Betelgeuse

This article 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″
Apparent magnitude (V)	0.42 (0.3 to 1.2)
Characteristics
Spectral type	M2Iab
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)	−6.05
Details
Mass	~18–19 M☉
Radius	~1180 R☉
Surface gravity (log g)	-0.5
Luminosity	~140,000 L☉
Temperature	3,500 K
Metallicity	0.05 Fe/H
Rotation	5 km/s
Age	~1.0×107 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, also known by its Bayer designation Alpha Orionis (α Orionis, α Ori), is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel (Beta Orionis) only rarely. Distinctly reddish-tinted, it is a semiregular variable star whose apparent magnitude varies between 0.2 and 1.2, the widest range of any first magnitude star. The star marks the upper right vertex of the Winter Triangle and center of the Winter Hexagon.

Classified as a red supergiant, Betelgeuse is one of the largest and most luminous stars known. If it were at the center of our 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. However, with distance estimates in the last century that have ranged anywhere from 180 to 1,300 light years from Earth, calculating its diameter, luminosity and mass have proven difficult. Betelgeuse is currently thought to lie around 640 light years away, yielding a mean absolute magnitude of about −6.05.

In 1920, Alpha Ori was the first star (after the Sun) to have its angular diameter measured. Since then, researchers have used a number of telescopes to measure this stellar giant, each with different technical parameters, often yielding conflicting results. Current estimates of the star's apparent diameter range from about 0.043 to 0.056 arcseconds. This is a moving target at best, as Betelgeuse appears to change shape periodically. Because of limb darkening, variability, and angular diameters that vary with wavelength, the star remains a perplexing mystery. To complicate matters further, Betelgeuse has a complex, asymmetric envelope caused by colossal mass loss involving huge plumes of gas being expelled from its surface. There is even evidence of stellar companions orbiting within this gaseous envelope, possibly contributing to the star's eccentric behavior.

Astronomers believe Betelgeuse is only 10 million years old, but has evolved rapidly because of its high mass. It is thought to be a runaway star from the Orion OB1 Association, which also includes the late type O and B stars in Orion's belt—Alnitak, Alnilam and Mintaka. Currently in a late stage of stellar evolution, Betelgeuse is expected to explode as a type II supernova, possibly within the next million years.

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 contrast, the historical record of Chinese astronomers during the first century BC mention Betelgeuse as having a yellow color. Prior to the modern systems of stellar classification, Angelo Secchi had created his own system of spectral analysis with Antares and Betelgeuse as the prototypes for his Class III (orange to red) stars.

With the history of astronomy intimately associated with mythology and astrology prior to 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. In South African mythology, Betelgeuse was a lion watching in a predatory manner the three zebras represented by Orion's belt.

Herschel's discovery

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; he noted an increase in activity from 1836–1840, followed by a subsequent reduction. 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 from 1957 to 1967. The records of the American Association of Variable Star Observers (AAVSO) show a maximum apparent magnitude (brightness) of 0.2 in the years 1933 and 1942, with a minimum fainter than magnitude 1.2 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 (20 ft) interferometer on the front of the 2.5 metre (100 inch) telescope at Mount Wilson Observatory. Helped by John Anderson, the trio measured the angular diameter of α Orionis at 0.047", a figure which resulted in a diameter of 3.84 × 10⁸ km (240 million miles or 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 conducted multiple investigations ranging from the ultraviolet to the mid infrared with controvertible 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 nascent computer technology 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

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 images, this ultraviolet image also contained a bright patch, indicating a hotter region of about 2,000K, in this case on the southwestern portion of the star's 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 α Ori's complex circumstellar shells. At the dawn of the millennium, Betelgeuse 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. He presented evidence that UC Berkeley's ISI atop Mt. Wilson Observatory had observed 15 consecutive years of stellar contraction. Despite the apparent diminution of Betelgeuse's size, Townes and his colleague, Edward Wishnow, pointed out that the star's visible brightness, or magnitude, which is monitored regularly by members of the American Association of Variable Star Observers (AAVSO), had shown no significant dimming over that same time frame. This finding of a diminishing radius coupled with a relatively constant flux puts into question some of the fundamental theories of stellar structure.

ESO's VLT image of nebula around Betelgeuse. The small red circle in the middle has a diameter about four and half times that of the Earth's orbit and represents the location of Betelgeuse's visible surface.

Enveloping this whole discussion have been numerous inquiries into the abstruse dynamics of Betelgeuse's extended atmosphere. For decades astronomers have understood that red giants dominate mass return to the Galaxy creating opaque outer shells, yet the actual mechanics of such stellar mass loss have remained 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 vast plumes 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 6 different shells surrounding Betelgeuse. As the century unfolds, 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

The location of Betelgeuse near the famous "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, the star is due south in the evening sky and visible to virtually every inhabited region of the globe, with only a few obscure research stations in Antarctica at latitudes south of 82° unable to see it. In large cities in the Southern Hemisphere (e.g., Sydney, Buenos Aires, and Cape Town) the star rises almost 49° above the horizon. Once May arrives, the red giant can be glimpsed but briefly on the western horizon just after the Sun sets.

The apparent magnitude of α Ori is listed in SIMBAD at 0.42, making it on average the ninth brightest star in the celestial sphere—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 eighth brightest star. As Rigel, with a nominal apparent magnitude of 0.12, has been reported to fluctuate slightly in brightness, by 0.03 to 0.3 magnitudes, it is also possible for Betelgeuse to occasionally outshine Rigel and become the seventh brightest star. At its faintest, it 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 apparently on radial velocity fluctuations in the photosphere. Only about 13% of the star's radiant energy is emitted in the form of visible light, with most of its radiation occurring in the infrared. If our eyes were sensitive to radiation at all wavelengths, Betelgeuse would appear as the brightest star in the sky. Betelgeuse is the brightest near-IR source in the sky with a J band magnitude of -2.99.

Parallax

Since the first successful parallax measurement was conducted in 1838 by Friedrich Bessel, astronomers have been puzzled by Betelgeuse's distance, the uncertainty of which has made reliable estimates of many stellar parameters difficult. An accurate distance and angular diameter will reveal a star's radius and effective temperature, leading to a clear understanding of bolometric luminosity; luminosity combined with an understanding of isotopic abundances provides an estimate 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 an ongoing inquiry as to the actual distance of this mysterious star with proposed distances as high as 400 parsecs or about 1,300 light years.

Before the publication of the Hipparcos Catalogue (1997), there were two respected publications with up-to-date parallax data on 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, a phenomenon which fueled much debate—not only in terms of the star's distance, but also its impact on other stellar parameters.

Image showing the NRAO Very Large Array located at Socorro, New Mexico, USA. Each of the 27 antennas weighs 209 metric tons and can be moved as needed on railroad tracks allowing the array to perform detailed studies using aperture synthesis interferometry.

The long-awaited results from the Hipparcos mission were finally released in 1997. Instead of resolving the issue, a new parallax figure was published of π = 7.63 ± 1.64 mas, which equated to a distance of 131 pc or roughly 430 ly. Because stars like Betelgeuse vary in brightness, they raise specific problems in quantifying their distance. As a result, the large cosmic error in the Hipparcos solution could well be of stellar origin, relating possibly to movements of the photocenter, of order 3.4 mas, in the Hipparcos photometric Hp band.

Recent advances in radio astronomy appear to have prevailed in this debate. New high spatial resolution, multi-wavelength radio positions of Betelgeuse conducted by Graham Harper and colleagues using NRAO's Very Large Array (VLA) have produced a more precise estimate, which combined with the recent Hipparcos data furnished a new astrometric solution: π = 5.07 ± 1.10 mas, a tighter error factor yielding a distance of 197 ± 45 pc or 643 ± 146 ly.

The next computational 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. The nearest stars, in fact, will have their distances measured to within an unprecedented 0.001% error factor. Even stars near the Galactic centre, some 30,000 light-years away, will have their distances measured to within a factor of 20%.

Variability

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

As a pulsating variable star with sub-classification "SRC", researchers have offered different hypotheses to explain α Ori's volatile choreography—a phenomenon which causes an absolute magnitude oscillation from −5.27 and −6.27. Our current understanding of stellar structure suggests 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—thus 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, which points to complex activity on the star's surface and in 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. In a landmark paper published in 1975, Martin Schwarzschild attributed these brightess 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 (yielding 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 of such granules lying on the Sun's photosphere, which because of their number produce a relatively constant flux. Beneath these granules, it is conjectured that there are 5 to 10 thousand supergranules, the average diameter of which is 30,000 km with a depth of about 10,000 km. 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 brightness variations of the integrated light of the star.

Schwarzschild's hypothesis of gigantic convection cells dominating the surface of red giants and supergiants seems to have stuck with 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 the year 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.

Angular size

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 diameter 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 showing the size of Betelgeuse's photosphere (circle) and the effect of convective forces on the star's asymmetric atmosphere as it expands beyond the orbit of Saturn.

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 edge of the photosphere;
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 problematic;
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. The concept of astronomical interferometry was first conceived by Hippolyte Fizeau in 1868. He proposed the observation of stars through two apertures to obtain interferences that would furnish information on the star's spatial intensity distribution. Since then, the science of interferometry has evolved considerably where multiple-aperture interferometers are now used consisting of numerous images superimposed on each other. These "speckled" images are then synthesized using Fourier analysis—a method used for a wide array of astronomical objects including the study of binary stars, quasars, asteroids and galactic nuclei. The emergence of adaptive optics since 1990 has revolutionized high angular resolution astronomy, while space observatories like Hipparcos, Hubble and Spitzer have produced other significant breakthroughs. Recently another instrument, the Astronomical Multi-BEam Recombiner (AMBER), is providing new insights. As part of the VLTI, AMBER is capable of combining the beams of three telescopes simultaneously, allowing researchers to achieve milliarcsecond spatial resolution. Also by combining three baselines instead of two, which is customary with conventional interferometry, AMBER enables astronomers to compute the closure phase—an important element in astronomical imaging.

The current debate revolves around which wavelength—the visible, near-infrared (NIR) or mid-infrared (MIR)—produces the most accurate angular measurement. The most widely adopted solution, it appears, is the one performed with the ISI in the mid-infrared by astronomers from the Space Sciences Laboratory at U.C. Berkeley. In the epoch year 2000, the group, under the leadership of John Weiner, published a paper showing Betelgeuse with a uniform disk of 54.7 ± 0.3 mas, ignoring any possible contribution from hotspots, which are less noticeable in the mid-infrared. The paper also included a theoretical allowance for limb darkening yielding a diameter of 55.2 ± 0.5 mas—a figure which equates to a radius of roughly 5.5 AU (1,180 R_☉), assuming a distance of 197.0 ± 45 pc. Nevertheless, given the angular error factor of ± 0.5 mas combined with a parallax error of ± 45 pc found in Harper's numbers, the photosphere's radius could actually be as small as 4.2 AU or as large as 6.9 AU.

Across the Atlantic, another team of astronomers led by Guy Perrin of the Observatoire de Paris produced a document in 2004 arguing that the near-infrared figure of 43.33 ± 0.04 mas was a more accurate photospheric measurement. "A consistent scenario to explain the observations of this star from the visible to the mid-infrared can be set-up", Perrin reports. "The star is seen through a thick, warm extended atmosphere that scatters light at short wavelengths thus slightly increasing its diameter. The scatter becomes negligible above 1.3 μm. The upper atmosphere being almost transparent in K and L—the diameter is minimum at these wavelengths where the classical photosphere can be directly seen. In the mid-infrared, the thermal emission of the warm atmosphere increases the apparent diameter of the star." The argument has yet to receive widespread support among astronomers.

More recent studies done in the near-infrared with the IOTA and VLTI have brought strong support to Perrin's analysis yielding diameters that range from 42.57 to 44.28 mas with minimal error factors less than 0.04 mas. Central to this discussion, however, is a second 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 horizon 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 angular separation 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 or roughly 1,000 km per hour. What is not fully known is whether this observation is evidence of a rhythmic expansion and contraction of the star as astronomers have theorized, and if so, what the periodic cycle might be, although Townes suggests 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 but the appearance of expansion and contraction as the star rotates on its axis. Consequently, until a full cycle of data has been gathered, we will not know whether the 1993 figure of 56.0 mas represents a maximum extension of the star or its mean, or whether the 2008 figure of 47.0 in fact represents a minimum. It will probably take another 15 years or longer (2025) before we know with any certainty, meaning that the Jovian orbit of 5.5 AU will probably serve as the star's "average" radius for some time.

Once considered as having the largest angular diameter of any star in the sky after the Sun, in 1997 Betelgeuse lost that distinction 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

Betelgeuse's spectral class is listed as M2Iab in the SIMBAD astronomical database, signifying that it is a red Class M star. The "ab" suffix is derived from the Yerkes spectral classification system, and indicates that it is an intermediate luminous supergiant, less bright than other supergiants like Deneb. However, given some of the recent findings, this classification may be outdated, as there is evidence Betelgeuse is actually much more luminous than Deneb and other stars in its class.

Assuming average radius of 5.5 AU and a distance of 197 pc, theoretical calculations would yield a luminosity figure in excess of 180,000 Suns (L_☉) at maximum. When the star contracts as it appears to have since 1993, its luminosity would diminish to about 130,000L_☉. Either way, that amount of electromagnetic energy dwarfs Deneb's output of about 50,000L_☉. But with most of the star's radiant energy occurring in the infrared and huge amounts being absorbed by circumstellar matter, the human eye simply cannot perceive the star's intrinsic brightness.

Given the many uncertainties surrounding Betelgeuse, no consensus has yet emerged regarding the star's mass. Estimates range from 5 to 30 solar masses (M_☉) with most investigators showing a preference for a relatively large mass ranging from 10 to 20M_☉. One model reports a mass at the lower end of the scale at 14M_☉, although a mass ranging from 18 to 20 is more commonplace.

Typical of red supergiants, Betelgeuse is a cool star with surface temperatures in the last decade reported between 3,500 to 3,600K. It 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—extremely slow when compared with a fast rotator like Pleione in the Pleiades star cluster, which turns on its axis once every 11.8 hours.

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 a Class M supergiant with an initial mass of 20M_☉ 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 where there is no star formation region—an implausible scenario. Although the radial velocity and proper motion for the 25 Ori subassociation have yet to be measured, α Ori's projected pathway across the heavens does not appear to intersect with it either. Also, formation close to the far younger Orion Nebula Cluster (ONC, also known as Ori OB1d) is doubtful. 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 of the young stars in Orion where masses greater than 10 solar can be found in abundance, 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 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 huge 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. Sirius, by contrast, with a radius of 1.71R_☉, would be roughly the size of a soccer ball. 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 approximately 41% or 680 million Suns.

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 vast and 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 18 to 19 Suns (M_☉), with certain 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. If we compare such star matter to the density of ordinary air at sea level, the ratio is roughly 1.286 × 10⁻⁵, a density so ethereal, one would have to soar above the noctilucent clouds in the Earth's mesosphere to experience it. 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 1M_☉ every 10,000 years, resulting in a complex circumstellar environment that is constantly in flux. All stars exhibit mass loss. Rates vary from about 10⁻¹⁴ to 10⁻⁴ $\begin{smallmatrix}M_{\odot} \end{smallmatrix}$ yr ^-1 depending on spectral type, luminosity class, rotation rate, companion proximity, and evolutionary stage. Exactly how this mass loss occurs, however, has been a mystery confronting astronomers for decades. When Schwarzschild first proposed his theory of monster convection cells, he argued it was the likely cause in red supergiants. Prior attempts to explain mass loss in terms of solar wind theory had proven unsuccessful as they led to a contradiction with observations involving circumstellar shells. Other theories that have been advanced include magnetic activity, global pulsations and shock structures as well as stellar rotation.

Artist's rendering from ESO showing Betelgeuse with a gigantic bubble boiling on its surface and a vast 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 in 2009, astronomers may be close to solving this mystery. What Kervella noticed was a large plume of gas extending outward at least six times the stellar radius indicating that the star is not shedding matter evenly in all directions. The plume's presence, in fact, implies that the spherical symmetry of its 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 the AMBER instrument on ESO's Very Large Telescope Interferometer, Keiichi Ohnaka from the Max Planck Institute in Bonn 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.

Evidence of circumstellar shells surrounding M supergiants was first proposed by Walter Adams and Elizabeth MacCormack in 1935 when they observed anomalies in the spectral signature of such stars and concluded that the likely cause was an expanding gaseous envelope. The first indication of vastly extended envelopes occurred in 1955 with the work of Armin Deutsch who noticed when studying the Rasalgethi system that spectroscopic peculiarities were mysteriously occurring in the G star companion, α² Her. This led him to conclude that the whole system had to be enveloped by a circumstellar shell composed of matter being ejected by the main star, M supergiant α¹ Her, and extending to at least 170 stellar radii. In the mid 1970s, Andrew Bernat undertook a detailed analysis of four circumstellar shells, Betelgeuse, Antares, Rasalgethi and Mu Cephei, concluding that red stars dominate mass return to the Galaxy.

In addition to the photosphere, researchers have now identified six other components of Betelgeuse's complex atmosphere. Extending outward, we find a compact 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). There is also evidence of coronal plasma in the star's extended atmosphere, a phenomenon that heretofore was not believed to exist in late stage stars off the main sequence. 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 .45 stellar radii (~2−3 AU) above the photosphere, the closest membrane appears to be the molecular layer known as the MOLsphere. Studies show it to be composed of water-vapor and carbon monoxide with an effective temperature of about 1500 ± 500K. 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.

Between two and seven stellar radii (~10−40 AU), astronomers have identified another region known as an asymmetric gaseous envelope composed of elemental abundances C, N and O. The radio-telescope images taken in 1998 confirm that Betelgeuse has a dense atmosphere with a "remarkably complex structure". Observations show the atmosphere to be boiling with a temperature of 3,450 ± 850K—similar to the temperature recorded on the star's surface but much lower than surrounding gas in the same region. The VLA images also showed this lower-temperature gas progressively decreasing in temperature as it extends outward—the existence of which, although unexpected, 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 CN and extending at least six photospheric radii in the southwest direction of the star, is believed to exist.

The chromosphere, as mentioned earlier, was resolved in 1996 at about 2.2 times the optical disk (~10 AU) at ultraviolet wavelengths and is reported to have a temperature no higher than 5,500K. The image was taken with the Faint Object Camera on-board the Hubble Space Telescope and also revealed a bright area in the southwest quadrant of the disk. 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 200 AU or seven times the Neptunian orbit. The CfA team led by Alex Lobel concluded that the spatially resolved STIS spectra directly demonstrate the co-existence of warm chromospheric plasma with cool gas in Betelgeuse's circumstellar dust envelope.

The first attestation of a dust shell surrounding Betelgeuse was put forth by Sutton et al. when the team noted in 1977 that dust shells around mature stars often emit large amounts of radiation in excess of the photospheric contribution. Using heterodyne interferometry, the team concluded that α Ori emits the majority of its excess outside 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, hence 25 to 50 stellar radii 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 hot spots. 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, (~0.055" diameter), the inner shell would extend from roughly 50 to 150 stellar radii (~300 to 800 AU) with the outer shell extending out as far as 250 stellar radii (~1400 AU). With the heliopause estimated at about 100 AU, the size of this outer shell is almost fourteen times the size of our Solar System.

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 10M_☉, 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 evolve to this stage. Also, because runaway stars are believed to be caused by supernova explosions, there is strong evidence that OB stars μ Columbae, AE Aurigae and 53 Arietis all originated with such an explosion 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.

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. The team realized that the observed polarization could be caused by a systemic asymmetry created by the close companion orbiting the supergiant inside its extended dust envelope. 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. The magnitude differences with respect to the primary, measured at 656.3 (Hα) and 656.8 nm (red continuum), were 3.4 and 3.0 for the close component and 4.6 and 4.3 for the distant component.

Illustration of an aperture mask as used by Buscher et al. in their discovery of hotspots—a finding which seems to have displaced the quest for stellar companions.

Images from NASA's Spitzer Space Telescope showing that asymmetrical envelopes can trigger the formation of tightly knit binary star systems.

In the years that followed, astronomers conducted a number of studies in the hope of obtaining additional confirmation. In 1987, Andrea Dupree, one of Karovska's colleagues at the CfA, observed: "Periastron of the recently discovered close optical companion to Alpha Ori is predicted to be 1986.7; detection of atmospheric disturbances similar to those found subsequent to the last periastron (~ 1984.6) would give strong support to the presence of a companion." However, in the years that followed no such confirmation was ever published. Rather, in 1990, David F. Buscher, John E. Baldwin and a team of collaborators from the Cavendish Astrophysics Group made a number of high-resolution images of the supergiant at wavelengths of 633, 700, and 710 nm using the nonredundant masking method. At all these wavelengths, they remarked, there was unambiguous evidence for an asymmetric feature on the surface of the star, which contributed 10−15 percent of the star's total observed flux. Their conclusion was that such a phenomenon could be caused by a close companion passing in front of the stellar disk, differential photospheric brightening due to the effects of stellar rotation or the more likely scenario of "large-scale convection in the stellar atmosphere" as suggested by Schwarzschild.

The Cavendish colleagues published another paper in 1992, this time under the helm of Richard. W. Wilson, 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.

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. She observed that any such correlation could be caused one of three factors: a companion star, tidal distortion of the supergiant's atmosphere, or by a photospheric bright spot. She concludes: "To determine the nature of the companion (which presently remains a puzzle), it is crucial to obtain further speckle observations using large aperture telescopes, coordinated with other ground-based observations and the observations from space."

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, we will likely see conclusive evidence, one way or another, 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 December 2012.

Name

Spelling

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.

Image of Islamic Celestial Globe, circa AD 1630, showing "al-Jauzā" (Betelgeuse) and belt of Orion in the foreground. The brass globe served both as a map of the heavens and a precision tool for making astronomical calculations.

Pronunciation

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)
• /ˈbɛtəldʒuːs/

Etymology

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.

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 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 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".

Allen linked Orion's association with stormy weather to that of this deity of storms. Bahu was its Sanskrit name, as part of a Hindu understanding of the constellation as a running antelope or stag. Other terms included the Persian Bašn "the Arm" via Brown, and Coptic Klaria "an Armlet".

A Sanskrit name for Betelgeuse was ãrdrã "the moist one", eponymous of the Ardra lunar mansion in Hindu astrology.^{[citation needed]} In traditional Chinese astronomy, Betelgeuse was known as 参宿四 (Shēnxiùsì, the Fourth (Star of the constellation) of Three (Stars)) as the constellation of 参宿 was at first a name only for the three stars in the girdle of Orion. Four stars were later added to this constellation, but the earlier name stuck.^{[citation needed]} In Japan, this star was called Heike-boshi (suggested by the red butterfly flag of the Heike clan), (平家星), "the Star of the Heike clan" or Kin-waki, (金脇), "the Gold (Star) beside (Mitsu-boshi)." In Malayalam and Tamil the star is named Thiruvathira.

In popular culture

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 they had received a suggestion the sequel be named Sanduleak-69 202 after the former star of SN 1987A. 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."

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.

Italian electronica band Overmamba released a song on Crawl Out, their debut album, called Betelgeuse and largely inspired by the star; the lyrics tell of a fantasy space-time warping trip around the star and its planets.

See also

• List of brightest stars
• List of largest known stars
• List of most luminous stars
• List of most massive stars
• List of supernova candidates
• Betelgeuse in fiction

External links

• Alpha Orionis (Betelgeuse) AAVSO Variable Star of the Month article, December 2000.
• Surface imaging of Betelgeuse with COAST and the WHT Interferometric images taken at different wavelengths.
• Detailed image of Betelgeuse on Astronoo
• 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 Stunning skyscape showing the relative brightness of Betelgeuse and Rigel.
2. The Spotty Surface of Betelgeuse A reconstructed image showing two hotspots, possibly convection cells.
3. Simulated Supergiant Star Freytag's "Star in a Box" illustrating the nature of Betelgeuse's "monster granules".
4. Why Stars Twinkle Image of Betelgeuse showing the effect of atmospheric twinkling in a microscope.
5. Canaries Sky The glowing nebulas surrounding Betelgeuse.

• Red supergiant movie Numerical simulation of a red supergiant star like Betelgeuse.
• Comments on Betelgeuse’s explosion in 2012