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.