alpha orionis

Rigel

[rahy-juhl, -guhl]

Rigel (β Ori / β Orionis / Beta Orionis) is the brightest star in the constellation Orion and the sixth brightest star in the sky, with visual magnitude 0.18. Although it has the Bayer designation "beta", it is almost always brighter than Alpha Orionis (Betelgeuse).

Physical properties

Rigel is well beyond the current range of accurate parallax measurements; spectroscopic estimates place its distance between , while Hipparcos' “best guess” is , with a margin of error of about 19%. Rigel is a B8Iab supergiant at 17 solar masses, it shines with approximately 40,000 times the luminosity of the Sun. Rigel is the most luminous star in our local region of the Milky Way; the nearest more powerful star is Deneb, as much as down the Orion Arm.

As it is so bright and it is moving through a region of nebulosity, Rigel lights up several dust clouds in its general vicinity, the most notable being the IC 2118 (the Witch Head Nebula). Rigel is also associated with the Orion Nebula, which—while more or less along the same line of sight as the star—is almost twice as far away from Earth. Despite the difference in distance, projecting Rigel's path through space 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 many of the other bright stars in that region of the sky; more specifically, it is a member of the Taurus-Orion R1 Association, with the OB1 Association reserved for stars closer to the nebula and more recently formed.

Rigel is slightly variable, in an irregular way common to supergiants, with a range from 0.03 to 0.3 of a magnitude over roughly 22-25 days. A fourth star in the system is sometimes proposed, but it is generally considered that this is a misinterpretation of the main star's variability, which may be caused by physical pulsation of the surface.

Spectroscopy

Studies done on Rigel, looking at the Hα lines, have shown a wide variety of configurations. It varies from large emission to large absorption. Current studies are underway to determine if there is a pattern.

Rigel is surrounded by a shell of expelled gas, perhaps shed by its pulsations, stellar wind, or both; the issue remains unsolved.

System

Rigel has been a known visual binary since at least 1831, when it was first measured by F.G.W. Struve. Though Rigel B is not particularly faint at magnitude 6.7, its closeness to Rigel A — which is over 500 times brighter — makes it a challenging target for telescopes smaller than . At Rigel's estimated distance, Rigel B is separated from its primary by over 2200 AU; not surprisingly, there has been no sign of orbital movement, though they share the same proper motion.

Rigel B is itself a spectroscopic binary system, consisting of two main sequence stars that orbit their center of gravity every 9.8 days. The stars both belong to the spectral class B9V; Rigel B is the more massive of the pair, at 2.5 versus 1.9 solar masses.

There was long-running controversy in the late 19th and early 20th century over the possible visible binarity of Rigel B. A number of experienced observers claimed to see it as a double, while others were unable to confirm it; indeed, the proponents themselves were sometimes unable to duplicate their results. Observations since have ruled out the likelihood of a visible companion to Rigel B.

Etymology and cultural significance

The star's name comes from its location at the "left foot" of Orion. It is a contraction of Riǧl Ǧawza al-Yusra, this being Arabic for "Left Foot of the Central One". Another Arabic name is رجل الجبار riǧl al-ǧabbār, "the foot of the great one" (giant, conqueror, etc.), which is also the source of the variant name Algebar. It also has the alternative traditional names Algebar or Elgebar, but these are rarely used.

It is known as 参宿七 (Shēnxiù Qī, "The Seventh of the Three Stars") in Chinese. This mathematically questionable name is due to the fact that the Asterism of Three Stars was originally composed of just three stars, all of them in the girdle of the Orion. Later, four more stars were added to this asterism, but the name remained unchanged.

In stellar navigation, Rigel is one of the most important navigation stars, since it is bright, easily located and equatorial, which means it is visible all around the world's oceans.