modern measures of distance place it considerably closer than the others, and it now seems to be independent of them. More important, Bellatrix was long taken as a standard for the measurement of stellar brightnesses, against which astronomers can follow the meanderings of variable stars (those that vary in brightness). But Bellatrix perversely seems to vary a bit too, by a few percent over an undetermined period. As our ability to measure stellar brightnesses gets better and better, it is becoming more and more difficult to find stars that are entirely stable.