RIGEL (Beta Orionis). Like its class M reddish rival in Orion, Betelgeuse, Rigel (Beta Orionis) is a supergiant, though a contrasting blue one (actually more blue-white) of class B (B8). Its name comes from the same root as Betelgeuse's, originally "rijl al-jauza," meaning the "foot" of al-jauza, the Arab's "Central One." For us, the star represents the mythical Hunter's left foot, as he looks down upon us. The foot is usually associated with a fainter star, Cursa (Beta Eridani), which (with others) represents his footstool. Though Rigel is Orion's Beta star, this brilliant zeroth-magnitude (0.12) star is somewhat brighter than Alpha Orionis, Betelgeuse, perhaps suggesting that variable Betelgeuse was at or near its brightest when it got its name. (Or that Bayer just used position instead.) Among the most brilliant stars of the sky, Rigel ranks 7th in visual brightness, just behind Auriga's Capella. At a distance of 860 light years (second Hipparcos reduction), it shines with the light of 85,000 Suns after account is taken of ultraviolet light from its 11,500 Kelvin surface. The two combine to tell of a radius swollen to 74 times that of the Sun, 0.34 Astronomical Units, nearly as big as the orbit of Mercury. Direct measure of angular diameter leads to a similar radius of 73 times solar, showing that the star's various properties are accurate. The theory of stellar structure and evolution shows that the star must carry a mass of close 18 times that of the Sun, and indicates that it has a dead helium core and is still in a swelling and cooling phase. Only about 10 million years old, Rigel should eventually expand to become a red supergiant very much like Betelgeuse is today, by which time it will be fusing helium into carbon and beyond in preparation for its eventual explosion as a supernova. (Given the various observational and theoretical uncertainties, it is also possible that the star has a somewhat lower mass of say 14 solar or so and that it is now fusing its helium after already having BEEN a red giant that shrank some and heated at its surface to return to blue supergianthood. In any case, a supernova seems more than likely.) If and when it does go, it would appear to us as bright as a quarter Moon. Rigel is accompanied by a fairly bright, seventh magnitude companion nine seconds of arc away. Normally, such a star is easily found in a small telescope, but Rigel's brilliance nearly overwhelms it. The companion, at a distance of at least 2500 AU (60 times farther from Rigel than Pluto is from the Sun), must take at least 25,000 years to make an orbit, and is itself double (making it Rigel BC), the components much less massive class B main sequence stars that are fusing hydrogen into helium. The stars of Rigel BC are roughly 100 AU from each other, implying an orbital period of about 400 years. Not done, there is 15th magnitude Rigel D, which also seems to belong to the system. At a separation of 44 seconds of arc (at least 11,500 AU), this probable class K dwarf would take some quarter-million years to orbit the inner trio. Once thought to be part of the vast Orion OB1 associationi of blue stars, Rigel now seems to be too close to us for real membership unless, like Betelgeuse, it was ejected from its birth system. Roughly 2.5 degrees to the northwest of the star lies an arc-like blue "reflection nebula" called IC 2118 (the Witch-Head Nebula). Even though at least 40 light years away, it visibly scatters Rigel's fierce light, giving mute testimony to the power of this amazing blue supergiant.
Written by Jim Kaler 2/27/98. Last updated 6/26/09. Return to STARS.