Betelgeuse

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/, /ˈbɛtəldʒuːz/[1] or /ˈbiːtəldʒuːs/[2]
Right ascension	05^h 55^m 10.30536^s[3]
Declination	+07° 24′ 25.4304″[3]
Characteristics
Evolutionary stage	Red supergiant
Spectral type	M1–M2 Ia–ab[4]
Apparent magnitude (V)	0.50[5] (0.0- 1.3[6])
Apparent magnitude (J)	-3.00[7]
U−B color index	+2.06[5]
B−V color index	+1.85[5]
Variable type	SRc[6]
Astrometry
Radial velocity (Rv)	+21.91[8] km/s
Proper motion (μ)	RA: 24.95 ± 0.08[9] mas/yr Dec.: 9.56 ± 0.15[9] mas/yr
Parallax (π)	5.07 ± 1.10[9] mas
Distance	643 ± 146[9] ly (197 ± 45[9] pc)
Absolute magnitude (MV)	−5.85[9]
Details
Mass	11.6+5.0 −3.9[10] M☉
Radius	955[10] – 1,200[11] R☉
Luminosity	90000 – 150000[11] L☉
Surface gravity (log g)	−0.5[12] cgs
Temperature	3300[11] – 3650[13] K
Metallicity	0.05 Fe/H[14]
Rotational velocity (v sin i)	5[15] km/s
Age	8.0 – 8.5[16] Myr
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

Coordinates: 05^h 55^m 10.3053^s, +07° 24′ 25.426″

Betelgeuse, also known by its Bayer designation Alpha Orionis (shortened to α Orionis or α Ori), is the ninth-brightest star in the night sky and second-brightest in the constellation of Orion. Distinctly reddish, it is a semiregular variable star whose apparent magnitude varies between 0.0 and 1.3, the widest range of any first-magnitude star. Betelgeuse is one of three stars that make up the Winter Triangle, and it marks the center of the Winter Hexagon. The star's name is derived from the Arabic إبط الجوزاء Ibt al-Jauzā' or يد الجوزاء Yad al-Jauzā', meaning "the hand of Orion".

The star is classified as a red supergiant of spectral type M1-2 and is one of the largest and most luminous observable stars. If Betelgeuse 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. Estimates of its mass are poorly constrained, but range from 5 to 30 times that of the Sun. Its distance from Earth was estimated in 2008 at 640 light-years, yielding a mean absolute magnitude of about −6. 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 includes the stars in Orion's Belt—this crimson runaway has been observed moving through the interstellar medium at a supersonic speed of 30 km/s, 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 life cycle before exploding as a type II supernova within the next million years. An observation by the Herschel Space Observatory in January 2013 revealed that the star's winds are crashing against the surrounding interstellar medium.^[17]

In 1920, Betelgeuse became the second star (after the Sun) to have the angular size of its photosphere measured. Since then, researchers have used telescopes with different technical parameters to measure the stellar giant, often with conflicting results. Studies since 1990 have produced an angular diameter (apparent size) ranging from 0.043 to 0.056 arcseconds, an incongruity largely caused by the star's tendency to periodically change shape. Due to limb darkening, variability, and angular diameters that vary with wavelength, many of the star's properties are not yet known with any certainty. Adding to these challenges, the surface of Betelgeuse is obscured by a complex, asymmetric envelope roughly 250 times the size of the star, caused by colossal mass loss.

Observational history

Betelgeuse and its red coloration have been noted since antiquity; the classical astronomer Ptolemy described its color as ὑπόκιρρος (hypókirrhos), a term that was later described by a translator of Ulugh Beg's Zij-i Sultani as rubedo, Latin for "ruddiness".^[18][19] 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.^[20] 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,^[21] a possibility given current research into the complex circumstellar environment of these stars.^[22]

Nascent discoveries

Sir John Herschel in 1867

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 when Betelgeuse outshone Rigel in October 1837 and again in November 1839.^[23] A 10-year quiescent period followed; then in 1849, Herschel noted another short cycle of variability, which peaked in 1852. Later observers recorded unusually high maxima with an interval of 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 1933 and 1942, and a minimum of 1.2, observed in 1927 and 1941.^[24][25] 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 rivaled the usually brighter Rigel (beta).^[26] From Arctic latitudes, Betelgeuse's red colour and higher location in the sky than Rigel meant the Inuit regarded it as brighter, and one local name was Ulluriajjuaq "large star".^[27]

In 1920, Albert Michelson and Francis Pease mounted a 6-meter interferometer on the front of the 2.5-meter 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 parallax value of 0.018".^[28] However, limb darkening and measurement errors resulted in uncertainty about the accuracy of these measurements.

The 1950s and 1960s saw two developments that would impact stellar convection theory in red supergiants: the Stratoscope projects and the 1958 publication of Structure and Evolution of the Stars, principally the work of Martin Schwarzschild and his colleague at Princeton University, Richard Härm.^[29][30] This book disseminated ideas on how to apply computer technologies to create stellar models, while the Stratoscope projects, by taking balloon-borne telescopes 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.^[29]

Imaging breakthroughs

Astronomers in the 1970s saw some major advances in astronomical imaging technology beginning with Antoine Labeyrie's invention of speckle interferometry, a process that significantly reduced the blurring effect caused by astronomical seeing. It increased the optical resolution of ground-based telescopes, allowing for more precise measurements of Betelgeuse's photosphere.^[31][32] With improvements in infrared telescopy atop Mount Wilson, Mount Locke and Mauna Kea in Hawaii, astrophysicists began peering into the complex circumstellar shells surrounding the supergiant,^[33][34][35] causing them to suspect the presence of huge gas bubbles resulting from convection.^[36] But it was not until the late 1980s and early 1990s, when Betelgeuse became a regular target for aperture masking interferometry, that breakthroughs occurred in visible-light and infrared imaging. Pioneered by John E. Baldwin and colleagues of the Cavendish Astrophysics Group, the new technique employed a small mask with several holes in the telescope pupil plane, converting the aperture into an ad-hoc interferometric array.^[37] The technique contributed some of the most accurate measurements of Betelgeuse while revealing bright spots on the star's photosphere.^[38][39][40] These were the first optical and infrared images of a stellar disk other than the Sun, taken first from ground-based interferometers and later from higher-resolution observations of the COAST telescope. The "bright patches" or "hotspots" observed with these instruments appeared to corroborate a theory put forth by Schwarzschild decades earlier of massive convection cells dominating the stellar surface.^[41][42]

In 1995, the Hubble Space Telescope's Faint Object Camera captured an ultraviolet image with a resolution superior to that obtained by ground-based interferometers—the first conventional-telescope image (or "direct-image" in NASA terminology) of the disk of another star.^[43] Because ultraviolet light is absorbed by the Earth's atmosphere, observations at these wavelengths are best performed by space telescopes.^[44] Like earlier pictures, this image contained a bright patch indicating a region in the southwestern quadrant 2000 K hotter than the stellar surface.^[45] 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°.^[46]

Recent studies

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

In a study published in December 2000, the star's diameter was measured with the Infrared Spatial Interferometer (ISI) at mid-infrared wavelengths producing a limb-darkened estimate of 55.2 ± 0.5 milliarcseconds (mas)—a figure entirely consistent with Michelson's findings eighty years earlier.^[28][47] 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 made at the Paranal Observatory in Chile argue for much tighter diameters. On 9 June 2009, Nobel Laureate Charles Townes announced that the star had shrunk by 15% since 1993 at an increasing rate without a significant diminution in magnitude.^[48][49] Subsequent observations suggest that the apparent contraction may be due to shell activity in the star's extended atmosphere.^[50]

In addition to the discussion of the star's diameter, questions have arisen about the complex dynamics of Betelgeuse's extended atmosphere. The mass that makes up galaxies is recycled as stars are formed and destroyed, and red supergiants are major contributors, yet the mechanics of stellar mass loss remain a mystery.^[51] With 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 from the star into the surrounding atmosphere with distances approximating 30 AU.^[15] This mass ejection was equal to the distance between the Sun and Neptune and is one of multiple events occurring in Betelgeuse's surrounding atmosphere. Astronomers have identified at least six shells surrounding Betelgeuse. Solving the mystery of mass loss in the late stages of a star's evolution may reveal those factors that precipitate the explosive deaths of these stellar giants.^[48]

Visibility

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

In the night sky, Betelgeuse is easy to spot with the naked eye owing to its distinctive orange-red color. In the Northern Hemisphere, beginning in January of each year, it can be seen rising in the east just after sunset. By mid-September to mid-March (best in mid-December), it is visible to virtually every inhabited region of the globe, except for a few research stations in Antarctica at latitudes south of 82°. In May (moderate northern latitudes) or June (southern latitudes), the red supergiant can be seen briefly on the western horizon after sunset, reappearing again a few months later on the eastern horizon before sunrise. In the intermediate period (June–July) it is invisible to the naked eye (visible only with a telescope in daylight), unless around midday (when the Sun is below horizon) on Antarctic regions between 70° and 80° south latitude.

The mean apparent magnitude of Betelgeuse is 0.50, making it on average the eighth brightest star in the celestial sphere excluding the Sun. Because Betelgeuse is a variable star whose brightness ranges between 0.0 and 1.3, 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 At its faintest, Betelgeuse will fall behind Deneb and Mimosa as 20th brightest star.

Image from ESO's Very Large Telescope showing the stellar disk and an extended atmosphere with a previously unknown plume of surrounding gas^[52]

Betelgeuse has a color index (B–V) of 1.85—a figure which points to its advanced "redness". The photosphere has an extended atmosphere, which displays strong lines of emission rather than absorption, a phenomenon that occurs when a star is surrounded by a thick gaseous envelope. This extended gaseous atmosphere has been observed moving 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.^[53] 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.^[25]

Parallax

Since the first successful parallax measurement by Friedrich Bessel in 1838, astronomers have been puzzled by Betelgeuse's apparent distance. Knowledge of the star's distance improves the accuracy of other stellar parameters, such as luminosity that, when combined with an angular diameter, can be used to calculate the physical radius and effective temperature; luminosity and isotopic abundances can also be used to estimate the stellar age and mass.^[9] In 1920, when the first interferometric studies were performed on the star's diameter, the assumed parallax was 0.0180 arcseconds. This equated to a distance of 56 parsecs (pc) or roughly 180 light-years (ly), producing not only an inaccurate radius for the star but every other stellar characteristic. Since then, there has been ongoing work to measure the distance of Betelgeuse, with proposed distances as high as 400 pc or about 1300 ly.^[9]

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.^[54] 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.^[55] Given this uncertainty, researchers were adopting a wide range of distance estimates, leading to significant variances in the calculation of the star's attributes.^[9]

NRAO's Very Large Array used to derive Betelgeuse's 2008 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.^[56] However, later evaluation of the Hipparcos parallax measurements for variable stars like Betelgeuse found that the uncertainty of these measurements had been underestimated.^[57] 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±20 pc or 520±73 ly.^[3]

In 2008, Graham Harper and colleagues, using the Very Large Array (VLA), produced a radio solution of π = 5.07±1.10 mas, equalling a distance of 197±45 pc or 643±146 ly.^[9] 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 also have systematic errors, the Harper solution combines the datasets in the hope of mitigating such errors.^[9] The European Space Agency's current Gaia mission may not improve over the measurements of Betelgeuse by the earlier Hipparcos mission because it is brighter than the approximately V=6 saturation limit of the mission's instruments.^[58]

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.^[24] Different hypotheses have been put forward to explain Betelgeuse's pulsations and their rhythm—which result in an absolute magnitude oscillation from −5.27 and −6.27.^{[note 1]} 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 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.^[24] 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.^[60][61]

The supergiant consistently displays irregular photometric, polarimetric and spectroscopic variations, phenomena pointing to complex activity on the star's surface and its extended atmosphere.^[38] 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.^[42][62] For the Sun, these convection cells, known as solar granules, represent the foremost mode of heat transfer—hence those convective elements dominate the brightness variations in the solar photosphere.^[42] The typical diameter for a solar granule is about 2000 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 on the Sun's photosphere, and this large number produces a relatively constant flux.^[63] By contrast, Schwarzschild argues that stars like Betelgeuse may only have a dozen granules with diameters of 180 million kilometers or more dominating the surface of the star with depths of about 60 million kilometers, which, due to the low temperatures and extremely low density found in red supergiant envelopes, result in convective inefficiency. Consequently, if only a third of these convective cells are visible at any one time, the variations in their observable light emission may result in the recorded irregular brightness variations of the overall light from the star.^[42]

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.^[64] 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."^[39] In 2000, another team of astronomers, led by Alex Lobel of the Harvard–Smithsonian Center for Astrophysics, noted that Betelgeuse exhibits raging storms of hot and cold gas in its turbulent atmosphere. The team surmised that large areas of the star's photosphere bulge out in different directions at times, ejecting long plumes of warm gas into the cold dust envelope. Another explanation is the occurrence of shock waves caused by warm gas traversing cooler regions of the star.^[61][65] Observing the atmosphere of Betelgeuse over a period of five years between 1998 and 2003 with the Space Telescope Imaging Spectrograph aboard Hubble, the team likened the rise and fall of convection cells in the chromosphere to the blobs in a lava lamp.^[65]

Diameter

On 13 December 1920, Betelgeuse became the first star outside the Solar System to have the angular size of its photosphere measured.^[28] Although interferometry was still in its infancy, the experiment proved a success. The researchers, using a uniform disk model, determined that Betelgeuse had a diameter of 0.047 arcseconds, although the stellar disk was likely 17% larger due to the limb darkening, resulting in an estimate for its angular diameter of about 0.055".^[28][49] Since then, other studies have produced angular diameters that range from 0.042 to 0.069 arcseconds.^[32][47][66] Combining these data with historical distance estimates of 180 to 815 ly yields a projected radius of the stellar disk of anywhere from 1.2 to 8.9 AU.^{[note 2]} Using the Solar System for comparison, the orbit of Mars is about 1.5 AU, Ceres in the asteroid belt 2.7 AU, Jupiter 5.5 AU—so, assuming Betelgeuse occupying the place of the Sun, its photosphere might extend beyond the Jovian orbit, not quite reaching 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. Betelgeuse is a pulsating star, so its diameter changes with time;
2. The star has no definable "edge" 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 ejected from the star—matter which absorbs and emits light—making it difficult to define the photosphere of the star;^[48]
4. Measurements can be taken at varying wavelengths within the electromagnetic spectrum and the difference in reported diameters can be as much as 30–35%, yet comparing one finding with another is difficult as the star's apparent size differs depending on the wavelength used.^[48] Studies have shown that the measured angular diameter is considerably larger at ultraviolet wavelengths, decreases through the visible to a minimum in the near-infrared, and increase again in the mid-infrared spectrum;^[43][59][67]
5. Atmospheric twinkling limits the resolution obtainable from ground-based telescopes since turbulence degrades angular resolution.^[38]

To overcome these challenges, researchers have employed various solutions. Astronomical interferometry, first conceived by Hippolyte Fizeau in 1868, was the seminal concept that has enabled major improvements in modern telescopy and led to the creation of the Michelson interferometer in the 1880s, and the first successful measurement of Betelgeuse.^[68] Just as human depth perception increases when two eyes instead of one perceive an object, 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 and multiple-aperture interferometers are now used to capture speckled images, which are synthesized using Fourier analysis to produce a portrait of high resolution.^[69] It was this methodology that identified the hotspots on Betelgeuse in the 1990s.^[70] Other technological breakthroughs include adaptive optics,^[71] space observatories like Hipparcos, Hubble and Spitzer,^[43][72] and the Astronomical Multi-BEam Recombiner (AMBER), which combines the beams of three telescopes simultaneously, allowing researchers to achieve milliarcsecond spatial resolution.^[73][74]

Which part of the electromagnetic spectrum—the visible, near-infrared (NIR) or mid-infrared (MIR)—produces the most accurate angular measurement is still debated.^{[note 2]} 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.^[47] 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 1200 R_☉, assuming the 2008 Harper distance of 197.0 ± 45 pc,^[11][75] 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.^[76][77]

A 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.^[59] 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.^[59]

Infrared image of Betelgeuse, Meissa and Bellatrix with surrounding nebulas^[78]

Studies with the IOTA and VLTI published in 2009 brought strong support to Perrin's analysis and yielded diameters ranging from 42.57 to 44.28 mas with comparatively insignificant margins of error.^[79][80] In 2011, Keiichi Ohnaka and colleagues 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.^[81] Consequently, if one combines 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_☉.^[75][82] A 2014 paper derives an angular diameter of 42.28 mas (equivalent to a 41.01 mas uniform disc) using H and K band observations made with the VLTI AMBER instrument.^[83]

Central to this discussion is another paper published in 2009 by the Berkeley team, led by Charles Townes, reporting that the radius of Betelgeuse had shrunk from 1993 to 2009 by 15%, with the 2008 angular measurement equal to 47.0 mas, not too far from Perrin's estimate.^[49][84] Unlike most earlier papers, 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.^[49] Other possible explanations are photospheric protrusions due to convection or a star that is not spherical but asymmetric causing the appearance of expansion and contraction as the star rotates on its axis.^[85]

The debate about differences 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, remains 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.^[50] 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_☉) assuming the shorter Hipparcos distance of 498 ± 73 ly in lieu of Harper's estimate at 643 ± 146 ly. The Gaia spacecraft may clarify assumptions presently used in calculating the size of Betelgeuse's stellar disk.

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 a diameter roughly one-third that of Betelgeuse.^[86]

Properties

Hertzsprung–Russell diagram identifying supergiants like Betelgeuse that have moved off the main sequence

Betelgeuse is a very large, luminous and cool star classified as a red supergiant of M1-2 Ia-ab class. The letter "M" in this designation means that it is a red star belonging to the M spectral class and therefore has a relatively low photospheric temperature; the "Ia-ab" suffix luminosity class indicates that it is an intermediate luminous supergiant, with properties partway between a normal supergiant and a luminous supergiant. Uncertainties regarding the star's surface temperature, angular diameter and distance make it difficult to achieve a precise measurement of Betelgeuse's luminosity. Research from 2012 gives Betelgeuse an average luminosity of 120000±30000 L_☉,^[87] assuming a median temperature of 3300 K and a radius of 1200 R_☉.^[75] However, because most of the star's radiation is in the near infrared, the human eye cannot perceive the star's intrinsic brightness. Since 1943, the spectrum of Betelgeuse has served as one of the stable anchor points by which other stars are classified.^[88]

The mass of Betelgeuse has never been measured because it has no known companion.^[10] A mass estimate is only possible using theoretical modeling, a situation which has produced mass estimates ranging from 5 to 30 M_☉ in the 2000s.^[89][90] Smith and colleagues calculated that Betelgeuse began its life as a star of 15 to 20 M_⊙, based on a luminosity of 90000–150000 L_☉.^[11] A novel method of determining the supergiant's mass was proposed in 2011 by Hilding Neilson and colleagues, arguing for a current 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_⊙.^[10]

Due to its variability and the presence of hotspots, the photospheric temperature of Betelgeuse is uncertain. Studies since 2001 report temperatures ranging from 3140^[87] to 3,641 K^[59] with a median of about 3300 K.^[87][91][15] The star is also a slow rotator and the most recent velocity recorded was 5 km/s.^[15] Depending on its photospheric radius, it could take the star from 25 to 32 years to turn on its axis—much slower than Antares, which has a rotational velocity of 20 km/s.^[92]

In 2002, astronomers using computer simulations speculated 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.^[93] 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 effect.^[94]

Motion

Orion OB1 Association

The kinematics of Betelgeuse are complex. The age of Class M supergiants with an initial mass of 20 $\begin{smallmatrix}M_\odot\end{smallmatrix}$ is roughly 10 million years.^[9][95] Given its motion, a corresponding projection back in time would take Betelgeuse around 290 parsecs farther from the galactic plane—an implausible location, as there is no star formation region there. Moreover, Betelgeuse's projected pathway does not appear to intersect with 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.^[9][96]

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 molecular clouds observed in Orion,^[97] but has evolved rapidly due to its high mass.^[9]

Like many young stars in Orion whose mass is greater than 10 $\begin{smallmatrix}M_\odot\end{smallmatrix}$, Betelgeuse will use its fuel quickly and not live 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 mass and luminosity are such 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.^[61][98]

Relative sizes of the planets in the Solar System and several stars, including Betelgeuse
1. Mercury < Mars < Venus < Earth
2. Earth < Neptune < Uranus < Saturn < Jupiter
3. Jupiter < Proxima Centauri < Sun < Sirius
4. Sirius < Pollux < Arcturus < Aldebaran
5. Aldebaran < Rigel < Antares < Betelgeuse
6. Betelgeuse < VY Canis Majoris < NML Cygni < UY Scuti.

Density

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 1180 times the radius of the Sun—able to 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 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.^{[note 3]} Moreover, observations from 2009 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%.^{[note 4]}

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 far denser than Betelgeuse.

Not only is the photosphere enormous, but the star is surrounded by a complex circumstellar environment where light could take over three years to escape.^[99] In the outer reaches of the photosphere the density is extremely low, yet the total mass of the star is believed to be no more than 20 M_☉. Consequently, the average density is less than twelve parts per billion (1.119×10⁻⁸) that of the Sun. Such star matter is so tenuous that Betelgeuse has often been called a "red-hot vacuum".^[24][25]

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 10000 years, resulting in a complex circumstellar environment that is constantly in flux. In a 2009 paper, stellar mass loss was cited as the "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".^[100] However, the physical mechanism is not well understood.^[82] When Schwarzschild first proposed his theory of huge convection cells, he argued it was the likely cause of mass loss in evolved supergiants like Betelgeuse.^[42] Recent work has corroborated this hypothesis, yet there are still uncertainties about 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.^[82] In 2001, Graham Harper estimated a stellar wind at 0.03 M_☉ every 10000 years,^[91] but research since 2009 has provided evidence of episodic mass loss making any total figure for Betelgeuse uncertain.^[101] Current observations 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, pass once again through a brief yellow supergiant phase and then explode as a blue supergiant or Wolf-Rayet star.^[22]

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. They 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.^[15] The plume's presence 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 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.^[15] Probing deeper with ESO's AMBER, Keiichi Ohnaka and colleagues 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.^[101]

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.^[79]

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.^[79][102] Water vapor had been originally detected in the supergiant's spectrum in the 1960s with the two Stratoscope projects but had been ignored for decades. 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-meter Unit Telescopes at ESO's VLT

Extending for several radii (~10–40 AU) about the photosphere exists another cooler 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.^[79][103]

Radio-telescope images taken in 1998 confirm that Betelgeuse has a highly complex atmosphere,^[104] with a temperature of 3450±850 K, similar to that recorded on the star's surface but much lower than surrounding gas in the same region.^[104][105] The VLA images also show this lower-temperature gas progressively cool 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 due to gas heated to high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere."^[104] 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.^[79]

The chromosphere was directly imaged by the Faint Object Camera on board the Hubble Space Telescope in ultraviolet wavelengths. The images also revealed a bright area in the southwest quadrant of the disk.^[106] 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 5500 K.^[64][79] 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.^[106] 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).^[79][106]

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.^[107][108]

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.^[33][79] Since then, there have been studies done of this dust envelope at varying wavelengths yielding decidedly different results. Studies from the 1990s have estimated the inner radius of the dust shell anywhere from 0.5 to 1.0 arcseconds, or 100 to 200 AU.^[109][110] These studies point out that the dust environment surrounding Betelgeuse is not static. In 1994, Danchi et al. reported that Betelgeuse undergoes sporadic dust production involving decades of activity followed by inactivity. In 1997, a group of astronomers led by Chris Skinner noticed significant changes in the dust shell's morphology in one year, suggesting that the shell is asymmetrically illuminated by a stellar radiation field strongly affected by the existence of photospheric hotspots.^[109] The 1984 report of a giant asymmetric dust shell 1 pc (206265 AU) from the star has not been corroborated by 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 Betelgeuse sheds its outer layers as it journeys.^[99][111]

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 and the other expands as far as 7.0 arcseconds.^[112] Assuming the Jovian orbit of 5.5 AU as the star radius, 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). The Sun's heliopause is estimated at about 100 AU, so the size of this outer shell would be almost fourteen times the size of the Solar System.

Supersonic bow shock

Betelgeuse is travelling supersonically through the interstellar medium at a speed of 30 km per second (i.e. ~6.3 AU per year) creating a bow shock.^[87][113] The shock is not created by the star, but its powerful stellar wind as it ejects vast amounts of gas into the interstellar medium at a rate of 17 km/s, heating up the material surrounding the star thereby making it visible in infrared light.^[114] 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 distance of 643 light-years.^[115]

3D hydrodynamic simulations of the bow shock made in 2012 indicate that it is very young—less than 30000 years old—suggesting two possibilities: one, that Betelgeuse moved into a region of the interstellar medium with different properties recently or two, that Betelgeuse has undergone a significant transformation as its stellar wind has changed.^[116] 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 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."^[87][117] Moreover, if future research bears out this hypothesis, Betelgeuse may prove to have traveled close to 200000 AU as a red supergiant scattering as much as 3 $\begin{smallmatrix}M_{\odot} \end{smallmatrix}$ along its trajectory.

Approaching supernova

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

The fate of Betelgeuse depends on its initial mass—a critical factor which is not well understood.^[90] Since most investigators posit a mass greater than 10 M_☉, the most likely scenario is that the supergiant will continue to burn and fuse elements until its core collapses, at which point Betelgeuse will explode as a supernova, leaving behind a neutron star remnant about 20 km in diameter.^[118] As of 2014, theoretical calculations for a non-rotating star suggest Betelgeuse has developed a 3–4 M_☉ carbon-oxygen core which is being enlarged by deposition of fusion products from the surrounding helium shell at about a fifth of the star's radius. It will explode as a type II supernova within 100000 years after going through carbon, neon, oxygen, and silicon burning in the core. The remnant would be a neutron star around 1.5 M_☉. The estimated age for the red supergiant is between 8 and 8.5 million years and its initial mass was 20+5
−3 M_☉.^[118] The exact mass, rotation rate, and mass loss are critical to Betelgeuse's fate; stellar evolution models show that initially rotating stars more massive than about 18 M_☉ do not explode as supernovae while they are red supergiants. Instead they lose all their outer hydrogen to become yellow hypergiants, luminous blue variables, or even Wolf-Rayet stars before producing a type II-L, IIb or Ib/c supernova.^[119]

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 in the next million years.^{[52][120][121]} Supporting this hypothesis are unusual features that have been observed in the interstellar medium of the Orion Molecular Cloud Complex, which suggest that there have been multiple supernovae in the recent past.^[96] 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 from such explosions in Ori OB1 2.2, 2.7 and 4.9 million years ago.^[96]

J. Craig Wheeler of the University of Texas at Austin predicts Betelgeuse's demise will emit 10⁴⁶ joules of neutrinos, which will pass through the star's hydrogen envelope in around an hour, then travel at near light speed to reach the Solar System six centuries later—providing the first evidence of the cataclysm. The supernova could brighten over a two-week period to an apparent magnitude of −12,^[122] outshining the Moon in the night sky and becoming easily visible in broad daylight. It would remain at that intensity for two to three months before rapidly dimming.^[122] 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.^[121] The flash of ultraviolet radiation from the explosion will likely be weaker than the ultraviolet output of the Sun. 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, it could produce gamma rays for thousands of years.^[122]

Due to misunderstandings caused by the 2009 publication of the star's 15% contraction,^[48][76] 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.^[123][124] The timing and prevalence of these rumors have been linked to broader misconceptions of astronomy, particularly to doomsday predictions relating to the Mayan calendar.^[125][126] In their 2012 study, physicists at the Space Sciences Laboratory point out that the apparent contraction in the star's diameter may be due to the complex dynamics in the star's surrounding nebula and not the star itself,^[50] reconfirming that until we better understand the nature of mass loss, predicting the timing of a supernova will remain a challenge. The latest studies project a supernova in 100000 years.^[127]

Star system

In 1985, Margarita Karovska, in conjunction with other astrophysicists at the Harvard–Smithsonian Center for Astrophysics, 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.^[128][129]

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.^[130] 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.^[131] 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.^[79] 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.^[132]

Ethnological attributes

Spelling and pronunciation

Betelgeuse has been known as Betelgeux,^[1] and in German Beteigeuze^[133] (according to Bode).^[134][135] Betelgeux and Betelgeuze were used until the early 20th century, when the spelling Betelgeuse became universal.^[136] There is no consensus for the correct pronunciation of the name,^[137] and pronunciations for the star are as varied as its spellings:

• /ˈbɛtəldʒuːz/ – Oxford English Dictionary^[1] and Royal Astronomical Society of Canada
• /ˈbiːtəldʒuːz/ or /ˈbiːtəldʒɜːz/ – Oxford English Dictionary^[1]
• /ˈbiːtəldʒuːs/ – (Canadian Oxford Dictionary, Webster's Collegiate Dictionary^[2])
• /ˈbɛtəldʒəz/ – (The Friendly Stars)^[138]

Etymology

Betelgeuse is often mistranslated as "armpit of the central one".^[139] 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 forms Betelgeuse, Betelguese, Betelgueze and Betelgeux. The star was named Beldengeuze in the Alfonsine Tables,^[140] and Italian Jesuit priest and astronomer Giovanni Battista Riccioli had called it Bectelgeuze or Bedalgeuze.^[18] 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.^[141] 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.^[142] Other writers have since accepted Kunitzsch's explanation.^[98]

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 modern 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.^[142] The 17th-century English translator Edmund Chilmead gave it the name Ied Algeuze ("Orion's Hand"), from Christmannus.^[18] 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",^[18] as منكب الجوزاء Mankib al Jauzā'.

Dunhuang Star Chart, circa AD 700, showing 参宿四 Shēnxiùsì (Betelgeuse), the Fourth Star of the constellation of Three Stars^[143]

Other names

Other names for Betelgeuse included the Persian Bašn "the Arm", and Coptic Klaria "an Armlet".^[18] Bahu was its Sanskrit name, as part of a Hindu understanding of the constellation as a running antelope or stag.^[18] In traditional Chinese astronomy, Betelgeuse was known as 参宿四 (Shēnxiùsì, the Fourth Star of the constellation of Three Stars)^[144] 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.^[145] In Japan, the Taira or Heike clan adopted Betelgeuse and its red color as its symbol, calling the star Heike-boshi, (平家星), while the Minamoto or Genji clan had chosen Rigel and its white color. The two powerful families fought a legendary war in Japanese history, the stars seen as facing each other off and only kept apart by the Belt.^[146][147]

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".^[148] A Hawaiian term for it was Kaulua-koko "brilliant red star".^[149] The Lacandon people of Central America knew it as chäk tulix "red butterfly".^[150]

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.^[18] 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 of ivory made for him, with Betelgeuse as the shoulder, its color reminiscent of the reddish yellow sheen of ivory.^[23]

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.^[23] The Wardaman people of northern Australia knew the star as Ya-jungin "Owl Eyes Flicking", its variable light signifying its intermittent watching of ceremonies led by the Red Kangaroo Leader Rigel.^[151] In South African mythology, Betelgeuse was perceived as a lion casting a predatory gaze toward the three zebras represented by Orion's Belt.^[152]

A Sanskrit name for Betelgeuse was ãrdrã "the moist one", eponymous of the Ardra lunar mansion in Hindu astrology.^[153] The Rigvedic God of storms Rudra presided over the star; this association was linked by 19th-century star enthusiast Richard Hinckley Allen to Orion's stormy nature.^[18] 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 am in late summer and autumn signified the time for village men to go to the fields and plough.^[154] To the Inuit, the appearance of Betelgeuse and Bellatrix high in the southern sky after sunset marked the beginning of spring and lengthening days in late February and early March. The two stars were known as Akuttujuuk "those (two) placed far apart", referring to the distance between them, mainly to people from North Baffin Island and Melville Peninsula.^[27]

The opposed locations of Orion and Scorpius, 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 Scorpius signify the death of Orion by the scorpion. In China they signify brothers and rivals Shen and Shang.^[23] 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 considered significant and their constellations were seen as a pair of scorpions. Scorpion days marked as nights that both constellations could be seen.^[155]

In popular culture

The star's unusual name inspired the title of the 1988 film Beetlejuice, and script writer Michael McDowell was impressed by 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.^[136] 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.^[156] The identity of the red star Borgil mentioned in Lord of the Rings was much debated; Aldebaran, Betelgeuse and the planet Mars were touted as candidates. Professor Kristine Larsen has concluded the evidence points to it being Aldebaran as it precedes Menelvagor (Orion).^[157] Astronomy writer Robert Burnham, Jr. proposed the term padparadaschah which denotes a rare orange sapphire in India, for the star.^[136] 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."^[156] In the poetic work Betelguese, a Trip Through Hell by Jean Louis De Esque, hell is on Betelgeuse because De Esque believed that it was "a celestial pariah, an outcast, the largest of all known comets or outlawed suns in the universe."^[158]

Two American navy ships were named after the star, both of them 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.^[159]

The Dave Matthews Band song "Black and Blue Bird" references the star.^[160]

Notes

Article	Year^1	Telescope	#	Spectrum	λ (μm)	∅ (mas)^2	Radii^3 @ 197±45 pc	Notes
Michelson[28]	1920	Mt-Wilson	1	Visible	0.575	47.0 ± 4.7	3.2–6.3 AU	Limb darkened +17% = 55.0
Bonneau[32]	1972	Palomar	8	Visible	0.422–0.719	52.0–69.0	3.6–9.2 AU	Strong correlation of ∅ with λ
Balega[66]	1978	ESO	3	Visible	0.405–0.715	45.0–67.0	3.1–8.6 AU	No correlation of ∅ with λ
	1979	SAO	4	Visible	0.575–0.773	50.0–62.0	3.5–8.0 AU
Buscher[38]	1989	WHT	4	Visible	0.633–0.710	54.0–61.0	4.0–7.9 AU	Discovered asymmetries/hotspots
Wilson[130]	1991	WHT	4	Visible	0.546–0.710	49.0–57.0	3.5–7.1 AU	Confirmation of hotspots
Tuthill[41]	1993	WHT	8	Visible	0.633–0.710	43.5–54.2	3.2–7.0 AU	Study of hotspots on 3 stars
	1992	WHT	1	NIR	0.902	42.6 ± 0:03	3.0–5.6 AU
Gilliland[43]	1995	HST		UV	0.24–0.27	104–112	10.3–11.1	FWHM diameters
					0.265–0.295	92–100	9.1–9.8
Weiner[47]	1999	ISI	2	MIR (N Band)	11.150	54.7 ± 0.3	4.1–6.7 AU	Limb darkened = 55.2 ± 0.5
Perrin[59]	1997	IOTA	7	NIR (K Band)	2.200	43.33 ± 0.04	3.3–5.2 AU	K&L Band, 11.5 μm data contrast
Haubois[79]	2005	IOTA	6	NIR (H Band)	1.650	44.28 ± 0.15^‡	3.4–5.4 AU	Rosseland diameter 45.03 ± 0.12
Hernandez[80]	2006	VLTI	2	NIR (K Band)	2.099–2.198	42:57 ± 0:02	3.2–5.2 AU	High precision AMBER results.
Ohnaka[101]	2008	VLTI	3	NIR (K Band)	2.280–2.310	43.19 ± 0.03	3.3–5.2 AU	Limb darkened 43.56 ± 0.06
Townes[49]	1993	ISI	17	MIR (N Band)	11.150	56.00 ± 1.00	4.2–6.8 AU	Systematic study involving 17 measurements at the same wavelength from 1993 to 2009
	2008	ISI		MIR (N Band)	11.150	47.00 ± 2.00	3.6–5.7 AU
	2009	ISI		MIR (N Band)	11.150	48.00 ± 1.00	3.6–5.8 AU
Ohnaka[81]	2011	VLTI	3	NIR (K Band)	2.280–2.310	42.05 ± 0.05	3.2–5.2 AU	Limb darkened 42.49 ± 0.06
Harper[9]	2008	VLA		Also noteworthy, Harper et al. in the conclusion of their paper make the following remark: "In a sense, the derived distance of 200 pc is a balance between the 131 pc (425 ly) Hipparcos distance and the radio which tends towards 250 pc (815 ly)"—hence establishing ± 815 ly as the outside distance for the star.

1. This range of absolute magnitudes assumes an apparent magnitude that varies from 0.2 to 1.2 and a distance of 197 pc (ignoring interstellar extinction which is thought to be very small for Betelgeuse^[59]).
2. The above table provides a non-exhaustive list of angular measurements conducted since 1920. Also included is a column providing a current range of radii for each study based on Betelgeuse's most recent distance estimate (Harper et al) of 197 ± 45 pc.
3.   Computations relating to Betelgeuse volume

   The analogy is based on the computation of certain ratios – specifically the diameter, radius and volume of the three celestial bodies in question, Betelgeuse, the Sun and Earth. Once these ratios are derived, the relative size of each as they relate to Wembley Stadium can be easily determined. The calculations begin with the formula for angular diameter as follows:

   • Betelgeuse diameter ≈ 0.0552 arcseconds × 197.0 pc ≈ 11.000 AU (rounded up) × 149597871 km ≈ 1.646×10⁹ km,
   • Betelgeuse radius ≈ 11.000 AU ÷ 2 ≈ 5.500 AU × 149597871 km ≈ 8.230×10⁸ km ≈ 823000000 km,
   • Betelgeuse volume ≈ (4π/3) × 823000000³ ≈ 2.335×10²⁷ km³.

   Also:

   • Solar radius ≈ 696000 km. Volume ≈ 1.412×10¹⁸ km³,
   • Earth radius ≈ 6371.0 km. Volume ≈ 1.083×10¹² km³.
   • Wembley Bowl Volume ≈ 1139100 m³. Spherical radius ≈ (1139100 m³ ÷ (4π/3))^1/3 ≈ 64.787 m or 64787 mm.

   Therefore:

   • Betelgeuse ≈ (2.335×10²⁷) ÷ (1.412×10¹⁸) ≈ 1.654×10⁹ Suns,
   • Betelgeuse ≈ (2.335×10²⁷) ÷ (1.083×10¹²) ≈ 2.156×10¹⁵ Earths.
   • Solar volume relative to Wembley ≈ 1139100 m³ ÷ (1.654×10⁹) × 10⁹{i.e. to convert to mm³} ≈ 689000 mm³ (rounded up)
   • Solar diameter relative to Wembley ≈ (689000 mm³ ÷ (4π/3))^1/3 ≈ 55.61845 × 2 ≈ 110 mm
   • Earth volume relative to Wembley ≈ 1139100 m³ ÷ (2.156×10¹⁵) × 10⁹{i.e. to convert to mm³} ≈ 0.528 mm³
   • Earth diameter relative to Wembley ≈ (0.528 mm³ ÷ (4π/3))^1/3 ≈ 0.501 × 2 ≈ 1.002 mm
   • Earth's orbital radius (1 AU) relative to Wembley ≈ 1 AU ÷ 5.5 AU × 64.787 m = 11.8 m

   Conclusion:

   • If the immense space of Wembley Stadium were Betelgeuse, the Earth would be a tiny pearl, 1.0 mm in diameter, orbiting a Sun, 11.0 cm in diameter (i.e. the size of an average mango or grapefruit), with an orbital distance of about 11.8 m.
4.   Computations relating to stellar contraction

   As pointed out in the Angular anomalies section, the observed contraction could be due to a shrinking of the star's radius or by other phenomena. Assuming the photosphere is spherical, calculating a reduction in volume begins with the formula for angular diameter as follows: Calculations for 1993 values:

   • Betelgeuse radius ≈ 0.056 arcseconds × 197.0 pc ≈ 11.032 AU ÷ 2 ≈ 5.516 AU × 149597871 km ≈ 825000000 km,
   • Betelgeuse volume ≈ (4π/3) × 825000000³ ≈ 2.352×10²⁷ km³.

   Calculations for 2008 values:

   • Betelgeuse radius ≈ 0.047 arcseconds × 197.0 pc ≈ 9.260 AU ÷ 2 ≈ 4.630 AU × 149597871 km ≈ 692500000 km,
   • Betelgeuse volume ≈ (4π/3) × 692500000³ ≈ 1.391×10²⁷ km³.

   Therefore:

   • Betelgeuse change in volume ≈ 2.352×10²⁷ km³ – 1.391×10²⁷ km³ ≈ −9.610×10²⁶ km³
   • Betelgeuse percent change in volume ≈ −9.610×10²⁶ km³ ÷ 2.352×10²⁷ km³ ≈ –40.86%
   • Betelgeuse volume change as a function of Solar volume ≈ 9.610×10²⁶ ÷ 1.412×10¹⁸ km³ ≈ −681000000 Suns.

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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.