Betelgeuse (Star) Friday, July 3, 2009

Betelgeuse, Alpha Ori

The pink arrow at the star on left labeled α indicates Betelgeuse in Orion.
Observation data
Epoch J2000.0 Equinox J2000.0
Constellation Orion
Pronunciation /ˈbiːtəldʒuːz/ or
/ˈbɛtəldʒuːz/[1]
Right ascension 05h 55m 10.3053s[2]
Declination +07° 24′ 25.426″[2]
Apparent magnitude (V) 0.58[2] (0.3 to 1.2)
Characteristics
Spectral type M2Iab[2]
U-B color index 2.06[2]
B-V color index 1.85[2]
Variable type SR c[2] (Semi-regular)
Astrometry
Radial velocity (Rv) +21.0[2] km/s
Proper motion (μ) RA: 24.95 ± 0.08[3] mas/yr
Dec.: 9.56 ± 0.15[3] mas/yr
Parallax (π) 5.07 ± 1.10[3] mas
Distance 640 [3] ly
(197 [3] pc)
Absolute magnitude (MV) −5.14
Details

Mass 20[4] M☉
Radius 936[5] R☉
Surface gravity (log g) -0.5[6]
Luminosity 135,000[4] L☉
Temperature 3,500[6] K
Rotation 17 years (14.6 km/s)[7]
Age 1.0 × 107[citation needed] years

Other designations

Betelgeuse (α Ori, α Orionis, Alpha Orionis) is a semiregular variable star located approximately 640 light-years away from Earth. [2][3]

It 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.

Betelgeuse is a red supergiant, and one of the largest and most luminous 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.[8] Some astronomers believe it may become a supernova within a timeframe where it could be observable by human civilization.

Etymology

The name is a corruption of the Arabic يد الجوزاء yad al-jawzā ("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 (ﻴ, with two dots underneath) being misread as a b (ﺒ, 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 ابط, transliterated as Ibţ,[9] hence in 1899 Richard Hinckley Allen mistakenly gave the origin as Ibţ al Jauzah).[10] 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:

In Chinese, Betelgeuse is known as 参宿四 (Shēnsùsì, 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

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.[11]

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 α 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.[12][13] 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.[14]. 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.[7]

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.[15]

Observed size

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.[16][17]
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,[18] 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 1,000 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.[4]

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.[4][19] Between 1993 and 2009, the star's diameter has contracted by over 15 percent.[20] Though the data is inconclusive, major news media have been reporting that this data could presage a supernova.[21]

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.[6]

Fate

It is possible that Betelgeuse will become a supernova, [4][22] which will be the brightest ever recorded, outshining the Moon in the night sky.[22] Considering its size and age of 8.5 million years, old for its size class, it may explode within the next thousand years.[22] Since its rotational axis is not toward the Earth and also because of its 640 light year distance,[22] Betelgeuse's supernova will not cause a gamma ray burst in the direction of Earth large enough to damage its ecosystem.

Nobel Laureate Charles Townes announced evidence that 15 consecutive years of stellar contraction has been observed by UC Berkeley's Infrared Spatial Interferometer (ISI) atop Mt. Wilson Observatory in Southern California. Reported on June 9, 2009, the star has shrunk 15% since 1993 with an increasing rate. The average speed at which the radius of the star is shrinking over the last 15 years is approximately 470-490 miles per hour.[23]

According to the university, Betelgeuse's radius is about 5.5 A.U.s, and the star's radius has shrunk by a distance equal to half an astronomical unit, or about the orbit of Venus.[24] Some theorists[who?] have speculated that this behavior is expected for a star at the beginning of the gravitational collapse at the end of its life.[citation needed] The mass of Betelgeuse puts it in range to become a neutron star or possibly a black hole.

See also

References

  1. ^ OED
  2. ^ a b c d e f g h i j "SIMBAD query result: V* alf Ori -- Semi-regular pulsating Star". Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-id?Ident=betelgeuse. Retrieved on 2007-06-20.
  3. ^ a b c d e f (5), Table 6, A New VLA-Hipparcos Distance to Betelgeuse and its Implications, Graham M. Harper, Alexander Brown, and Edward F. Guinan, The Astronomical Journal 135, #4 (April 2008), pp. 1430–1440, Bibcode: 2008AJ....135.1430H, doi:10.1088/0004-6256/135/4/1430.
  4. ^ a b c d e astro.uiuc.edu, Betelgeuse (Alpha Orionis) - Update 2008, Stars, Jim Kaler, retrieved 9 October 2008.
  5. ^ http://www.astro.illinois.edu/~jkaler/sow/betelgeuse.html
  6. ^ a b c Lobel, A.; Dupree, A. K. (2000). "Modeling the Variable Chromosphere of α Orionis". The Astrophysical Journal 545: 454–474. doi:10.1086/317784. http://www.journals.uchicago.edu/doi/full/10.1086/317784. Retrieved on 2007-02-04.
  7. ^ a b Uitenbroek, H.; Dupree, A. K.; Gilliland, R. L. (1998). "Spatially Resolved Hubble Space Telescope Spectra of the Chromosphere of α Orionis". The Astronomical Journal 116: 2501–2512. http://www.iop.org/EJ/article/1538-3881/116/5/2501/980184.html. Retrieved on 2007-06-20.
  8. ^ "Betelgeuse". SolStation. http://www.solstation.com/x-objects/betelgeuse.htm. Retrieved on 2005-11-11.
  9. ^ Kunitzsch, Paul, and Smart, Tim (2006). A Dictionary of Modern Star Names. Cambridge, MA: Sky Publishing. pp. 45. ISBN 9781931559447.
  10. ^ a b c d e f g h Allen, Richard Hinckley (1963). Star Names: Their Lore and Meaning (Revised ed.). New York: Dover Publications. ISBN 0486210790.
  11. ^ Burnham, Robert (1978). Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System, Volume 2. New York: Courier Dover Publications. pp. 1290. ISBN 0486235688.
  12. ^ Michelson, A. A.; Pease, F. G. (1921). "Measurement of the diameter of alpha Orionis with the interferometer". Astrophysical Journal 53: 249–259. doi:10.1086/142603. http://adsabs.harvard.edu/abs/1921ApJ....53..249M. Retrieved on 2007-06-20.
  13. ^ Staff (November 2000). "Pease, Francis G (1881–1938)". Encyclopedia of Astronomy and Astrophysics. http://eaa.crcpress.com/default.asp?action=summary&articleId=3915. Retrieved on 2007-06-20.
  14. ^ D. Buscher et al. (1990). "Detection of a bright feature on the surface of Betelgeuse". Monthly Notices of the Royal Astronomical Society 245: 7. http://adsabs.harvard.edu/cgi-bin/bib_query?1990MNRAS.245p...7B. Retrieved on 2007-08-07. R. Wilson et al. (1997). "The changing face of Betelgeuse". Monthly Notices of the Royal Astronomical Society 291: 819. http://adsabs.harvard.edu/cgi-bin/bib_query?1997MNRAS.291..819W. Retrieved on 2007-08-07.
  15. ^ J. Weiner et al. (2000). "Precision Measurements of the Diameters of α Orionis and ο Ceti at 11 Microns". The Astrophysical Journal 544 (2): 1097–1100. doi:10.1086/317264. http://www.journals.uchicago.edu/doi/full/10.1086/317264. Retrieved on 2007-06-23.
  16. ^ The yellow/red "image" or "photo" of Betelgeuse usually seen is actually not a picture of the red giant but rather a mathematically generated image based on the photograph. The photograph was actually of much lower resolution: The entire Betelgeuse image fit entirely within a 10x10 pixel area on the Hubble Space Telescopes Faint Object Camera. The actual images were oversampled by a factor of 5 with bicubic spline interpolation, then deconvolved.
  17. ^ Gilliland, Ronald; L.; Dupree, A. K.. "First Image of the Surface of a Star with the Hubble Space Telescope". Astrophysical Journal Letters v.463, p.L29. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1996ApJ...463L..29G.
  18. ^ T. R. Bedding et al. (1997). "The angular diameter of R Doradus: a nearby Mira-like star". Monthly Notices of the Royal Astronomical Society 286 (4): 957–962. http://adsabs.harvard.edu/abs/1997astro.ph..1021B. Retrieved on 2007-06-20.
  19. ^ D. Burns et al. (1997). "The surface structure and limb-darkening profile of Betelgeuse". Monthly Notices of the Royal Astronomical Society 290 (1): L11–L16.
  20. ^ Ron Cowen (2009-06-10). "Betelgeuse shrinks: The red supergiant has lost 15 percent of its size". http://www.sciencenews.org/view/generic/id/44573/title/Betelgeuse_shrinks. "The shrinkage corresponds to the star contracting by a distance equal to that between Venus and the sun, researchers reported June 9 at an American Astronomical Society meeting and in the June 1 Astrophysical Journal Letters."
  21. ^ "Nearby Star May Be Getting Ready to Explode". 2009-06-10. http://www.foxnews.com/story/0,2933,525695,00.html. "The nearby, well-known and very bright star may soon explode in a supernova, according to data released by U.C. Berkeley researchers Tuesday."
  22. ^ a b c d Betelgeuse could explode as a supernova » Radio Podcasts | Earth & Sky
  23. ^ http://esciencenews.com/articles/2009/06/09/red.giant.star.betelgeuse.mysteriously.shrinking
  24. ^ http://www.berkeley.edu/news/media/releases/2009/06/09_betelim.shtml

Further reading

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