User:Sadalsuud/Beetle1geuse

This page is about the star. For the film, see Beetlejuice. For the oil tanker, see Betelgeuse incident.

Betelgeuse (α 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/
Right ascension	05h 55m 10.3053s
Declination	+07° 24′ 25.426″
Characteristics
Spectral type	M2Iab
Apparent magnitude (V)	0.42 (0.3 to 1.2)
Apparent magnitude (J)	-2.99 ± 0.10
U−B color index	2.06
B−V color index	1.85
Variable type	SR c (Semi-Regular)
Astrometry
Radial velocity (Rv)	+21.91 km/s
Proper motion (μ)	RA: 24.95 ± 0.08 mas/yr Dec.: 9.56 ± 0.15 mas/yr
Parallax (π)	5.07 ± 1.10 mas
Distance	643 ± 146  ly (197 ± 45  pc)
Absolute magnitude (MV)	−6.02
Details
Mass	7.7–20 M☉
Radius	950–1200 R☉
Luminosity	120,000±30,000 L☉
Surface gravity (log g)	-0.5 cgs
Temperature	3,140-3,641 K
Metallicity	0.05 Fe/H
Rotation	5 km/s
Age	~7.3×106  years
Other designations
Betelgeuse, α Ori, 58 Ori, HR 2061, BD +7° 1055, HD 39801, FK5 224, HIP 27989, SAO 113271, GC 7451, CCDM J05552+0724AP, AAVSO 0549+07
Database references
SIMBAD	data

Betelgeuse (/ˈbiːtəldʒuːz/ or /ˈbɛtəldʒuːz/), also known by its Bayer designation Alpha Orionis (α Orionis, α Ori), is the eighth brightest star in the night sky and second brightest in the constellation of Orion, outshining Rigel (Beta Orionis) only rarely. Distinctly reddish, it is a semiregular variable star whose apparent magnitude varies between 0.2 and 1.2, the widest range of any first-magnitude star. Betelgeuse marks the upper right vertex of the Winter Triangle asterism and the center of the Winter Hexagon.

The star is classified as a red supergiant of spectral type M2Iab and is one of the largest and most luminous known stars. If it were at the center of the Solar System its surface would extend past the asteroid belt, possibly to the orbit of Jupiter and beyond, wholly engulfing Mercury, Venus, Earth and Mars. Its distance in 2008 was estimated at 640 light-years, yielding a mean absolute magnitude of about −6.02. Less than 10 million years old, Betelgeuse has evolved rapidly because of its high mass. Having been ejected from its birthplace in the Orion OB1 Association—which also includes the late type O and B stars in Orion's belt, Alnitak, Alnilam and Mintaka—this crimson runaway has been observed racing through the interstellar medium at a supersonic speed of 30 km/sec, creating a bow shock over 4 light-years wide. Currently in a late stage of stellar evolution, the supergiant is expected to proceed through its expected life cycle before exploding as a type II supernova within the next million years.

In 1920, Betelgeuse was the first star (after the Sun) to have its photosphere measured. Since then, researchers have used a number of telescopes to measure this stellar giant, each with different technical parameters, often yielding conflicting results. Studies since 1990 have produced an apparent diameter ranging from 0.043 to 0.056 arcseconds, an incongruity largely caused by the star's perceived tendency to periodically change shape. Because of limb darkening, variability, and angular diameters that vary with wavelength, the star remains a perplexing mystery. Adding to the computational challenges, Betelgeuse has a complex, asymmetric envelope caused by colossal mass loss obscuring its surface—an envelope that is roughly 250 times the size of the star itself—with stellar companions possibly orbiting within this circumstellar nebula magnifying the star's eccentric behavior.

Observational history

Betelgeuse and its red coloration have been noted since antiquity; the classical astronomer Ptolemy described its color as ὑπόκιρρος (hypókirros), a term which was later described by a translator of Ulugh Beg's Zij-i Sultani as rubedo, Latin for "ruddiness". In the nineteenth century, before modern systems of stellar classification, Angelo Secchi included Betelgeuse as one of the prototypes for his Class III (orange to red) stars. By contrast, three centuries before Ptolemy, Chinese astronomers observed Betelgeuse as having a yellow coloration, suggesting that the star may have spent time as a yellow supergiant around the beginning of the common era, an intriguing possibility given current research into the complex circumstellar environment of these stars.

Nascent discoveries

Portrait of Sir John Herschel a few years before his death

The variation in Betelgeuse's brightness was first described in 1836 by Sir John Herschel, when he published his observations in Outlines of Astronomy. From 1836 to 1840 he noticed significant changes in magnitude with Betelgeuse outshining Rigel in October 1837 and again in November 1839. A quiescent period followed lasting about 10 years, then in 1849, Herschell noted another short cycle of variability which peaked in 1852. Later observers recorded unusually high maxima with an interval of several years, but only small variations from 1957 to 1967. The records of the American Association of Variable Star Observers (AAVSO) show a maximum brightness of 0.2 in the years 1933 and 1942, with a minimum of 1.2, observed in both 1927 and 1941. This variability in brightness may explain why Johann Bayer, with the publication of his Uranometria in 1603, designated the star alpha as it may have rivalled the usually brighter Rigel (beta).

In 1920, Albert Michelson and Francis Pease mounted a 6-metre interferometer on the front of the 2.5-metre telescope at Mount Wilson Observatory. Helped by John Anderson, the trio measured the angular diameter of Betelgeuse at 0.047", a figure which resulted in a diameter of 3.84 × 10⁸ km (2.58 AU) based on the then-current parallax value of 0.018". However there was known uncertainty owing to limb darkening and measurement errors—a central theme which would be the focus of scientific inquiry for almost a century. Beginning with this first angular measurement at visible wavelengths, researchers have since tried the measurement at other wavelengths with varying results.

The 1950s and '60s saw important scientific developments, the two Stratoscope projects and the 1958 publication of Structure and Evolution of the Stars, principally the work of Martin Schwarzschild and his close colleague at Princeton University, Richard Härm. This book taught a generation of astrophysicists how to use budding computer technologies to create stellar models while the Stratoscope projects, by taking instrumented balloons above the Earth's turbulence, produced some of the finest images of solar granules and sunspots ever seen, thus confirming the existence of convection in the solar atmosphere. Both developments would prove to have a significant impact on our understanding of the structure of red supergiants like Betelgeuse.

Aperture masking

File:Betelgeuse star (Hubble).jpg

Betelgeuse imaged in ultraviolet light by the Hubble Space Telescope and subsequently enhanced by NASA

The 1970s saw several notable advances in interferometry from the Berkeley Space Sciences Laboratory working in the infrared and Antoine Labeyrie in the visible, when researchers began to combine images from multiple telescopes and later invented "fringe-tracking" technology. But it was not until the late 1980s and early 1990s, when Betelgeuse became a regular target for aperture masking interferometry, that significant breakthroughs occurred in visible-light and infrared imaging. Pioneered by John E. Baldwin and other colleagues of the Cavendish Astrophysics Group, the new technique contributed some of the most accurate measurements of Betelgeuse to date while revealing a number of bright spots on the star's photosphere. These were the first optical and infrared images of a stellar disk other than the Sun, first from ground-based interferometers and later from higher-resolution observations of the COAST telescope, with the "bright patches" or "hotspots" potentially corroborating a theory put forth by Schwarzschild decades earlier of massive convection cells dominating the stellar surface.

In 1995, the Hubble Space Telescope's Faint Object Camera captured an ultraviolet image of comparable resolution—the first conventional-telescope image (or "direct-image" in NASA terminology) of the disk of another star. The image was taken at ultraviolet wavelengths since ground-based instruments cannot produce images in the ultraviolet with the same precision as Hubble. Like earlier pictures, this ultraviolet image also contained a bright patch indicating a region in the southwestern quadrant 2,000K hotter than the stellar surface. Subsequent ultraviolet spectra taken with the Goddard High Resolution Spectrograph suggested that the hot spot was one of Betelgeuse's poles of rotation. This would give the rotational axis an inclination of about 20° to the direction of Earth, and a position angle from celestial North of about 55°.

Recent studies

AAVSO V-band light curve of Betelgeuse (Alpha Orionis) from Dec. 1988 – Aug. 2002

The first decade of the 21st century has witnessed major advances on multiple fronts, the most central of which have been the imaging of the star's photosphere at different wavelengths and the study of Betelgeuse's complex circumstellar shells. At the dawn of the millennium, the star's diameter was measured in the mid-infrared using the Infrared Spatial Interferometer (ISI) producing a limb darkened estimate of 55.2 ± 0.5 milliarcseconds (mas)—a figure entirely consistent with Michelson's findings eighty years earlier. At the time of its publication, the estimated parallax from the Hipparcos mission was 7.63 ± 1.64 mas, yielding an estimated radius for Betelgeuse of 3.6 AU. However, numerous interferometric studies in the near-infrared have appeared since from the Paranal Observatory in Chile arguing for much tighter diameters. Nevertheless, on June 9, 2009, Nobel Laureate Charles Townes announced that the star had shrunk 15% since 1993 at an increasing rate without a significant diminution in magnitude Subsequent observations suggest that the apparent contraction may be due to shell activity in the star's extended atmosphere.

Enveloping this whole discussion have been numerous inquiries into the abstruse dynamics of Betelgeuse's extended atmosphere. The mass that makes up galaxies is recycled as stars are formed and destroyed. For decades astronomers have understood that the outer shells of red giants are central to this process, yet the actual mechanics of stellar mass loss remain a mystery. With recent advances in interferometric methodologies, astronomers may be close to resolving this conundrum. In July 2009, images released by the European Southern Observatory, taken by the ground-based Very Large Telescope Interferometer (VLTI), showed a vast plume of gas being ejected into the surrounding atmosphere with distances approximating 30 AU. Comparable to the distance between the Sun and Neptune, this mass ejection is but one of multiple dynamics occurring in the surrounding atmosphere. Astronomers have identified at least six different shells surrounding Betelgeuse. Solving the mystery of mass loss in the late stages of a star's evolution may reveal those factors which precipitate the explosive deaths of these stellar giants.

Visibility

Photograph from Rogelio Bernal Andreo showing Betelgeuse in relationship to the dense nebulas of the Orion Molecular Cloud Complex and the Belt of Orion

Betelgeuse is easy to spot in the night sky, as it appears in proximity to the famous belt of Orion and has a distinctive orange-red color to the naked eye. In the Northern Hemisphere, beginning in January of each year, it can be seen rising in the east just after sunset. By mid-March, it is visible to virtually every inhabited region of the globe, with only a few research stations in Antarctica at latitudes south of 82° unable to see it. Once May arrives, the red giant can be glimpsed but briefly on the western horizon just after sunset, reappearing again a few months later on the eastern horizon before sunrise.

The apparent magnitude of Betelgeuse is listed in SIMBAD at 0.42, making it on average the eighth brightest star in the celestial sphere excluding the Sun—just ahead of Achernar. Because Betelgeuse is a variable star whose brightness ranges between 0.2 and 1.2, there are periods when it will surpass Procyon to become the seventh brightest star. Occasionally it can even outshine Rigel and become the sixth brightest star, as the latter star, with a nominal apparent magnitude of 0.12, has been reported to fluctuate slightly in brightness, by 0.03 to 0.3 magnitudes. At its faintest, Betelgeuse will fall behind Deneb as the 19th brightest star and compete with Mimosa for the 20th position.

Image from ESO's Very Large Telescope showing not only the stellar disk, but also an extended atmosphere with a previously unknown plume of surrounding gas

Betelgeuse has a color index (B–V) of 1.85—a figure which points to the advanced "redness" of this celestial object. The photosphere has an extended atmosphere which displays strong lines of emission rather than absorption, a phenomenon which occurs when a star is surrounded by a thick gaseous envelope. This extended gaseous atmosphere has been observed moving both away from and towards Betelgeuse, depending on radial velocity fluctuations in the photosphere. Betelgeuse is the brightest near-infrared source in the sky with a J band magnitude of −2.99. As a result, only about 13% of the star's radiant energy is emitted in the form of visible light. If human eyes were sensitive to radiation at all wavelengths, Betelgeuse would appear as the brightest star in the sky.

Parallax

Since the first successful parallax measurement by Friedrich Bessel in 1838, astronomers have been puzzled by Betelgeuse's distance. Solving this enigma holds the key to understanding other stellar parameters, since an accurate "distance leads to the luminosity and when combined with an angular diameter gives the physical radius and effective temperature; luminosity and isotopic abundances provide estimates of the stellar age and mass". In 1920, when the first interferometric studies were performed on the star's diameter, the assumed parallax was 0.0180 arcseconds. That equated to a distance of 56 parsecs (pc) or roughly 180 light-years (ly) and produced not only an inaccurate radius for the star, but every other stellar characteristic. Since then, there has been ongoing work to measure the actual distance of Betelgeuse, with proposed distances as high as 400 pc or about 1,300 ly.

Before the publication of the Hipparcos Catalogue (1997), there were two conflicting parallax measurements for Betelgeuse. The first was the Yale University Observatory (1991) with a published parallax of π = 9.8 ± 4.7 mas, yielding a distance of roughly 102 pc or 330 ly. The second was the Hipparcos Input Catalogue (1993) with a trigonometric parallax of π = 5 ± 4 mas, a distance of 200 pc or 650 ly—almost twice the Yale estimate. With such uncertainty, researchers were adopting a wide range of distance estimates, leading to significant variances in the calculation of the star's attributes.

NRAO's Very Large Array used to derive Betelgeuse's current distance estimate

The results from the Hipparcos mission were released in 1997. The measured parallax of Betelgeuse was π = 7.63 ± 1.64 mas, which equated to a distance of 131 pc or roughly 430 ly, and had a smaller reported error than previous measurements. However, later evaluation of the Hipparcos parallax measurements for variable stars like Betelgeuse found that the uncertainty of these measurements were underestimated. In 2007, Floor van Leeuwen improved upon the Hipparcos parallax, producing a new figure of π = 6.55 ± 0.83, hence a much tighter error factor yielding a distance of roughly 152 ± 20pc or 520 ± 73ly.

In 2008, Graham Harper, using the Very Large Array (VLA), produced a radio solution of π = 5.07 ± 1.10 mas, equaling a distance of 197 ± 45 pc or 643 ± 146 ly. As Harper points out: "The revised Hipparcos parallax leads to a larger distance (152 ± 20 pc) than the original; however, the astrometric solution still requires a significant cosmic noise of 2.4 mas. Given these results it is clear that the Hipparcos data still contain systematic errors of unknown origin." Although the radio data suffer from systematic errors as well, the Harper solution combines both datasets in the hope of mitigating such errors.

The next breakthrough will likely come from the European Space Agency's upcoming Gaia mission when it undertakes a detailed analysis of physical properties for each star observed, revealing luminosity, temperature, gravity and composition. Gaia will achieve this by repeatedly measuring the positions of all objects down to magnitude 20, and those brighter than magnitude 15, to an accuracy of 24 microarcseconds—akin to measuring the diameter of a human hair from 1000 km away. On-board detection equipment will ensure that variable stars like Betelgeuse will all be detected to this faint limit, thus addressing most of the limitations of the earlier Hipparcos mission.

Variability

Ultraviolet image of Betelgeuse showing the star's asymmetrical pulsations, expansion and contraction

Betelgeuse is classified as a semiregular variable star of subgroup SRc; these are pulsating red supergiants with low amplitude variations and periods of stable brightness. Different hypotheses have been put forward to explain Betelgeuse's volatile choreography—a phenomenon which causes an absolute magnitude oscillation from −5.27 and −6.27. Established theories of stellar structure suggest that the outer layers of this supergiant gradually expand and contract, causing the surface area (photosphere) to alternately increase and decrease, and the temperature to rise and fall—thereby eliciting the measured cadence in the star's brightness between its dimmest magnitude of 1.2, seen as early as 1927, and its brightest of 0.2, seen in 1933 and 1942. A red supergiant like Betelgeuse will pulsate this way because its stellar atmosphere is inherently unstable. As the star contracts, it absorbs more and more of the energy that passes through it, causing the atmosphere to heat up and expand. Conversely, as the star expands, its atmosphere becomes less dense allowing the energy to escape and the atmosphere to cool, thus initiating a new contraction phase. Calculating the star's pulsations and modeling its periodicity have been difficult, as it appears there are several cycles interlaced. As discussed in papers by Stebbins and Sanford in the 1930s, there are short-term variations of around 150 to 300 days that modulate a regular cyclic variation with a period of roughly 5.7 years.

An illustration of the structure of the Sun showing photospheric granules :
1. Core
2. Radiative zone
3. Convective zone
4. Photosphere
5. Chromosphere
6. Corona
7. Sunspot
8. Granules
9. Prominence

In fact, the supergiant consistently displays irregular photometric, polarimetric and spectroscopic variations, phenomena which point to complex activity on the star's surface and its extended atmosphere. In marked contrast to most giant stars that are typically long period variables with reasonably regular periods, red giants are generally semiregular or irregular with pulsating characteristics. Martin Schwarzschild in 1975 attributed these brightness fluctuations to the changing granulation pattern formed by a few giant convection cells covering the surface of these stars. For the Sun, these convection cells, otherwise known as solar granules, represent the foremost mode of heat transfer—hence those convective elements which dominate the brightness variations in the solar photosphere. The typical diameter for a solar granule is about 2,000 km (a surface area roughly the size of India), with an average depth of 700 km. With a surface of roughly 6 trillion km², there are about 2 million such granules lying on the Sun's photosphere, which because of their number produce a relatively constant flux. By contrast, Schwarzschild argues that stars like Betelgeuse may have only a dozen monster granules with diameters of 180 million km or more dominating the surface of the star with depths of about 60 million km, which, because of the very low temperatures and extremely low density found in red giant envelopes, result in convective inefficiency. Consequently, if only a third of these convective cells are visible to us at any one time, the time variations in their observable light may well be reflected in the irregular brightness variations of the integrated light of the star.

The hypothesis that gigantic convection cells dominate the surface of red giants and supergiants remains accepted by the astronomical community. When the Hubble Space Telescope captured its first direct image of Betelgeuse in 1995 revealing a mysterious hot spot, astronomers attributed it to convection. Two years later, astronomers observed intricate asymmetries in the brightness distribution of the star revealing at least three bright spots, the magnitude of which was "consistent with convective surface hotspots". Then in 2000, another team of astronomers led by Alex Lobel of the Harvard–Smithsonian Center for Astrophysics (CfA) noted that Betelgeuse exhibits raging storms of hot and cold gas in its turbulent atmosphere. The team surmised that huge areas of the star's photosphere vigorously bulge out in different directions at times, ejecting long plumes of warm gas into the cold dust envelope. Another explanation that was also given was the occurrence of shock waves caused by warm gas traversing cooler regions of the star. The team investigated the atmosphere of Betelgeuse over a period of five years between 1998 and 2003 with the Space Telescope Imaging Spectrograph aboard Hubble. They found that the bubbling action of the chromosphere tosses gas out one side of the star, while it falls inward at the other side, similar to the slow-motion churning of a lava lamp.

Diameter

See also: List of largest known stars

A third challenge that has confronted astronomers has been measuring the star's angular diameter. On December 13, 1920, Betelgeuse became the first star outside the Solar System to ever have its photosphere measured. Although interferometry was still in its infancy, the experiment proved a success and Betelgeuse was found to have a uniform disk of 0.047 arcseconds. The astronomers' insights on limb darkening were noteworthy; in addition to a measurement error of 10%, the team concluded that the stellar disk was likely 17% larger due to the diminishing intensity of light around the edges—hence an angular diameter of about 0.055". Since then, there have been other studies conducted, which have produced angles that range from 0.042 to 0.069 arcseconds. Combining that data with historical distance estimates of 180 to 815 ly yields a projected diameter of the stellar disk of anywhere from 2.4 to 17.8 AU, hence a radius of 1.2 to 8.9 AU respectively. Using the Solar System as a yardstick, the orbit of Mars is about 1.5 AU, Ceres in the asteroid belt 2.7 AU, Jupiter 5.5 AU—consequently a photosphere which, depending on Betelgeuse's actual distance from Earth, could well extend beyond the Jovian orbit but not quite as far as Saturn at 9.5 AU.

Radio image from 1998 (pre-Harper) showing the size of Betelgeuse's photosphere (circle) and the effect of convective forces on the star's atmosphere

The precise diameter has been hard to define for several reasons:

1. The rhythmic expansion and contraction of the photosphere, as theory suggests, means the diameter is never constant;
2. There is no definable "edge" to the star as limb darkening causes the optical emissions to vary in color and decrease the farther one extends out from the center;
3. Betelgeuse is surrounded by a circumstellar envelope composed of matter being ejected from the star—matter which both absorbs and emits light—making it difficult to define the actual surface of the star;
4. Measurements can be taken at varying wavelengths within the electromagnetic spectrum, with each wavelength revealing something different. Studies have shown that angular diameters are considerably larger at visible wavelengths, decrease to a minimum in the near-infrared, only to increase again in the mid-infrared. The difference in reported diameters can be as much as 30–35%, yet because each wavelength measures something different, comparing one finding with another is difficult;
5. Atmospheric twinkling limits the resolution obtainable from ground-based telescopes since turbulence degrades angular resolution.

To overcome these challenges, researchers have employed various solutions. Astronomical interferometry, first conceived by Hippolyte Fizeau in 1868, was the seminal concept that revolutionized modern telescopy and lead to the creation of the Michelson interferometer in the 1880s, and the first successful measurement of Betelgeuse. Just as human depth perception increases when two eyes perceive an object instead of one, Fizeau proposed the observation of stars through two apertures instead of one to obtain interferences that would furnish information on the star's spatial intensity distribution. The science evolved quickly where today multiple-aperture interferometers are now used to capture numerous speckled images, which are then synthesized using Fourier analysis to produce a portrait of extraordinary resolution. It was this methodology that identified the hotspots on Betelgeuse in the 1990s. Other technological breakthroughs include adaptive optics, space observatories like Hipparcos, Hubble and Spitzer, and the Astronomical Multi-BEam Recombiner (AMBER), which combines the beams of three telescopes simultaneously, allowing researchers to achieve milliarcsecond spatial resolution.

The current debate revolves around which wavelength—the visible, near-infrared (NIR) or mid-infrared (MIR)—produces the most accurate angular measurement. In 1996, Manfred Bester, working with the ISI in the mid-infrared, led a team at the Space Sciences Laboratory (SSL) at U.C. Berkeley to produce a solution showing Betelgeuse with a uniform disk of 56.6 ± 1.0 mas. In 2000, the SSL team produced another measure of 54.7 ± 0.3 mas, ignoring any possible contribution from hotspots, which are less noticeable in the mid-infrared. Also included was a theoretical allowance for limb darkening yielding a diameter of 55.2 ± 0.5 mas. The Bester estimate equates to a radius of roughly 5.6 AU or 1,200 R_☉, assuming the 2008 Harper distance of 197.0 ± 45 pc, a figure roughly the size of the Jovian orbit of 5.5 AU, published in 2009 in Astronomy Magazine and a year later in NASA's Astronomy Picture of the Day.

Across the Atlantic, another team of astronomers working in the near-infrared and led by Guy Perrin of the Observatoire de Paris produced a 2004 document arguing that the more accurate photospheric measurement was 43.33 ± 0.04 mas. The study also put forth an explanation as to why varying wavelengths from the visible to mid-infrared produce different diameters. The star is seen through a thick, warm extended atmosphere. At short wavelengths (the visible spectrum) the atmosphere scatters light thus slightly increasing the star's diameter. At near-infrared wavelengths (K and L bands), the scattering is negligible, so the classical photosphere can be directly seen; in the mid-infrared the scattering increases once more causing the thermal emission of the warm atmosphere to increase the apparent diameter. Comparing studies, however, is problematic since each wavelength produces a different view of the star, requiring different interpretations.

Infrared image of Betelgeuse, Meissa and Bellatrix with surrounding nebulas

Studies done in 2009 with the IOTA and VLTI brought strong support to Perrin's analysis yielding diameters ranging from 42.57 to 44.28 mas with comparatively insignificant margins of error. In 2011, Keiichi Ohnaka from the Max Planck Institute for Radio Astronomy produced a third estimate in the near-infrared corroborating Perrin's numbers, this time showing a limb-darkened disk diameter of 42.49 ± 0.06 mas. Consequently, if we combine the smaller Hipparcos distance from van Leeuwen of 152 ± 20 pc with Perrin's angular measurement of 43.33 mas, a near-infrared photospheric estimate would equate to about 3.4 AU or 730 R_☉.

Central to this discussion is another paper published by the Berkeley team in 2009, this time led by Charles Townes, reporting that the radius of Betelgeuse had actually shrunk from 1993 to 2009 by 15%, with the 2008 angular measurement equal to 47.0 mas, not too far from Perrin's estimate. Unlike most papers heretofore published, this study encompassed a 15-year period at one specific wavelength. Earlier studies have typically lasted one to two years by comparison and have explored multiple wavelengths, often yielding vastly different results. The diminution in Betelgeuse's apparent size equates to a range of values between 56.0 ± 0.1 mas seen in 1993 to 47.0 ± 0.1 mas seen in 2008—a contraction of almost 0.9 AU in 15 years. What is not fully known is whether this observation is evidence of a rhythmic expansion and contraction of the star's photosphere as astronomers have theorized, and if so, what the periodic cycle might be, although Townes suggested that if a cycle does exist, it is probably a few decades long. Other possible explanations are photospheric protrusions due to convection or a star that is not spherical but rather asymmetric causing the appearance of expansion and contraction as the star rotates on its axis.

In conclusion, the current debate between measurements in the mid-infrared, which suggest a possible expansion and contraction of the star, and the near-infrared, which advocates a relatively constant photospheric diameter, is yet to be resolved. In a paper published in 2012, the Berkeley team reported that their measurements were "dominated by the behavior of cool, optically thick material above the stellar photosphere", indicating that the apparent expansion and contraction may be due to activity in the star's outer shells and not the photosphere itself. This conclusion, if further corroborated, would suggest an average angular diameter for Betelgeuse closer to Perrin's estimate at 43.33 arcseconds, hence a stellar radius of about 3.4 AU (730 R_☉), if we assume the shorter Hipparcos distance of 498 ± 73 ly in lieu of Harper's estimate at 643 ± 146 ly. The Gaia spacecraft will likely clarify many of the assumptions presently used in calculating the size of Betelgeuse's stellar disk, yielding a much wider consensus among astronomers.

Once considered as having the largest angular diameter of any star in the sky after the Sun, Betelgeuse lost that distinction in 1997 when a group of astronomers measured R Doradus with a diameter of 57.0 ± 0.5 mas. Betelgeuse is now considered to be in third place, although R Doradus, being much closer to Earth at about 200 ly, has an actual diameter roughly one-third that of Betelgeuse.

Properties

Since 1943, the spectrum of Betelgeuse has served as one of the stable anchor points by which other stars are classified. Its spectral class of M2Iab, signifies that it is a red Class M star; the "ab" suffix, derived from the Yerkes spectral classification system, indicates that it is an intermediate luminous supergiant. Uncertainties regarding the star's surface temperature, angular diameter and distance, make it very difficult to achieve a precise measurement of Betelgeuse's luminosity. Current research gives Betelgeuse an average luminosity of 120,000 ± 30,000 L_☉, assuming a median temperature of 3,300 K and a radius of 1,200 R_☉. However, with most of the star's radiant energy occurring in the infrared and a substantial amount being absorbed by circumstellar matter, the human eye simply cannot perceive the star's intrinsic brightness.

Because the existence of stellar companions has never been confirmed, there is no direct method of measuring Betelgeuse's mass. A mass estimate is only possible using theoretical modeling, a situation which has produced mass estimates ranging from 5—30 M_☉ in the last decade. Smith and colleagues calculated that Betelgeuse began its life as a star of 15 to 20 M_⊙, based on a photospheric measurement of 5.6 AU or 1,200 R_⊙. However, a novel method of determining the supergiant's mass was proposed in 2011 by Hilding Neilson and colleagues, arguing for a stellar mass of 11.6 M_⊙ with an upper limit of 16.6 and lower of 7.7 M_⊙, based on observations of the star’s intensity profile from narrow H-band interferometry and using a photospheric measurement of roughly 4.3 AU or 955 R_⊙. How the debate will be resolved is still open to question—at least until a companion star is identified allowing for a direct calculation of stellar mass.

Typical of red supergiants, Betelgeuse is a cool star. Because of its variability and the presence of hotspots, the photospheric temperature is somewhat uncertain. Studies in the last decade report temperatures ranging from 3,140 to 3,641 K with a median of about 3,300K. The star is also a slow rotator, with the most recent velocity recorded at 5 km/s. Depending on its photospheric radius, it could take the star anywhere from 25 to 32 years to turn on its axis—much slower than Antares which has a rotational velocity of 20 km/s.

In 2002, astronomers using sophisticated computer simulations began to speculate that Betelgeuse might exhibit magnetic activity in its extended atmosphere, a factor where even moderately strong fields could have a meaningful influence over the star's dust, wind and mass-loss properties. A series of spectropolarimetric observations obtained in 2010 with the Bernard Lyot Telescope at Pic du Midi Observatory revealed the presence of a weak magnetic field at the surface of Betelgeuse, suggesting that the giant convective motions of supergiant stars are able to trigger the onset of a small-scale dynamo.

Space motion

Orion OB1 Association

The kinematics of Betelgeuse are not easily explained. The age of Class M supergiants with an initial mass of 20 M_☉ is roughly 10 million years. Given its current space motion, a projection back in time would take Betelgeuse around 290 parsecs farther from the galactic plane—an implausible hypothesis, as there is no star formation region there. Moreover, Betelgeuse's projected pathway does not appear to intersect with either the 25 Ori subassociation or the far younger Orion Nebula Cluster (ONC, also known as Ori OB1d), particularly since Very Long Baseline Array astrometry yields a distance to the ONC between 389 and 414 parsecs. Consequently, it is likely that Betelgeuse has not always had its current motion through space and has changed course at one time or another, possibly the result of a nearby stellar explosion.

The most likely star-formation scenario for Betelgeuse is that it is a runaway star from the Orion OB1 Association. Originally a member of a high-mass multiple system within Ori OB1a, Betelgeuse was probably formed about 10–12 million years ago from the molecular clouds observed in Orion, but has evolved rapidly due to its unusually high mass.

Like many young stars in Orion whose mass is greater than 10 M_☉, Betelgeuse will use its fuel quickly and not live very long. On the Hertzsprung-Russell diagram, Betelgeuse has moved off the main sequence and has swelled and cooled to become a red supergiant. Although young, Betelgeuse has probably exhausted the hydrogen in its core—unlike its OB cousins born about the same time—causing it to contract under the force of gravity into a hotter and denser state. As a result, it has begun to fuse helium into carbon and oxygen producing enough radiation to unfurl its outer envelopes of hydrogen and helium. Its extreme luminosity is being generated by a mass so large that the star will eventually fuse higher elements through neon, magnesium, sodium, and silicon all the way to iron, at which point it will probably collapse and explode as a type II supernova.

Density

Relative sizes of the planets in the Solar System and several well-known stars, including Betelgeuse
1. Mercury < Mars < Venus < Earth
2. Earth < Neptune < Uranus < Saturn < Jupiter
3. Jupiter < Wolf 359 < Sun < Sirius
4. Sirius < Pollux < Arcturus < Aldebaran
5. Aldebaran < Rigel < Antares < Betelgeuse
6. Betelgeuse < Mu Cephei < VV Cephei A < VY Canis Majoris.

As an early M-type supergiant, Betelgeuse is one of the largest, most luminous and yet one of the most ethereal stars known. A radius of 5.5 AU is roughly 1,180 times the radius of the Sun—a sphere so imposing that it could contain over 2 quadrillion Earths (2.15 × 10¹⁵) or more than 1.6 billion (1.65 × 10⁹) Suns. That is the equivalent of Betelgeuse being a giant football stadium like Wembley Stadium in London with the Earth a tiny pearl, 1 millimeter in diameter, orbiting a Sun the size of a mango. Moreover, recent observations of Betelgeuse exhibiting a 15% contraction in angular diameter would equate to a shortening of the star's radius from about 5.5 to 4.6 AU, assuming that the photosphere is a perfect sphere. A reduction of this magnitude would correspond to a diminution in photospheric volume of about 41%.

Bowl volume of Wembley Stadium. The center circle (9.15 m radius) is a close analogy for the Earth's orbit around the Sun, while the air in the stadium is actually far more dense than the star itself.

Not only is the photosphere enormous, but the star is surrounded by a complex circumstellar environment where light could take over three years just to escape. In the outer reaches of the photosphere, the density is extremely low. In volume, Betelgeuse exceeds the Sun by a factor of about 1.6 billion. Yet the actual mass of the star is believed to be no more than 20 M_☉, with mass loss estimates projected at one to two Suns since birth. Consequently, the average density of this stellar mystery is less than twelve parts-per-billion (1.119 × 10⁻⁸) that of the Sun. Such star matter is so tenuous, in fact, that Betelgeuse has often been called a "red-hot vacuum".

Circumstellar dynamics

In the late phase of stellar evolution, massive stars like Betelgeuse exhibit high rates of mass loss, possibly as much as 1 M_☉ every 10,000 years, resulting in a complex circumstellar environment that is constantly in flux. In a recent paper, Stephen Ridgeway points out that "stellar mass loss is key to understanding the evolution of the universe from the earliest cosmological times to the current epoch, and of planet formation and the formation of life itself." The physical mechanism of this mass loss, however, is not well understood. When Schwarzschild first proposed his theory of monster convection cells, he argued it was the likely cause in evolved supergiants like Betelgeuse. Recent work has corroborated this hypothesis, yet certain mysteries remain to be solved—specifically the structure of their convection, the mechanism of their mass loss, the way dust forms in their extended atmosphere, and the conditions which precipitate their dramatic finale as a type II supernova. Current observations in fact suggest that a star like Betelgeuse may spend a portion of its lifetime as a red supergiant, but then cross back across the H-R diagram, passing once again through a brief yellow supergiant phase and then exploding as either a blue supergiant or Wolf-Rayet star.

Artist's rendering from ESO showing Betelgeuse with a gigantic bubble boiling on its surface and a radiant plume of gas being ejected to at least six photospheric radii or roughly the orbit of Neptune

As a result of work done by Pierre Kervella and his team at the Paris observatory, astronomers may be close to solving this mystery. Kervella noticed a large plume of gas extending outward at least six times the stellar radius indicating that Betelgeuse is not shedding matter evenly in all directions. The plume's presence, in fact, implies that the spherical symmetry of the star's photosphere, often observed in the infrared, is not preserved in its close environment. Asymmetries on the stellar disk had been reported many times at different wavelengths. However, due to the refined capabilities of the NACO adaptive optics on the VLT, these asymmetries have come into focus. The two mechanisms that could cause such asymmetrical mass loss, Kervella noted, were large-scale convection cells or polar mass loss, possibly due to rotation. Probing deeper still with ESO's AMBER, Keiichi Ohnaka observed that the gas in the supergiant's extended atmosphere is vigorously moving up and down, creating bubbles as large as the supergiant itself, leading his team to conclude that such stellar upheaval is behind the massive plume ejection observed by Kervella.

Asymmetric shells

In addition to the photosphere, six other components of Betelgeuse's atmosphere have now been identified. They are a molecular environment otherwise known as the MOLsphere, a gaseous envelope, a chromosphere, a dust environment and two outer shells (S1 and S2) composed of carbon monoxide (CO). Some of these elements are known to be asymmetric while others overlap.

Exterior view of ESO's Very Large Telescope (VLT) in Paranal, Chile

At about 0.45 stellar radii (~2–3 AU) above the photosphere there may lie a molecular layer known as the MOLsphere or molecular environment. Studies show it to be composed of water vapor and carbon monoxide with an effective temperature of about 1500 ± 500 K. Water vapor had been originally detected in the supergiant's spectrum back in the 1960s with the two Stratoscope projects but had been ignored for decades. Recent studies suggest that the MOLsphere may also contain SiO and Al₂O₃—molecules which could explain the formation of dust particles.

Interior view of one of the four 8.2-metre Unit Telescopes at ESO's VLT

Extending for several radii (~10–40 AU) about the photosphere there exists another cool region known as an asymmetric gaseous envelope. It is enriched in oxygen and especially in nitrogen relative to carbon. These composition anomalies are likely caused by contamination by CNO processed material from the inside of Betelgeuse.

The radio-telescope images taken in 1998 confirm that Betelgeuse has a dense highly complex atmosphere, with a temperature of 3,450 ± 850K—similar to that recorded on the star's surface but much lower than surrounding gas in the same region. The VLA images also show this lower-temperature gas progressively cools as it extends outward. Although unexpected, it turns out to be the most abundant constituent of Betelgeuse's atmosphere. "This alters our basic understanding of red-supergiant star atmospheres", explained Jeremy Lim, the team's leader. "Instead of the star's atmosphere expanding uniformly because of gas heated to very high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere." This is the same region in which Kervella's 2009 finding of a bright plume, possibly containing carbon and nitrogen and extending at least six photospheric radii in the southwest direction of the star, is believed to exist.

The chromosphere, as mentioned earlier, was directly imaged by the Faint Object Camera on board the Hubble Space Telescope in the ultraviolet wavelengths. The images also revealed a bright area in the southwest quadrant of the disk. The average radius of the chromosphere in 1996 was about 2.2 times the optical disk (~10 AU) and was reported to have a temperature no higher than 5,500K. However in 2004 observations with the STIS, Hubble's high-precision spectrometer, pointed to the existence of warm chromospheric plasma at least one arcsecond away from the star. At a distance of 197 pc, the size of the chromosphere could be up to 200 AU The observations have conclusively demonstrated that the warm chromospheric plasma spatially overlaps and coexists with cool gas in Betelgeuse's gaseous envelope as well as with the dust in its circumstellar dust shells (see below).

ESO's VLT image of a complex nebula around Betelgeuse; the tiny red circle in the middle represents the photosphere, as it ejects its plume of gas into the immediate atmosphere ultimately creating a dust environment that extends ~400 AU from the star.

The first attestation of a dust shell surrounding Betelgeuse was put forth by Sutton and colleagues, who noted in 1977 that dust shells around mature stars often emit large amounts of radiation in excess of the photospheric contribution. Using heterodyne interferometry, they concluded that the red supergiant emits most of its excess beyond 12 stellar radii or roughly the distance of the Kuiper belt at 50 to 60 AU, depending on the assumed stellar radius. Since then, there have been numerous studies done of this dust envelope at varying wavelengths yielding decidedly different results. More recent studies have estimated the inner radius of the dust shell anywhere from 0.5 to 1.0 arcseconds, or 100 to 200 AU. What these studies point out is that the dust environment surrounding Betelgeuse is anything but static. In 1994, Danchi et al. reported that Betelgeuse undergoes sporadic dust production involving decades of activity followed by inactivity. A few years later, a group of astronomers led by Chris Skinner noticed significant changes in the dust shell's morphology in just one year, suggesting that the shell is asymmetrically illuminated by a stellar radiation field strongly affected by the existence of photospheric hotspots. The 1984 report of a giant asymmetric dust shell located 1 pc (206,265 AU) from the star has not been corroborated in recent studies, although another report published the same year said that three dust shells were found extending four light-years from one side of the decaying star, suggesting that, like a snake, Betelgeuse sheds its outer layers as it journeys across the heavens.

Although the exact size of the two outer CO shells remains elusive, preliminary estimates suggest that one shell extends from about 1.5 to 4.0 arcseconds with the other expanding as far as 7.0 arcseconds. Using the Jovian orbit of 5.5 AU as the "average" radius for this gargantuan star, the inner shell would extend roughly 50 to 150 stellar radii (~300 to 800 AU) with the outer one as far as 250 stellar radii (~1400 AU). With the sun's heliopause estimated at about 100 AU, the size of this outer shell is almost fourteen times the size of the Solar System.

Supersonic bow shock

Studies since the beginning of the millennium have revealed that Betelgeuse is travelling supersonically through the interstellar medium (ISM) at a speed of 30 km per second (i.e. ~6.3 AU per year) creating a bow shock. The shock is not created by the star itself, but rather a powerful stellar wind as it ejects vast amounts of gas into the ISM at a rate of 17 km/sec, heating up the material surrounding the star thereby making it visible in infrared light. Because Betelgeuse is so bright, it was only in 1997 that the bow shock was first imaged. The cometary structure is estimated to be at least 1 parsec wide, assuming a current distance of 643 light-years.

Recent 3D hydrodynamic simulations of the bow shock indicate that it is very young—less than 30,000 years old—suggesting either of two possibilities: one, that Betelgeuse moved into a region of the ISM with very different properties recently or two, that Betelgeuse itself has undergone a significant transformation as its stellar wind has changed. In their 2012 paper, Mohamed et al. propose that this phenomenon was caused by Betelgeuse transitioning from a blue supergiant (BSG) to a red supergiant (RSG). In fact, in the late evolutionary stage of a star like Betelgeuse, evidence suggests that stars "may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks." Moreover, if future research bears out this hypothesis, Betelgeuse may well prove to have traveled close to 200,000 AU as a red supergiant scattering as much as 3 ${\begin{smallmatrix}M_{\odot }\end{smallmatrix}}$ along its trajectory.

Approaching supernova

The future fate of Betelgeuse depends on its mass—a critical factor which is not well understood. Since most investigators concede a mass greater than 10 M_☉, the most likely scenario is that the supergiant will continue to burn and fuse elements until its core is iron, at which point Betelgeuse will explode as a type II supernova. During this event the core will collapse, leaving behind a neutron star remnant some 20 km in diameter.

Celestia's computerized depiction of Orion as it might appear from Earth should Betelgeuse explode as a supernova

Artist depiction of a Gamma-ray burst showing jets and supernova shell

Betelgeuse is already old for its size class and is expected to explode relatively soon compared to its age. Solving the riddle of mass-loss will be the key to knowing when a supernova may occur, an event expected anytime in the next million years. Supporting this hypothesis are a number of unusual features that have been observed in the interstellar medium of the Orion Molecular Cloud Complex, which suggest that there have been multiple supernova explosions in the recent past. Betelgeuse's suspected birthplace in the Orion OB1 Association is the probable location for such supernovae. Since the oldest subgroup in the association has an approximate age of 12 million years, the more massive stars likely had sufficient time to reach the end of their lifespan and explode already. Also, because runaway stars are believed to be caused by supernovae, there is strong evidence that OB stars μ Columbae, AE Aurigae and 53 Arietis all originated with such explosions in Ori OB1 2.2, 2.7 and 4.9 million years ago.

At its current distance from Earth, such a supernova explosion would be the brightest recorded, outshining the Moon in the night sky and becoming easily visible in broad daylight. Professor J. Craig Wheeler of The University of Texas at Austin predicts the supernova will emit 10⁵³ ergs of neutrinos, which will pass through the star's hydrogen envelope in around an hour, then reach the Solar System several centuries later. Since its rotational axis is not pointed toward the Earth, Betelgeuse's supernova is unlikely to send a gamma ray burst in the direction of Earth large enough to damage ecosystems. The flash of ultraviolet radiation from the explosion will likely be weaker than the ultraviolet output of the Sun. The supernova could brighten to an apparent magnitude of −12 over a two-week period, then remain at that intensity for 2 to 3 months before rapidly dimming. The year following the explosion, radioactive decay of cobalt to iron will dominate emission from the supernova remnant, and the resulting gamma rays will be blocked by the expanding envelope of hydrogen. If the neutron star remnant becomes a pulsar, then it could produce gamma rays for thousands of years.

Due to misunderstandings caused by the 2009 publication of the star's 15% contraction, Betelgeuse has frequently been the subject of scare stories and rumors suggesting that it will explode within a year, leading to exaggerated claims about the consequences of such an event. The timing and prevalence of these rumors have been linked to broader misconceptions of astronomy, particularly in reference to doomsday predictions relating to the Mayan calendar. In their 2012 study, the Space Sciences Laboratory physicists point out that the apparent contraction in the star's diameter may actually be due to the complex dynamics in the star's surrounding nebula and not the star itself, reconfirming the fact that until we really understand the nature of mass loss, predicting the timing of a supernova will remain a mystery.

Star system

In 1985, Margarita Karovska, in conjunction with other astrophysicists at the CfA, announced the discovery of two close companions orbiting Betelgeuse. Analysis of polarization data from 1968 through 1983 indicated a close companion with a periodic orbit of about 2.1 years. Using speckle interferometry, the team concluded that the closer of the two companions was located at 0.06 ± 0.01" (~9 AU) from the main star with a position angle (PA) of 273 degrees, an orbit that would potentially place it within the star's chromosphere. The more distant companion was estimated at 0.51 ± 0.01" (~77 AU) with a PA of 278 degrees.

In the years that followed no confirmation of Karovska's discovery was published. In 1992, a team of collaborators from the Cavendish Astrophysics Group questioned the finding. They published a paper noting that the brightness features on the surface of Betelgeuse appear to be "too bright to be associated with a passage of the suggested companions in front of the red giant." They also noticed that these features were fainter at 710 nanometers compared to 700 by a factor of 1.8, indicating that such features would have to reside within the molecular atmosphere of the star. Despite this, that same year Karovska published a new paper reconfirming her team's exegesis, but also noting that there was a meaningful correlation between the calculated position angles of the orbiting companion and the reported asymmetries, suggesting a possible connection between the two. Since then, researchers have turned their attention to analyzing the intricate dynamics of the star's extended atmosphere and little else has been published on the possibility of orbiting companions, although as Xavier Haubois and his team reiterate in 2009, the possibility of a close companion contributing to the overall flux has never been fully ruled out. Dommanget's double star catalog (CCDM) lists at least four adjacent stars, all within three arcminutes of this stellar giant, yet aside from apparent magnitudes and position angles, little else is known. As the decade unfolds and new technologies are brought to unraveling the star's enigmatic past, conclusive evidence will likely emerge of any potential star system. Given the planned capabilities of the upcoming Gaia mission, a confirmation could occur any time after the mission's scheduled launch in August 2013.

Ethnological attributes

Spelling and pronunciation

Betelgeuse has been known variously as Betelgeux, and Beteigeuze in German (according to Bode). Betelgeux and Betelgeuze were used until the early 20th century, when the spelling Betelgeuse became universal. There is no consensus for the correct pronunciation of the name, and pronunciations for the star are as varied as its spellings:

• /ˈbɛtəldʒuːz/ Oxford English Dictionary and Royal Astronomical Society of Canada
• /ˈbiːtəldʒuːz/ Oxford English Dictionary
• /ˈbɛtəldʒəz/ (The Friendly Stars)
• /ˈbiːtəldʒuːs/ (Canadian Oxford Dictionary, Webster's Collegiate Dictionary)

Etymology

There is some uncertainty surrounding the first element of the name, rendered as "Bet-". However, "abet" or إبط is the Arabic word for "armpit", which is where the star is located in the Orion constellation. Betelgeuse is often mistranslated as "armpit of the central one". In his 1899 work Star-Names and Their Meanings, American amateur naturalist Richard Hinckley Allen stated the derivation was from the ابط الجوزاء Ibṭ al-Jauzah, which he claimed degenerated into a number of forms including Bed Elgueze, Beit Algueze, Bet El-gueze, Beteigeuze and more, to the (then) current forms Betelgeuse, Betelguese, Betelgueze and Betelgeux. The star was named Beldengeuze in the Alfonsine Tables, and Italian Jesuit priest and astronomer Giovanni Battista Riccioli had called it Bectelgeuze or Bedalgeuze. Paul Kunitzsch, Professor of Arabic Studies at the University of Munich, refuted Allen's derivation and instead proposed that the full name is a corruption of the Arabic يد الجوزاء Yad al-Jauzā' meaning "the Hand of al-Jauzā'", i.e., Orion. European mistransliteration into medieval Latin led to the first character y (ﻴ, with two dots underneath) being misread as a b (ﺒ, with only one dot underneath). During the Renaissance, the star's name was written as بيت الجوزاء Bait al-Jauzā' ("house of Orion") or بط الجوزاء Baţ al-Jauzā', incorrectly thought to mean "armpit of Orion" (a true translation of "armpit" would be ابط, transliterated as Ibţ). This led to the modern rendering as Betelgeuse. Other writers have since accepted Kunitzsch's explanation.

The last part of the name, "-elgeuse", comes from the Arabic الجوزاء al-Jauzā', a historical Arabic name of the constellation Orion, a feminine name in old Arabian legend, and of uncertain meaning. Because جوز j-w-z, the root of jauzā', means "middle", al-Jauzā' roughly means "the Central One". Later, al-Jauzā' was also designated as the scientific Arabic name for Orion and for Gemini. The current Arabic name for Orion is الجبار al-Jabbār ("the Giant"), although the use of الجوزاء al-Jauzā' in the name of the star has continued. The 17th century English translator Edmund Chilmead gave it the name Ied Algeuze ("Orion's Hand"), from Christmannus. Other Arabic names recorded include Al Yad al Yamnā ("the Right Hand"), Al Dhira ("the Arm"), and Al Mankib ("the Shoulder"), all appended to "of the giant", as منكب الجوزاء Mankib al Jauzā'. In Persian, however, the name is اِبطالجوزا, derived from the Arabic ابط الجوزاء Ibţ al-Jauzā', "armpit of Orion".

Dunhuang Star Chart, circa AD 700, showing 参宿四 Shēnxiùsì (Betelgeuse), the Fourth Star of the constellation of Three Stars

Other names

Other terms for Betelgeuse included the Persian Bašn "the Arm", and Coptic Klaria "an Armlet". Bahu was its Sanskrit name, as part of a Hindu understanding of the constellation as a running antelope or stag. In traditional Chinese astronomy, Betelgeuse was known as 参宿四 (Shēnxiùsì, the Fourth Star of the constellation of Three Stars) as the Chinese constellation 参宿 originally referred to the three stars in the girdle of Orion. This constellation was ultimately expanded to ten stars, but the earlier name stuck. In Japan, this star was called Heike-boshi, (平家星), and alludes to a legendary war in Japanese history between two powerful families, the Taira or Heike clan, who had adopted Betelgeuse and its red color as its symbol, and the Minamoto or Genji clan who chose the white star Rigel.

In Tahitian lore, Betelgeuse was one of the pillars propping up the sky, known as Anâ-varu, the pillar to sit by. It was also called Ta'urua-nui-o-Mere "Great festivity in parental yearnings". A Hawaiian term for it was Kaulua-koko "brilliant red star". The Lacandon people of Central America knew it as chäk tulix "red butterfly".

Mythology

With the history of astronomy intimately associated with mythology and astrology before the scientific revolution, the red star, like the planet Mars that derives its name from a Roman war god, has been closely associated with the martial archetype of conquest for millennia, and by extension, the motif of death and rebirth. Other cultures have produced different myths. Stephen R. Wilk has proposed the constellation of Orion could have represented the Greek mythological figure Pelops, who had an artificial shoulder made of ivory made for him, with Betelgeuse as the shoulder, its color reminiscent of the reddish yellow sheen of ivory.

In the Americas, Betelgeuse signifies a severed limb of a man-figure (Orion)—the Taulipang of Brazil know the constellation as Zililkawai, a hero whose leg was cut off by his wife, with the variable light from Betelgeuse linked to the severing of the limb. Similarly, the Lakota people of North America see it as a chief whose arm has been severed.

A Sanskrit name for Betelgeuse was ãrdrã "the moist one", eponymous of the Ardra lunar mansion in Hindu astrology. It was linked via Orion's association with stormy weather to the Rigvedic God of storms Rudra. The constellations in Macedonian folklore represented agricultural items and animals, reflecting their village way of life. To them, Betelgeuse was Orach "the ploughman", alongside the rest of Orion which depicted a plough with oxen. The rising of Betelgeuse at around 3 a.m. in late summer and autumn signified the time for villager menfolk to go to the fields and plough. In South African mythology, Betelgeuse was perceived as a lion casting a predatory gaze toward the three zebras represented by Orion's belt.

The opposed locations of Orion and Scorpio, with their corresponding bright variable red stars Betelgeuse and Antares, were noted by ancient cultures around the world. The setting of Orion and rising of Scorpio signify the death of Orion by the scorpion. In China they signify brothers and rivals Shen and Shang. The Batak of Sumatra marked their New Year with the first new moon after the sinking of Orion's Belt below the horizon, at which point Betelgeuse remained "like the tail of a rooster". The positions of Betelgeuse and Antares at opposite ends of the celestial sky were significant, with their corresponding constellations seen as a pair of scorpions. Scorpion days marked as nights that both constellations could be seen.

In popular culture

See also: Betelgeuse in fiction

The star's unusual name inspired the 1988 film Beetlejuice, and script writer Michael McDowell was impressed at how many people made the connection. He added that they had received a suggestion the sequel be named Sanduleak-69 202 after the former star of SN 1987A. The correspondence between the star's mythological motif of death and rebirth, the fact that Betelgeuse is believed to be a dying star and the film's theme of ghosts haunting the living is particularly poignant, especially in light of Carl Jung's theory of synchronicity. In August Derleth's short story "The Dweller in the Darkness" set in H. P. Lovecraft's Cthulhu Mythos, Betelgeuse is the home of the 'benign' Elder Gods. There has been much debate over the identity of the red star Borgil mentioned in Lord of the Rings, with Aldebaran, Betelgeuse and even the planet Mars touted as candidates. Professor Kristine Larsen has concluded the evidence points to it being Aldebaran as it precedes Menelvagor (Orion). Astronomy writer Robert Burnham, Jr. proposed the term padparadaschah which denotes a rare orange sapphire in India, for the star. In the popular science fiction series "The Hitchhiker's Guide to the Galaxy" by Douglas Adams, Ford Prefect was from "a small planet somewhere in the vicinity of Betelgeuse." In the poetic work "Betelguese, a trip through hell" by Jean Louis De Esque, hell is located on Betelguese because De Esque believed that it was "a celestial pariah, an outcast, the largest of all known comets or outlawed suns in the universe."

There have been two American navy ships named after the star, both World War II vessels, the USS Betelgeuse (AKA-11) launched in 1939 and USS Betelgeuse (AK-260) launched in 1944. In 1979, a French supertanker named Betelgeuse was moored off Whiddy Island discharging oil when it exploded, killing 50 people in one of the worst disasters in Ireland's history.

External links

• Surface imaging of Betelgeuse with COAST and the WHT Interferometric images taken at different wavelengths.
• Near, Mid and Far Infrared Infrared Processing and Analysis Center (IPAC) webpage showing pictures at various wavelengths.
• APOD Pictures:

1. Mars and Orion Over Monument Valley Skyscape showing the relative brightness of Betelgeuse and Rigel.
2. Orion: Head to Toe Breathtaking vista the Orion Molecular Cloud Complex from Rogelio Bernal Andreo.
3. The Spotty Surface of Betelgeuse A reconstructed image showing two hotspots, possibly convection cells.
4. Simulated Supergiant Star Freytag's "Star in a Box" illustrating the nature of Betelgeuse's "monster granules".
5. Why Stars Twinkle Image of Betelgeuse showing the effect of atmospheric twinkling in a microscope.

• Red supergiant movie Numerical simulation of a red supergiant star like Betelgeuse.

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