Rigel

The famous blue star Rigel forms the western foot of the Hunter Orion, and indeed its Arabic name means 'leg of the giant'.

The pattern of Rigel's variability suggests that it is turbulent and energetic, with the star's internal processes sending complex patterns of waves and ripples across its surface regions.

The galactic position and direction of Rigel relative to Earth's Sun. Note that, at this extreme scale, the two stars are effectively in the same place.

The blue star that forms the western foot of Orion the Hunter is among the brightest in the entire sky. It appears in the far south of Orion, southward of Orion's Belt, and close to Orion's border with neighbouring Eridanus. Its name means 'leg' or 'foot', deriving from an old Arabic name Rijl Jauzah al Yusrā, the 'left leg of Orion' or 'leg of the giant'.

A highly luminous blue supergiant star, Rigel shines brightly in the sky despite lying more than 860 light years from the Sun. Its exact luminosity is unknown, but it likely generates at least a hundred thousand times as much light as the Sun, and possibly rather more than this. This makes Rigel almost certainly the most luminous star within a thousand light years of the Solar System. Its powerful light reflects from nearby clouds of dust and gas to form nebulae, notably the tattered arc of the Witch Head Nebula, IC 2118, to the west of the star.

As seen from Earth, Rigel is usually the brightest star in Orion (and the seventh brightest in the entire sky), but the fact that its brightness is variable means that this status can change at times. Betelgeuse, the next brightest star in Orion, is also variable, and this combination means that at times Rigel and Betelgeuse can change places in magnitude order. This phenomenon perhaps accounts for the fact that Rigel is designated Beta Orionis (and Betelgeuse Alpha Orionis) whereas the more usual comparative brightness of the stars would rank them the other way round.

The pulsating blue supergiant Rigel has a diameter of about a hundred million kilometres, some seventy times that of the Sun. In the far distance a multiple blue star system is visible - Rigel's much less luminous companions.

Rigel belongs to the Alpha Cygni class of variables, a type of pulsating variable in which the star's luminosity pulsates in unpredictable and irregular patterns. The effect accounts for the changes in Rigel's visual magnitude as seen from Earth. The star shows significant activity, expelling material from its surface at times; it is thought that Rigel produces huge coronal loops, with matter arcing away from the star before falling back to its surface.

Rigel is some seventeen times more massive than the Sun, and more than seventy times the diameter (the process of expansion into a supergiant stage means that the star is much less dense than a main sequence star like the Sun). This is a star in the later stages of its existence, and it is likely that hydrogen fusion has ceased altogether within Rigel's core, which is instead fusing helium atoms to form carbon and oxygen. The star's most likely fate will be to erupt into a supernova explosion, and if it does so, it will briefly become one of the brightest objects in the skies of Earth, at an estimated magnitude of -11 (several hundred times brighter than Venus).

While the massive star dominates its system, it is not alone, and Beta Orionis is in fact a multiple system. Orbiting the supergiant at an estimated distance of 2,200 AU is a group of three blue main sequence stars. The inner pair of this system (Beta Orionis B) forms a close binary in which the companions circle each other in a period of ten days. Meanwhile the third star, Beta Orionis C, orbits this binary pair more distantly, with a period of sixty-three years. This entire triple system follows a long slow path around the supergiant, completing each orbit every 24,000 years.