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Betelgeuse

Betelgeuse is the upper left star in the rectangle of bright stars in Orion.
Observation data Epoch J2000.0
Constellation (pronunciation)	Orion
Right ascension	05h 55m 10.3053s[1]
Declination	+07° 24′ 25.426″[1]
Apparent magnitude (V)	0.58[1] (0.3 to 1.2)
Characteristics
Spectral type	M2Iab[1]
U-B color index	2.06[1]
B-V color index	1.85[1]
Variable type	SR c[1] (Semi-regular)
Astrometry
Radial velocity (Rv)	+21.0[1] km/s
Proper motion (μ)	RA: 27.33[1] mas/yr Dec.: 10.86[1] mas/yr
Parallax (π)	7.63 ± 1.64[1] mas
Distance	approx. 430 ly (approx. 130 pc)
Absolute magnitude (MV)	−5.14
Details
Mass	14 M☉
Radius	630 R☉
Surface gravity (log g)	-0.5[3]
Luminosity	63,000 (40,000–100,000) L☉
Temperature	3,500[3] K
Rotation	17 years (14.6 km/s)[2]
Age	1.0 × 107 years
Other designations
Alpha Orionis, 58 Ori, HR 2061, BD+7°1055, HD 39801, SAO 113271, FK5 224, HIP 27989.[1]

Betelgeuse (pronounced /ˈbiːtəldʒuːz/) (Alpha (α) Orionis) is a semiregular variable star located 427 light-years away.^[1] It is the second brightest star in the constellation Orion, and the ninth brightest star in the night sky. Although it has the Bayer designation "alpha", it is not as bright as Rigel (Beta Orionis). It is a vertex of the Winter Triangle asterism.

Betelgeuse is a red supergiant, one of the largest stars known. If it were placed at the center of our solar system, its outer surface would extend between the orbit of Mars and Jupiter, completely engulfing Earth. 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.

Contents 1 Etymology 2 Observation history 3 Distinguishing characteristics 4 The star's future 5 References 6 See also 7 External links

The name is a corruption of the Arabic يد الجوزاء yad al-jawzā, or "hand of the central one". Jauza, the central one, initially referred to Gemini among the Arabs, but later referred to Orion instead. During the Middle Ages the first character of the name, y (ﻴ, with two dots under it), was misread as a b (ﺒ, with one dot under it) when transliterating into Latin, and Yad al-Jauza became Bedalgeuze. Then, during the Renaissance, it was theorized that the name was originally written as Bait al-Jauza, thought to mean "armpit of the central one" in Arabic, which led to the modern rendering as "Betelgeuse"; however, the actual translation of "armpit" would be ابط ("Ibţ").^[4] Thus in 1899 Richard Hinckley Allen gave the origin of the name as "Ibţ al Jauzah".^[5]

Because of its rich reddish color the star has frequently been referred to as the "martial one", and in astrology portends military or civic honors. Other names are:

• Al Dhira (the Arm)^[5]
• Al Mankib (the Shoulder),^[5]
• Al Yad al Yamma (the Right Hand),^[5]
• Ardra (Hindi,^[5] and name of Hindu Nakshatra),
• Bahu (Sanskrit),^[5]
• Bašn (Persian) (the Arm),
• Gula (Euphratean),
• Ied Algeuze (Orion's Hand),^[5]
• Klaria (Coptic) (an Armlet)^[5]
• Yedelgeuse

It is known as 参宿四 (Shēnsùsì, the Fourth Star of the Constellation of Three Stars) in Chinese. The confusing name is due to the fact that the Constellation of Three Stars was originally composed of just three stars, all of them in the girdle of the Orion. Later, four more stars were added to this constellation, but the name remained unchanged.

The variability of Betelgeuse was first observed by Sir John Herschel in 1836. He published his observations of the star in his work Outlines of Astronomy, noting that the variations increased in the interval 1836–1840 and subsequently decreased. In 1849 he noted an increased cycle of variability, which peaked in 1852. Subsequently observers have recorded unusually high maxima with an interval of several years, but only minor variations in the period 1957–1967. Records of the American Association of Variable Star Observers show a maximum brightness of magnitude 0.2 was achieved in 1933 and 1942, with a minimum of below magnitude 1.2 in 1927 and 1941.^[6]

In 1919, Albert Michelson and Francis Pease mounted a 6-metre (20-foot) interferometer on the front of the 2.5 m (100-inch) telescope at the Mount Wilson Observatory. Assisted by John A. Anderson, in December, 1920 Pease measured the angular diameter of α Orionis as 0.047 arcseconds. Given the then-current parallax value of 0.018 arcsecond, this resulted in an estimated radius of 3.84 × 10⁸ km (240 million miles). However there was some known uncertainty due to limb darkening and measurement errors.^[7][8]. 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 imaging, revealing a number of bright spots on the star's surface, which were thought to result from convection ^[11]. In 1995 the Faint Object Camera of the Hubble Space Telescope was used to capture the first conventional-telescope image (or "direct-image" in NASA terminology) of Betelgeuse, the first of a star other than the Sun. Processing of the ultra-violet image revealed a bright patch on the southwestern portion of the surface. This patch had a higher temperature than the surrounding stellar photosphere. The hotspot in the Hubble image was conjectured to be at the pole (rotation axis)^[2].

Recent 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 limb darkening, where the intensity of a star's image diminishes near the edge. For Betelgeuse, some adjustment is needed to compensate for gas near the photosphere.^[12]

A white disk showing the apparent size of Betelgeuse viewed from a distance of about 8 AU.

By comparison, this is our own Sun viewed from the same distance and field of view.

Several features of Betelgeuse 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,^[13] smaller only than the Sun and R Doradus, and one of only a dozen or so stars that telescopes have imaged as a visible disk. It was one of the first stars to have its angular diameter measured with an astronomical interferometer; the apparent diameter was found to be variable. The distance to Betelgeuse is not precisely known, but if it is assumed to be 427 light years then the actual diameter is 800 times the Sun's diameter. It has a color index (B-V) of 1.86. It is thought to have a mass of about 14 solar masses.

The precise diameter is not easy to define, as the optical emission decreases very gradually with radius from the center of the star, and the color of the emission also varies with radius. Though only 14 times more massive than the Sun, it is as much as 300 million times greater in volume; a difference in volume much like a beach ball compared to a large stadium. It was also the first star to have starspots on its disk resolved in optical images by a telescope, first from Aperture Masking Interferometry and later from the Hubble Space Telescope and more detailed observations by the COAST telescope.^[14]

Surrounding the photosphere of Betelgeuse is an extended atmosphere that displays strong lines of emission (rather than absorption). This chromosphere has a temperature no higher than 5,500 K, and extends outward to as many as 7 times the diameter of the star. This extended gaseous atmosphere has been observed moving away from the star, and sometimes toward it, depending on the radial velocity fluctations of the photosphere.^[3]

Visual observation of this star shows that the rotation axis of Betelgeuse has an inclination of about 20° to the direction of the Earth, and a position angle of about 55°. If so, then the hot spot that was seen in 1995 is located at the approximate position of one of the star's poles.^[2]

Astronomers predict that Betelgeuse will ultimately undergo a type II supernova explosion although it is possible that the mass is low enough for Betelgeuse to leave a rare oxygen-neon white dwarf. Opinions are divided as to the likely timescale for this event. Although Betelgeuse is only around 10 million years old, some regard the star's current variability as suggesting that it is already in the carbon burning phase of its life cycle, and will therefore undergo a supernova explosion at some time in the next thousand years or so. Skeptics dispute this contention and regard the star as being likely to survive much longer. There is a consensus that such a supernova would be a spectacular astronomical event, but would not — being so distant — represent any significant threat to life on Earth.

Even so, Betelgeuse would brighten at least 10,000 times as a supernova, causing it to shine with the luminosity of a crescent Moon. Some sources predict a maximum apparent magnitude equal to about that of the full Moon (mv = -12.5). This would likely last for several months. It would look like a brilliant point, the brightness of a full Moon with the color of an incandescent bulb at night, and easily visible in daylight. After that period it would gradually diminish until after some months or years it would disappear from naked eye view. Then Orion's right shoulder would vanish for a time until, in a few centuries, a splendid nebula would develop. However, if Betelgeuse's axis (one of its poles) is pointed towards Earth there would be tangible effects here. A shower of gamma rays and other cosmic particles would be directed at Earth. There would be spectacular aurorae and possibly a measurable diminution of the ozone layer with consequent adverse radiation effects on life. In such an orientation towards the solar system it would also appear many times brighter than if its axis were pointed away.

Curiously, in 1980, Shu-ren, Jianmin and Jin-Yi unearthed 1st century BC Chinese records that refer to the color of Betelgeuse as being white or yellow.^{[citation needed]} However, Ptolemy writing in 150 CE, calls it a red star. (It should be noted, however, that Ptolemy also calls Sirius red, despite the fact that it is recognized as white.)^[15] Therefore, Fang Lizhi, a Chinese astrophysicist, proposed that Betelgeuse could have turned into a red giant star during that period.^[16] It is known that as stars use up the hydrogen fuel in their cores, their color changes from white to yellow to red. Shu-ren et al. suggest that Betelgeuse could have changed its color when it expelled a shell of dust and gas, that, even now, can be seen to be expanding away from it. Thus, if their theory is right, it is unlikely that Betelgeuse will become a supernova any time soon because a star usually stays a red giant for tens of thousands of years.

• Peter G. Tuthill, Chris A. Haniff, John E. Baldwin, Hotspots on late-type supergiants, Monthly Notices of the Royal Astronomical Society, Volume 285, Issue 3, pp. 529-539 (1997).
• "When did it change to red?", The Hindu (a popular English language newspaper in India), Sunday Supplement, January 31-1982.
• Davis, Kate (2000-12-01). Variable Star of the Month—December, 2000: Alpha Orionis (English). AAVSO. Retrieved on 2006-08-13.
• Interferometric observations of the supergiant stars α Orionis and α Herculis with FLUOR at IOTA, February 2004
• Magnetic activity in late-type giant stars: Numerical MHD simulations of non-linear dynamo action in Betelgeuse
• Invisible Giant: Chandra's Limits on X-rays from Betelgeuse

• List of largest known stars

• Young, John (November 24 2006). Surface imaging of Betelgeuse with COAST and the WHT. University of Cambridge. Retrieved on 2007-06-21. — Images of hotspots on the surface of Betelgeuse taken at visible and infra-red wavelengths using high resolution ground-based interferometers.
• Betelgeuse. SolStation. Retrieved on 2005-11-11.