Betelgeuse is the second brightest star in Orion, with a magnitude of 0.45, and is the ninth brightest star in the night sky. Although Betelgeuse has the Bayer designation α Ori, Rigel (β Ori) is slightly brighter. Betelgeuse marks the shoulder of the great Hunter, Orion, and appears distinctly orange in color through a telescope, or even to the naked eye.
History and Mythology
There is some confusion as to the origin of the name, Betelgeuse. According to some sources, it is derived from the Arabic phrase "bayt al jauza", meaning "house of the twins", since Betelgeuse may once have been regarded as part of the constellation Gemini, the Twins. The Arabs originally called Gemini Jauza ("the central one"), but later transferred that name to Orion. In fact, the name "Betelgeuse" seems most likely to be a corruption of the Arabic "yad al-jawza", meaning "hand of the central one." Europeans during the Middle Ages mistransliterated the first character ('y') into a Latin 'b'. Thus throughout the Renaissance, the star's name was written as "Bait al-Jauza", and thought to mean "armpit of the central one". In the venerable 1899 "Star Names: Their Lore and Meaning", Richard Hinckley Allen mistakenly gave the origin of the name as "Ibt al Jauzah", since a true transliteration of "armpit" would be "Ibt". In German, the star's name was corrupted even further: it is called Beteigeuze, because the letter 'l' in the Latinized name was mistaken for an 'i'.
Because of its rich reddish color, Betelgeuse has also been called "the martial one"; it portends military or civic honors in astrology. In Chinese, Betelgeuse is known as Shensusi, the Fourth Star of the Constellation of Three Stars. The Constellation of Three Stars at first referred only to the three stars of Orion's belt; later, four more stars were later added, but its earlier name stuck.
Physical Properties
Betelgeuse is a red supergiant, originally measured by the Hipparcos satellite to be about 425 light years distant. More recent, and contradictory, measurements place its distance from 500 to 640 light years. This makes Betelgeuse one of the largest stars known, with a diameter over 650 times the Sun's. In fact, after the Sun, Betelgeuse has the second-largest angular diameter of any star in the sky. Its apparent size was first measured at 0.047" by Albert Michelson and Francis Pease in 1920 and 1921, using an interferometer on the 100 inch telescope at Mount Wilson Observatory. More recent visible-light observations have found the angular diameter of Betelgeuse to vary between 0.0568 and 0.0592". If Betelgeuse were at the center of our solar system, its surface would extend out to 3.4 AU: more than halfway to Jupiter, and wholly engulfing Mercury, Venus, Earth and Mars.
Betelgeuse has a spectral class of M1-2 Ib, and a surface temperature about 3,100 K. It has an absolute magnitude of about -5.5, and a visual luminosity 9,400 times the Sun's. Betelgeuse radiates mostly at infrared wavelengths, however, so its total luminosity is around 100,000 times the Sun's. Betelgeuse's mass is thought to be around 14 times the Sun's. But because of its large size, it has a mean density thousands of times less than the Earth's atmosphere.
Betelgeuse is the first of only a dozen or so stars that telescopes have imaged as a visible disk. In 1995, Betelgeuse became the target of the Hubble Telescope's Faint Object Camera, which captured the first direct image of any star other than the Sun. The ultra-violet image revealed a bright patch on the star's surface, about 2,000 Kg hotter than the surrounding photosphere. The hot spot is likely one of the star's poles, showing that Betelgeuse's rotation axis has an inclination of about 20° to the direction of the Earth.
Betelgeuse has several "companion" stars lying along the same line of sight, but these are physically unrelated; Betelgeuse appears to be single.
Variability
Like most other red giants and supergiants, Betelgeuse is a pulsating variable, varying in magnitude from about 0.4 to 1.3. Betelgeuse's variability was first described by Sir John Herschel in 1836. According to the American Association of Variable Star Observers, the star had a peak magnitude of 0.2 in 1933 and 1942, and a minimum below magnitude 1.2 in 1927 and 1941 - a range of more than a factor of 2 in total brightness. Betelgeuse is a semi-regular, pulsating variable, whose main period appears to be around 5.7 years, with shorter periods of 150 to 300 days superimposed.
Astronomers now know that these brightness variations are caused by changes in the actual size of the star's atmosphere. Betelgeuse varies from 550 to 920 times the diameter of the Sun, ranging in total luminosity from 40,000 to 100,000 suns. Red supergiants pulsate this way because their atmospheres are not stable. When the star is smallest, its atmosphere absorbs too much of the energy passing through it, so it heats and expands. As it expands, it becomes thinner; energy then passes through it more easily, so it cools and shrinks again. In 2000, the star was found to pulsate asymmetrically, possibly because of hot spots from a few, unusually large convection cells.
The star's variability may also be related to periods of mass ejection, and the formation of dust grains. Betelgeuse is surrounded by shells of dust and gas that it has blown off through a very strong stellar wind. There is a partial ring of dust lying at around three times its radius; more is found at around 650 AU, and around 12,000 AU. At about 36,000 AU, there is abundant gas; and a great, asymmetric shell of dust extends to at least 3.3 light years. The dust shells, extended atmosphere, and pulsations make it difficult to locate an actual "surface" and tell just how large Betelgeuse actually is. But in fact, the massive amounts of dust and gas expelled from red supergiant stars are an important source of chemical enrichment for the interstellar medium.
Evolution
Like many of the stars in Orion, Betelgeuse probably formed a few million years ago from the molecular clouds observed in the constellation. Betelgeuse is a member of the "Orion OB1 Association," where massive young stars with over 10 times the sun's mass can be found in abundance. Betelgeuse may only be around six million years old, but has evolved rapidly because of its large mass. Massive stars use their fuel quickly and do not live very long. Betelgeuse has already run out hydrogen at its core, and is fusing helium into carbon and oxygen. From theory, its initial mass should have fallen somewhere between 12 and 17 times the Sun's. If Betelgeuse only contains 12 to 14 solar masses, it may become one of the exceedingly rare neon-oxygen white dwarfs.
However, it is more likely that Betelgeuse contains 15 to 17 solar masses. If this is so, then eventually the star will fuse higher elements - neon, magnesium, sodium, and silicon - all the way to iron. It will then collapse and explode as a Type II supernova, leaving behind a compact neutron star about the size of a small town. Some astronomers believe that, considering its size and apparent age, Betelgeuse may explode within the next thousand years - or may have done so already! At its relatively close distance of 425 light years, Betelgeuse's supernova would cause a gamma ray burst large enough to damage Earth's ecosystem if its rotational axis were pointed toward the Earth. (Fortunately, it is not.) But a Betelgeuse supernova would be seen easily in full daylight, and outshine the Moon in the night sky.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]