Betelgeuse

Betelgeuse

A highly evolved, red supergiant, Betelgeuse has an immense but highly variable, outer atmosphere that can swell beyond the width of Neptune's orbit

System Summary

Betelgeuse, or Alpha Orionis, is now estimated to be located around 430 +/- 100 light-years from Sol and it lies in the north-eastern part of the Orion Constellation, the Hunter. To east of Bellatrix (Gamma Orionis); northeast of the belt stars of Alnitak (Zeta Orionis), Alnilam (Epsilon Orionis), and Mintaka (Delta Orionis), and Rigel (Beta Orionis); and southeast of the Crab Nebula and Pulsar.

In the venerable Star Names: Their Lore and Meaning, Richard Hinckley Allen noted that its proper name, Betelgeuse, is derived from the Arabic for "Ibt al Jauzah" (the Armpit of the Central One). Betelgeuse may be the brightest M-type star in Earth's night sky. Although the star is the closest so-called red supergiant to Sol, its colour appears more orange than red to Human eyes.

Betelgeuse is the bright, orange-coloured star in Constellation Orion, at top left.

The Star

Betelgeuse is a red supergiant star of spectral and luminosity type M1-2 Ib. The star may have between 12 and 17 times Sol's mass (ESA), over 650 times Sol's diameter, and around 40,000 to 100,000 times its bolometric luminosity. The European Space Agency's astrometry satellite HIPPARCOS has measured the star's distance from Earth to be around 430 light years, giving it an absolute visual magnitude of -5.1 (based on a HIPPARCOS Vmag= 0.45) and a visual luminosity of 9,400 times Sol's. Its 3,100-degree-Kelvin surface, however, radiates mostly infrared wavelengths, so that only 13 percent of its radiant energy is emitted in the form of visible light.

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 brightness's, 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 brightness's gets better and better, it is becoming more and more difficult to find stars that are entirely stable.