Beta Orionis - Rigel

Rigel, or Beta Orionis, is the brightest star in the constellation Orion, the Hunter. Although it has the Bayer designation β Ori, it is almost always brighter than α Ori (Betelgeuse), which is a variable red giant. Rigel is at the southern boundary of Orion, 15 degrees south of Betelgeuse. These two stars, being near the celestial equator, are visible from most locations on Earth. With visual magnitude 0.18, Rigel is the seventh brightest star in the sky.

History and Mythology

Rigel is located near Orion's left foot; in fact, in Arabic, Rigel means "foot". Another Arabic name is "rigl al-gabbar", meaning "the foot of the great one", which is the source of the variant names Algebar or Elgebar, but these are rarely used. It is known as Shenxiu Qi, "The Seventh of the Three Stars", in Chinese. This mathematically questionable name is due to the fact that in ancient China, the Constellation of Three Stars (Orion) was originally just the three stars in Orion's belt. Later, four more stars were added to the constellation, but its name remained unchanged.

Rigel is one of the most important stars in navigation, since it is bright and equatorial, easily located from all around the world. It is also a popular setting in science fiction; Rigel is the name of at least twelve planets in the Star Trek Universe. And on "The Simpsons", Rigel 7 is the home of Kang and Kodos, whose spoken Rigellian language is, by an astonishing coincidence, identical to English.

Physical Properties

Rigel is a blue-white supergiant of spectral type B8 Iab, with a surface temperature (11,000 K) about double that of our Sun. Its warmer temperature gives it a bluish-white light that contrasts beautifully with Betelgeuse. Rigel's distance is beyond the current range of accurate parallax measurements. Hipparcos' best guess is 775 light years, with a 19% margin of error. Spectroscopic estimates place it between 700 and 900 light years. If at a distance of 775 light years, Rigel actually shines with the luminosity of 40,000 Suns. If the hot star's invisible ultraviolet radiation is considered, its luminosity climbs to 66,000 suns. If Rigel were at the same distance as Sirius (8.6 ly), it would have an apparent magnitude of -10.0: as bright as the quarter Moon. Rigel's radiation pours from a star 17 times the Sun's mass, and 70 times its radius. Rigel is the most luminous star in our local region of the Milky Way; the nearest more powerful star is Deneb, which is in fact several times the luminosity of Rigel, but as much as 3200 light years away, down the Orion Arm.

Being so bright, Rigel lights up several dust clouds in its general vicinity; the most notable is IC 2118, the Witch Head Nebula.

Variability and Companions

Rigel is slightly variable, caused by the physical pulsation of its surface, as is common to supergiants. It is semi-regular, with a period of roughly 22-25 days, and an amplitude of 0.03 to 0.3 magnitudes. Rigel is surrounded by a shell of expelled gas, perhaps shed by its pulsations, stellar wind, or both.

Rigel has a companion, first measured in 1831 by F. G. W. Struve. Rigel B is a bluish star appearing 9" away from Rigel A at magnitude 6.7. Though Rigel B is not particularly faint, it is difficult to separate the pair in small telescopes because of the glare from the brilliant Rigel A, which is over 500 times brighter. At Rigel's estimated distance, Rigel B is separated from its primary by over 2200 AU. Rigel B shows no orbital motion, but is clearly physically related, having the same radial velocity. Rigel B is itself a spectroscopic binary, consisting of two main sequence stars that orbit their mutual center of mass every 9.8 days. The stars are of spectral class B5 V and B9 V, and contain 2.5 and 1.9 solar masses, respectively.

Evolution

Rigel is also associated with the Orion Nebula (M 42). While more or less along the same line of sight as the star, M 42 is almost twice as far away from Earth. Despite the difference in distance, projecting Rigel's motion through space backward for its expected age brings it close to the nebula. As a result, Rigel is sometimes classified as an outlying member of the Orion OB1 Association, along with the stars of Orion's Belt, the illuminating stars in the Orion Nebula, and many of the other hot blue-white stars in the constellation.

With an original mass around 17 times the Sun's, Rigel is in the process of dying. It has most likely finished fusing hydrogen into helium in its core, and is instead fusing helium into carbon and oxygen. Rigel seems fated to explode, though it might contain just little enough mass to become a rare heavy neon-oxygen white dwarf, rather than a supernova. Its less massive, main sequence companions are currently fusing hydrogen into helium, and will evolve into red giants long after whatever fate awaits Rigel A.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]