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Betelgeuse is located in the constellation of Orion.

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Betelgeuse is a semiregular variable star. Betelgeuse is located approximately 600 light-years away from Earth. Betelgeuse is the second brightest star in the constellation Orion and the ninth brightest star in the night sky. Although Betelgeuse has the Bayer designation alpha, Rigel (Beta Orionis) is usually brighter (Betelgeuse is a variable star and is on occasion brighter than Rigel). The star is a vertex of the Winter Triangle asterism.

The pink arrow at the star on left indicates Betelgeuse in Orion.

Betelgeuse is a red supergiant, relatively luminous, and one of the largest stars known. For comparison, if the star were at the center of our solar system its surface might extend out to between the orbits of Mars and Jupiter, wholly engulfing Mercury, Venus, Earth and Mars. The angular diameter of Betelgeuse was first measured in 1920–1921 by Michelson and Pease using an astronomical interferometer on the Mount Wilson 100 inch telescope.

Astronomers believe Betelgeuse is only a few million years old, but has evolved rapidly because of its high mass. Some astronomers believe it may become a supernova within a timeframe where it could be observable by human civilization.

Betelgeuse etymology.

The name is a corruption of the Arabic yad al-jawza ("hand of the central one"). The Arabs had earlier called Gemini Jauza ("the central one") but later switched this name to Orion instead. European mistransliteration into Latin during the Middle Ages led to the first character y (with two dots underneath) being misread as a b (with only one dot underneath). Thus throughout the Renaissance the star's name was written as Bait al-Jauza ("house of the central one") and thought to mean "armpit of the central one" in Arabic. This led to the modern rendering as Betelgeuse (although a true translation of "armpit" would be, transliterated as Ibt, hence in 1899 Richard Hinckley Allen mistakenly gave the origin as Ibt al Jauzah). In German, the star's name was corrupted even further: it is called Beteigeuze, because the letter l in the romanised name was mistaken for the letter i.

Because of its rich reddish color the star has also been called "the martial one" and in astrology Betelgeuse portends military or civic honors. This bright star has had many other names:

  • Al Dhira ("the Arm") .
  • Al Mankib ("the Shoulder"), .
  • Al Yad al Yamma ("the Right Hand"), .
  • Ardra (Hindi, Bengali and name of Hindu Nakshatra), .
  • Bahu (Sanskrit), .
  • Bašn (Persian) ("the Arm"), .
  • Ied Algeuze ("Orion's Hand"), .
  • Klaria (Coptic) ("an Armlet").
  • Yedelgeuse .

In Chinese, Betelgeuse is known as (Shensùsì, the Fourth Star of the Constellation of Three Stars) because the Constellation of Three Stars was at first a name for only three stars in the girdle of the Orion. Four more stars were later added to this constellation but the earlier name stuck.

Observation of Betelgeuse

Betelgeuse's variability in brightness was first described by Sir John Herschel in 1836 when he published his observations of the star in Outlines of Astronomy, noting the variations increased between 1836-1840, then decreased again. In 1849 he noted a shorter cycle of variability which peaked in 1852. Later observers recorded unusually high maxima with an interval of several years but only small variations between 1957 and 1967. Records of the American Association of Variable Star Observers show maximum brightnesses of magnitude 0.2 in 1933 and 1942, with minimums below magnitude 1.2 in 1927 and 1941.

In 1919 Albert Michelson and Francis Pease mounted a 6-metre (20 ft) interferometer on the front of the 2.5 metre (100-inch) telescope at Mount Wilson Observatory. Helped by John A. Anderson, in December 1920 Pease measured the angular diameter of a Orionis as 0.047 arcseconds. Given the then-current parallax value of 0.018 arcseconds, this resulted in an estimated radius of 3.84 × 108 km (240 million miles). However there was known uncertainty owing to limb darkening and measurement errors. More recent visible-light observations of Betelgeuse have found the diameter to vary between 0.0568 and 0.0592 arcseconds.

In the late 1980s and early 1990s Betelgeuse became a regular target for Aperture Masking Interferometry visible-light and infrared imaging, revealing a number of bright spots on the star's surface, which were thought to result from convection. These were the first optical and infrared images of the disk of a star other than our Sun and generally showed one or more bright patches indicating the location of hotspots in the stellar photosphere. In 1995 the Faint Object Camera on the Hubble Space Telescope was used to capture an ultraviolet image with comparable resolution - this was the first conventional-telescope image (or "direct-image" in NASA terminology) of the disk of another star. The image was made at ultraviolet wavelengths as at ultraviolet wavelengths ground-based instruments cannot produce images with resolution as high as Hubble. Like earlier images, this ultraviolet image also had a bright patch indicating a higher temperature region, in this case on the southwestern portion of the star's surface. Visual observation have shown Betelgeuse's rotation axis has an inclination of about 20° to the direction of Earth and a position (or height) angle of about 55°. Hence, it was hypothesized that the hot spot seen in the ultraviolet image could be one of the star's poles.

Recent ground-based infrared measurements of the disk of Betelgeuse gave a mid-infrared angular diameter of 54.7 ± 0.3 milli-arcseconds in November 1999, slightly smaller than the typical visible-light angular diameter. These measurements ignored any possible contribution from hotspots (which are less-noticeable in the mid-infrared) but factored-in some limb darkening, whereby the intensity of a star's image diminishes near the edge, as the photospheric gas gets thinner. It is difficult to define the precise diameter of Betelgeuse as the photosphere has no "edge" - instead the gas making up the photosphere gets gradually thinner with distance from the star

Observed size of Betelgeuse.

Betelgeuse Hubble Space Telescope.
Betelgeuse imaged in ultraviolet light by the Hubble Space Telescope and subsequently enhanced by NASA. The bright white spot is likely one of its poles. NASA/ESA credit.
Betelgeuse from Dec.
AAVSO V-band light curve of Betelgeuse (Alpha Orionis) from Dec. 1988 - Aug. 2002.

Betelgeuse has several features which are of particular interest to astronomers. Because of the size and proximity of this star it has the third largest angular diameter as viewed from Earth, smaller only than the Sun and R Doradus. Moreover, it is one of only a dozen or so stars telescopes have imaged as a visible disk. The angular diameter of Betelgeuse was one of the first to be measured with an astronomical interferometer and the apparent diameter was found to be variable. The distance to Betelgeuse is not known with precision but if this is assumed to be 640 light years, the star's diameter would be about 950 to 1000 times that of the Sun. Betelgeuse has a color index (B-V) of 1.86 and is thought to have a mass of about 20 solar masses.

The precise diameter is hard to define since optical emissions decrease very gradually with radius from the center of Betelgeuse and the color of these emissions also vary with radius. Though only 20 times more massive than the Sun, this star could be hundreds of millions times greater in volume (as with a beach ball compared to a large stadium). Betelgeuse was the first star on which starspots were resolved in optical images by a telescope, first from ground-based Aperture Masking Interferometry and later from the Hubble Space Telescope, followed by higher-resolution observations by the ground-based COAST telescope.

Betelgeuse's photosphere has an extended atmosphere which displays strong lines of emission (rather than absorption). This chromosphere has a temperature no higher than 5,500 K and may stretch outward to 7 times the diameter of the star. This extended gaseous atmosphere has been observed moving both away from and towards Betelgeuse, apparently depending on radial velocity fluctations in the photosphere

Betelgeuse's fate.

It is likely that Betelgeuse will become a supernova. Considering its size and age of 8.5 million years – old for its size class – it may explode within the next thousand years-if it hasn't already. Since its rotational axis is not toward the Earth, Betelgeuse's supernova would not cause a gamma ray burst in the direction of Earth large enough to damage its ecosystem even from a relatively close proximity of 640 light years.

A Betelgeuse supernova could easily outshine the Moon in the night sky. It will likely be the brightest supernova in recorded Human history, easily outshining SN 1006. After it explodes, it will likely linger for several months, being visible in the daytime sky and lighting up nighttime skies in the Solar System for a long time, after which the "right shoulder" of Orion will disappear forever.

Observation data of Betelgeuse.

Epoch J2000.0 - Equinox J2000.0

  • Betelgeuse Constellation Orion.
  • Right ascension - 05h 55m 10.3053s.
  • Declination - +07° 24' 25.426.
  • Apparent magnitude - (V) 0.58 (0.3 to 1.2).

Characteristics of Betelgeuse.

  • Spectral type - M2Iab.
  • U-B color index - 2.06.
  • B-V color index - 1.85.
  • Variable type - SR c (Semi-regular).

Astrometry of Betelgeuse.

  • Radial velocity (Rv) - +21.0 km/s.
  • Proper motion - RA: 24.95 ± 0.08 mas/yr Dec.: 9.56 ± 0.15 mas/yr.
  • Parallax - 5.07 ± 1.10 mas.
  • Distance approx. - 600 ly (approx. 200 pc).
  • Absolute magnitude (Mv) -5.14.

Details of Betelgeuse.

  • Mass - 20.
  • Radius - 950-1000.
  • Surface gravity (log g) -0.5.
  • Luminosity - 135,000.
  • Temperature - 3,500 K.
  • Rotation - 17 years (14.6 km/s).
  • Age - 1.0 × 107 years.

Other designations of Betelgeuse.

  • Alpha Orionis, 58 Ori, HR 2061, BD+7°1055, HD 39801, SAO 113271, FK5 224, HIP 27989.

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