Rigel - Properties

Properties

Spectroscopic estimates of Rigel's distance place its distance between 700 and 900 light-years (210 and 280 pc), while Hipparcos's measurement of its parallax gives a distance of 860 light-years (260 pc), with a margin of error of about 9%. It is a blue supergiant, at about 18 solar masses, shining with approximately 117,000 times the luminosity of the Sun. The interferometer-measured angular diameter of this star, after correction for limb darkening, is 2.75 ± 0.01 mas. At its estimated distance, this yields a physical size of about 74 times the radius of the Sun. If viewed from a distance of 1 astronomical unit, it would span an angular diameter of 35° and shine at magnitude −38.

As it is both bright and moving through a region of nebulosity, Rigel lights up several dust clouds in its vicinity, most notably 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 a variable supergiant. The reason of this is stellar pulsations. After the Deneb—the prototype of the class of Alpha Cygni pulsating stars—Rigel shows similar complex behavior. The radial velocity variations of Rigel proves that it simultaneously oscillates in at least 19 non-radial modes. The variability periods range broadly from about 1.2 to 74 days. It is a noble star among other blue supergiant stars in a sense that its pulsations are powered by the nuclear reactions in the hydrogen burning shell. The Rigel system is known to be composed of three stars. 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.