Rigel

Rigel is a blue supergiant star in the equatorial constellation of Orion. It has the Bayer designation β Orionis, which is Latinized to Beta Orionis and abbreviated Beta Ori or β Ori. Rigel is the brightest and most massive component – and the eponym – of a star system of at least four stars that appear as a single blue-white point of light to the naked eye. This system is located at a distance of approximately 850 light-years (260 pc). A star of spectral type B8Ia, Rigel is calculated to be anywhere from 61,500 to 363,000 times as luminous as the Sun, and 18 to 24 times as massive, depending on the method and assumptions used. Its radius is more than seventy times that of the Sun, and its surface temperature is 12,100 K. Due to its stellar wind, Rigel's mass-loss is estimated to be ten million times that of the Sun. With an estimated age of seven to nine million years, Rigel has exhausted its core hydrogen fuel, expanded, and cooled to become a supergiant. It is expected to end its life as a Type II supernova, leaving a neutron star or a black hole as a final remnant, depending on the initial mass of the star. Rigel varies slightly in brightness, its apparent magnitude ranging from 0.05 to 0.18. It is classified as an Alpha Cygni variable due to the amplitude and periodicity of its brightness variation, as well as its spectral type. Its intrinsic variability is caused by pulsations in its unstable atmosphere. Rigel is generally the seventh-brightest star in the night sky and the brightest star in Orion, though it is occasionally outshone by Betelgeuse, which varies over a larger range. A triple-star system is separated from Rigel by an angle of 9.5 arc seconds. It has an apparent magnitude of 6.7, making it 1/400th as bright as Rigel. Two stars in the system can be seen by large telescopes, and the brighter of the two is a spectroscopic binary. These three stars are all blue-white main-sequence stars, each three to four times as massive as the Sun. Rigel and the triple system orbit a common center of gravity with a period estimated to be 24,000 years. The inner stars of the triple system orbit each other every 10 days, and the outer star orbits the inner pair every 63 years. A much fainter star, separated from Rigel and the others by nearly an arc minute, may be part of the same star system.

Nomenclature

In 2016, the International Astronomical Union (IAU) included the name "Rigel" in the IAU Catalog of Star Names. According to the IAU, this proper name applies only to the primary component A of the Rigel system. The system is listed variously in historical astronomical catalogs as H II 33, Σ 668, β 555, or ADS 3823. For simplicity, Rigel's companions are referred to as Rigel B, C, and D; the IAU describes such names as "useful nicknames" that are "unofficial". In modern comprehensive catalogs, the whole multiple star system is known as WDS 05145-0812 or CCDM 05145–0812. The designation of Rigel as β Orionis (Beta Orionis) was made by Johann Bayer in 1603. The "beta" designation is commonly given to the second-brightest star in each constellation, but Rigel is almost always brighter than α Orionis (Betelgeuse). Astronomer James B. Kaler has speculated that Rigel was designated by Bayer during a rare period when it was outshone by the variable star Betelgeuse, resulting in the latter star being designated "alpha" and Rigel designated "beta". Bayer did not strictly order the stars by brightness, instead grouping them by magnitude. Rigel and Betelgeuse were both considered to be of the first magnitude class, and in Orion the stars of each class are thought to have been ordered north to south. Rigel is included in the General Catalogue of Variable Stars, but since it already has a Bayer designation it has no separate variable star designation. Rigel has many other stellar designations taken from various catalogs, including the Flamsteed designation 19 Orionis (19 Ori), the Bright Star Catalogue entry HR 1713, and the Henry Draper Catalogue number HD 34085. These designations frequently appear in the scientific literature, but rarely in popular writing.

Observation

Rigel is an intrinsic variable star with an apparent magnitude ranging from 0.05 to 0.18. It is typically the seventh-brightest star in the celestial sphere, excluding the Sun, although occasionally fainter than Betelgeuse. It is fainter than Capella, which may also vary slightly in brightness. Rigel appears slightly blue-white and has a B-V color index of −0.06. It contrasts strongly with reddish Betelgeuse. Culminating every year at midnight on 12 December, and at 9:00 pm on 24 January, Rigel is visible on winter evenings in the Northern Hemisphere and on summer evenings in the Southern Hemisphere. In the Southern Hemisphere, Rigel is the first bright star of Orion visible as the constellation rises. Correspondingly, it is also the first star of Orion to set in most of the Northern Hemisphere. The star is a vertex of the "Winter Hexagon", an asterism that includes Aldebaran, Capella, Pollux, Procyon, and Sirius. Rigel is a prominent equatorial navigation star, being easily located and readily visible in all the world's oceans (the exception is the area north of the 82nd parallel north).

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