Betelgeuse

alpha orionis

Betelgeuse, Alpha Orionis (α Ori), is a red supergiant star of the spectral type M1-2 located in the constellation Orion, the Hunter.  It has an estimated radius about 887 to 955 times that of the Sun and is one of the largest stars that can be seen without binoculars. Betelgeuse lies at an approximate distance between 640 and 724 light years from Earth.

alpha orionis
Betelgeuse (Alpha Orionis), image: Wikisky

With an apparent magnitude that varies between 0.0 and 1.3, Betelgeuse is usually the 2nd brightest star in Orion and the 10th brightest star in the sky. Most of the time, it is only slightly fainter than Achernar (Alpha Eridani) in the constellation Eridanus and slightly brighter than the Southern Pointer Hadar (Beta Centauri) in Centaurus. In infrared wavelengths, Betelgeuse is brighter than any other star in the sky.

Even though it is only 8 to 8.5 million years old, Betelgeuse is a massive star that has evolved quickly and is already approaching the end of its life cycle. It will explode as a supernova in the relatively near future, at some point in the next million years. When it does, it will release its store of raw material needed to create a new generation of stars.

Star type

Betelgeuse has the stellar classification M1-2 Ia–ab, indicating a supergiant star appearing red in colour. Betelgeuse has served as a spectral standard for its class since 1943, along with the red supergiant Mu Cephei (which is also nearing the end of its life). The “Ia-ab” suffix means that it is an intermediate-luminosity supergiant, halfway between a normal and luminous supergiant star.

betelgeuse star
Betelgeuse, image: Wikisky

Mass estimates for the star range from just under 10 solar masses to a little over 20 solar masses.

Betelgeuse started its life as a hot O-type star less than 10 million years ago and, like other extremely massive stars, it has evolved very quickly and lost about 18 or 19 solar masses from its initial mass. It has exhausted the hydrogen supply in its core, causing the core to contract into a hotter, denser state, while its outer layers have swelled outward. Betelgeuse likely started expanding into a red giant about 40,000 years ago. It is now believed to be fusing helium into carbon and oxygen in its core and will continue to fuse elements through neon, magnesium, silicon and finally iron. At that point, its core will collapse, triggering a supernova explosion that will be visible even in daylight and leave a neutron star in its wake.

The star is surrounded by a vast envelope of dust and gas expelled from its surface. Images taken by European Southern Observatory’s Very Large Telescope in June 2011 revealed that the nebulosity stretches about 400 astronomical units across. The star itself is believed to be only about 4.5 astronomical units wide.

This picture of the dramatic nebula around the bright red supergiant star Betelgeuse was created from images taken with the VISIR infrared camera on ESO’s Very Large Telescope (VLT). This structure, resembling flames emanating from the star, forms because the behemoth is shedding its material into space. The earlier NACO observations of the plumes are reproduced in the central disc. 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. The black disc corresponds to a very bright part of the image that was masked to allow the fainter nebula to be seen. Image: ESO/P. Kervella

Betelgeuse is classified as a semiregular variable star. Its magnitude ranges from 0.0 and 1.3, the largest variation seen in any first-magnitude star.

Size

Betelgeuse has an estimated radius of 887 solar radii, with a margin of error of 203 R.  If it were placed at the centre of our solar system, the star’s surface would engulf the orbit of Mars and come within, or possibly even extend past, the orbit of Jupiter (1,064 – 1,173 solar radii).

This image, made with the Atacama Large Millimeter/submillimeter Array (ALMA), shows the red supergiant Betelgeuse — one of the largest stars known. In the millimeter continuum the star is around 1400 times larger than our Sun. The overlaid annotation shows how large the star is compared to the Solar System. Betelgeuse would engulf all four terrestrial planets — Mercury, Venus, Earth and Mars — and even the gas giant Jupiter. Only Saturn would be beyond its surface. Credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

The star’s precise diameter has been difficult to measure because Betelgeuse is a pulsating variable and its radius keeps changing. There is also a circumstellar envelope of material surrounding the star, a product of increasing mass loss, which makes measurements more difficult. As a result, measurements taken at different wavelengths vary up to 30 to 35%. Limb darkening complicates things even further, with optical emissions varying in colour and fading toward the star’s edge, making the edge hard to define.

However, advances in technology and methodology – mainly in astronomical interferometry – as well as the arrival of space telescopes and observatories (Hipparcos, Hubble and Spitzer), have made it possible to study the star in a number of ways and give some good estimates regarding its properties.

This collage shows the Orion constellation in the sky (Betelgeuse is identified by the marker), a zoom towards Betelgeuse, and the sharpest ever image of this supergiant star, which was obtained with NACO on ESO’s Very Large Telescope., image: ESO, P.Kervella, Digitized Sky Survey 2 and A. Fujii

In a study published in The Astrophysical Journal in December 2000, measurements of the star’s diameter gave a value of 55.2 ± 0.5 milliarcseconds. At the time, the accepted value for the star’s parallax, obtained from the Hipparcos satellite, was 7.63 ± 1.64 milliarcseconds, giving a radius of 3.6 astronomical units. The measurements were carried out in November 1999 with the Infrared Spatial Interferometer (ISI) at Mount Wilson, California and the findings were not inconsistent with the values given by the American physicist Albert Michelson and astronomers Francis Pease and John Anderson in 1920.

Infrared image of Betelgeuse made using WISE and Akari data, image: Giuseppe Donatiello (CC0 1.0)

However, in 2009, a team at the Space Sciences Laboratory and Department of Physics, University of California, Berkeley published an infrared interferometric study that revealed that Betelgeuse had shrunk in size by 15% since 1993. Initial measurements revealed a size similar to Michelson’s result, but over the 15 years of observations, the star was shrinking at an increasing rate without showing significant changes in brightness. The angular measurement in 2008 was 47.0 ± 0.1 milliarcseconds, down from the 56.0 ± 0.1 milliarcseconds observed in 1993. In other words, the star had shrunk by nearly 0.9 astronomical units in 15 years. What is uncertain is whether the measurement reflects the star’s real size or it is the result of its photosphere expanding and contracting.

Ongoing observations of the star’s mass loss may provide insight into what happens in the last stages of a red supergiant’s evolution that brings about a supernova explosion.

uy scuti size,betelgeuse size,vy canis majoris size
UY Scuti compared to planets and other large stars, image: Wikimedia Commons/IStoleThePies (CC BY-SA 4.0)

Subsequent observations with the Berkeley Infrared Spatial Interferometer (ISI), published in 2011, found a strong variability in the star’s size, effective surface temperature, and the location and degree of asymmetry between 2006 and 2009, possibly indicating an evolving shell of cool, optically thick material near the star’s photosphere that interfered with the measurements.

Images obtained from the Very Large Telescope Interferometer (VLTI), operated by the European Southern Observatory (ESO) in the Atacama Desert in Chile, revealed a complex and asymmetric envelope surrounding the star, with a vast, bright plume of gas extending to a radius of about six times the star’s photosphere (about 30 astronomical units, or the average distance from the Sun to Neptune) in the southwest quadrant. The team of astronomers who made the observations suggested that the plume may be related either to the star’s rotation or to the presence of a convective hot spot on the photosphere.

Image of the supergiant star Betelgeuse obtained with the NACO adaptive optics instrument on ESO’s Very Large Telescope. The use of NACO combined with a so-called “lucky imaging” technique, allowed the astronomers to obtain the sharpest ever image of Betelgeuse, even with Earth’s turbulent, image-distorting atmosphere in the way. The resolution is as fine as 37 milliarcseconds, which is roughly the size of a tennis ball on the International Space Station (ISS), as seen from the ground. The image is based on data obtained in the near-infrared, through different filters. The field of view is about half an arcsecond wide, North is up, East is left. Credit: ESO/P. Kervella

Brightness  

Betelgeuse is a semiregular variable star, classified in the subgroup SRc, which also includes Mu Cephei (Herschel’s Garnet Star), RW Cygni, VX Sagittarii, and CE Tauri. These are cool luminous stars of late spectral classes – mostly pulsating red supergiants – that show variations of about 1 magnitude over periods that range from 30 days to several thousand days.

Betelgeuse usually shows only minor changes in brightness close to magnitude 0.5, but can at times shine at magnitude 0.0 or dim to magnitude +1.3. The period given for Betelgeuse in the General Catalogue for Variable Stars (GCVS) is 2,335 days.

A more detailed study published in October 2006 that analysed brightness variations of red supergiants using data collected by the American Association of Variable Star Observers (AAVSO) over the last century, revealed that Betelgeuse showed two different time-scales of variations, a slow one lasting a few thousand days and a faster one of several hundred days. During the observations, Betelgeuse showed two distinct periods, one of 388 ± 30 days and another of 2,050 ± 460 days. The variable red supergiant TV Geminorum in the constellation Gemini showed similar periods. The shorter periods are believed to be the result of fundamental and first overtone pulsations, while the source of the longer periods is unknown, but it is not related to radial pulsations. Various theories suggest that the longer periods are caused by the evolution of massive convection cells in combination with the star’s rotation, or by interactions with a close companion, or by non-radial g-modes, or chromospheric magnetic activity.

The visual magnitude of Betelgeuse varies between magnitude 0.0 and +1.3. At times the star rivals Vega (Alpha Lyrae) and Capella (Alpha Aurigae), the 5th and 6th brightest stars in the sky, while at other times it is fainter than Deneb (Alpha Cygni) and Mimosa (Beta Crucis), the 19th and 20th brightest stars.

If we could see at all wavelengths, Betelgeuse would be the brightest star in the sky. Only 13% of its energy is emitted in visible light, but it is the brightest near-infrared source in the sky. Its J-band magnitude of -2.99 puts it ahead of Antares (-2.7), R Doradus (-2.6), Arcturus (-2.2) and Aldebaran (-2.1).

Betelgeuse’s variability has probably been known since antiquity, but the first person to give an account of it was the English astronomer and mathematician Sir John Herschel in 1836. Herschel observed the star from the 1830s to the 1860s and noticed that its brightness varied significantly. Betelgeuse outshone Rigel twice in the 1830s – in October 1837 and November 1839 – and then, after a quiet decade, its variability peaked again in 1852. In December of that year, Herschel described it as the brightest star in the northern hemisphere.

Based on the data from the American Association of Variable Star Observers (AAVSO), Betelgeuse reached a maximum brightness in 1933 and 1942, peaking at magnitude 0.2, and it was at its faintest in 1927 and 1941, dropping to magnitude 1.2.

The star’s variability may be the reason why German uranographer Johann Bayer assigned it the Greek letter Alpha in his Uranometria (1603). Betelgeuse may have been brighter than Rigel (Beta Orionis) at the time.

Colour

With a B-V colour index of 1.85, Betelgeuse is distinctly red. The star and its distinctly red colour have been documented since ancient times. The Greco-Roman astronomer Ptolemy described Betelgeuse as “the bright, reddish star on the right shoulder” in his Almagest in the 2nd century CE. However, three centuries earlier, Chinese astronomers made a reference to the star’s yellow colour. If their observations were correct, this could indicate that Betelgeuse was not a red supergiant at the time, but still in the yellow supergiant evolutionary stage.

Italian astronomer Angelo Secchi, a pioneer in astronomical spectroscopy, collected the spectra of about 4,000 stars in the 19th century and developed the first system of stellar classification. He named Betelgeuse one of the prototypes for his Class III stars. These were stars orange to red in colour, corresponding to what we know today as class M. Antares in Scorpius was the other prototype for the class.

Mass

Betelgeuse is in the late stage of its life and, like other evolved massive stars, it is losing mass at a high rate, possibly up to 1 solar mass every 10,000 years.

Betelgeuse is not losing mass evenly, which is evident in the large plume of gas extending across a distance six or more times the star’s radius. The uneven mass loss is suspected to be caused either by very large convection cells in the star’s photosphere or by mass loss along polar plumes, possibly as a result of rotation.

This artist’s impression shows the supergiant star Betelgeuse as it was revealed thanks to different state-of-the-art techniques on ESO’s Very Large Telescope, which allowed two independent teams of astronomers to obtain the sharpest ever views of the supergiant star Betelgeuse. They show that the star has a vast plume of gas almost as large as our Solar System and a gigantic bubble boiling on its surface. These discoveries provide important clues to help explain how these mammoths shed material at such a tremendous rate. The scale in units of the radius of Betelgeuse as well as a comparison with the Solar System is also provided. Image: ESO/L. Calçada

Modern estimates of the star’s mass have been in the range from 9.5 to 21 solar masses. The star’s initial mass is believed to have been between 15 and 20 solar masses, based on its estimated luminosity of 90,000 to 150,000 solar luminosities. In 2011, a team of astronomers from the University of Bonn, University of Toronto and University of São Paulo calculated a current mass of 11.6 solar masses (allowing for error in the range from 7.7 to 16.6 solar masses), with an effective temperature of 3,590 ± 55 K, an angular diameter of 44.93 ± 0.15 milliarcseconds, and a radius of 955 ± 217 solar radii.

In 2016, a study using evolutionary models and adopting the VLA-Hipparcos distance of  197 ± 45 parsecs estimated a progenitor mass of 20 solar masses (17 – 25 solar masses), a current mass of 19.4 – 19.7 solar masses, a radius of 887 ± 203 solar radii, and a current age between 8 and 8.5 million years.

Distance

After Rigel, Orion’s brightest star, Betelgeuse is the most distant of the 10 brightest stars in the sky.

The currently accepted distance for Betelgeuse is between 643 and 724 light years, depending on the source. Since the first measurements were made in 1920, estimates of the star’s distance went as high as 1,300 light years (400 parsecs), but more recent studies indicate that the star is significantly closer to us. The observations in 1920 yielded a value of about 180 light years (56 parsecs) with the incorrect parallax of 0.0180 arcseconds. Before the release of the data collected by the Hipparcos satellite in the 1990s, there were two conflicting values for the star’s parallax, one of 9.8 ± 4.7 milliarcseconds, which gave a distance of about 330 light years (102 parsecs) and another of 5 ± 4 milliarcseconds and a distance of 650 light years (200 parsecs).

In 1997, the Hipparcos data was released with the measured parallax of 7.63 ± 1.64 milliarcseconds for Betelgeuse, which yielded a distance of about 430 light years (131 parsecs). Taking into account the uncertainty of the data collected for variable stars, this value was recalculated to 6.55 ± 0.83 milliarcseconds, giving a distance of 520 ± 73 light years (152 ± 20 parsecs).

Observations with the Very Large Array (VLA) in 2008 yielded a value of 5.07 ± 1.10 mas, with a distance of 643 ± 146 light years (197 ± 45 parsecs).

The most recent observations combining the VLA and new e-MERLIN and ALMA radio positions with the 2007 Hipparcos Intermediate Astrometric Data (HIAD), using data collected over 34 years (1982-2016) and including radio Cosmic Noise of 2.4 mas, produced a parallax of 4.51 ± 0.80 mas, which gives a distance of about 724 light years (222 parsecs), with a margin of error that allows for the range from 567.5 to 864.3 light years (174 – 256 parsecs).

This orange blob shows the nearby star Betelgeuse, as seen by the Atacama Large Millimeter/submillimeter Array (ALMA). This is the first time that ALMA has ever observed the surface of a star and this first attempt has resulted in the highest-resolution image of Betelgeuse available. Betelgeuse is one of the largest stars currently known — with a radius around 1400 times larger than the Sun’s in the millimeter continuum. About 600 light-years away in the constellation of Orion (The Hunter), the red supergiant burns brightly, causing it to have only a short life expectancy. The star is just about eight million years old, but is already on the verge of becoming a supernova. When that happens, the resulting explosion will be visible from Earth, even in broad daylight. The star has been observed in many other wavelengths, particularly in the visible, infrared, and ultraviolet. Using ESO’s Very Large Telescope astronomers discovered a vast plume of gas almost as large as our Solar System. Astronomers have also found a gigantic bubble that boils away on Betelgeuse’s surface. These features help to explain how the star is shedding gas and dust at tremendous rates (eso0927, eso1121). In this picture, ALMA observes the hot gas of the photosphere of Betelgeuse at sub-millimeter wavelengths — where localised increased temperatures explain why it is not symmetric. Scientifically, ALMA can help us to understand the extended atmospheres of these hot, blazing stars. Image: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella, 2017

Supernova

Betelgeuse is a massive star that will inevitably end its life in a supernova explosion. The red supergiant was once a hot, luminous class O star and, like all massive stars, it will live a short life. Its mass, rotation rate and other properties will ultimately determine the date and details of its end. The star’s core will eventually collapse, triggering a supernova which will leave behind a dense remnant, either a neutron star of about 1.5 solar masses or a black hole.

The star’s initial mass is uncertain and can only be estimated based on its current properties. Most estimates have been in the range between 10 to 25 solar masses, with recent studies narrowing it down to between 15 and 20 solar masses. A star with 15 solar masses would take between 11.5 and 15 million years to reach Betelgeuse’s current red supergiant phase, while one with 20 masses would take between 8.1 and 9.3 million years, depending on its rotation rate. (Slow rotators would take less time, while fast spinners would take the longest.)

It is also uncertain how long the star has been a red supergiant, with estimates ranging between 20,000 years and 140,000 years. A study published in 2016 using evolutionary models estimated an initial mass of 20 solar masses and an age of 8.5 million years, concluding that Betelgeuse has only recently begun burning helium and will likely begin carbon burning within 100,000 years and explode as a supernova shortly thereafter.

If the star explodes in the current red supergiant stage, it will likely produce a Type II-P supernova. If it first passes through another yellow supergiant stage and evolves blueward (into a blue supergiant or a Wolf-Rayet star), it may explode as a Type II-L supernova. If it explodes as a type II-P supernova, the explosion will have an estimated magnitude of -12.4, possibly outshining the full Moon for a couple of months before rapidly dimming.

The exact date of the explosion cannot be predicted. How much time Betelgeuse has left depends on its initial properties and on the time it has already spent as a red supergiant. The total time from the beginning of the red supergiant stage to the collapse of the core varies from 300,000 years to up to a million years. The model for a non-rotating star with 20 solar masses predicts less than 100,000 years, while rotating or lower-mass stars would take considerably longer.

When Betelgeuse does finally explode, the supernova will not affect the solar system because the star is too distant for its expelled material, ultraviolet or X-ray radiation to affect us.

Facts

Betelgeuse has the second largest apparent size of all stars except the Sun and R Doradus, a red giant star in the southern constellation Dorado, classified as a Mira variable. Betelgeuse was the star with the largest angular diameter known until 1997, when the diameter measurements of R Doradus produced a value of 57.0 ± 0.5 mas.

Betelgeuse was the first star other than the Sun to have the angular size of its photosphere measured. In 1920, American physicist Albert Michelson and astronomers Francis Pease and John Anderson used a 6-metre interferometer with the 2.5-metre telescope at the Mount Wilson Observatory in California to measure the star’s angular diameter. They got the value of 0.047 arcseconds, which translated into a diameter of 2.58 astronomical units using the parallax value of 0.018 arcseconds. However, the results of their measurements are not dependable. Later observations yielded an angular diameter between 0.042 and 0.056 arcseconds.

Betelgeuse is one of the four Orion stars selected for navigation. The other three navigational stars in the constellation are Rigel, Bellatrix and Alnilam.

Studies in the late 1980s and 1990s brought the first images of Betelgeuse’s stellar disk in optical and infrared wavelengths. These were the first such images of a star other than the Sun at the time.

In February 1989, high-resolution images of Betelgeuse revealed an asymmetric feature on its surface, possibly the result of a close companion passing in front of it or due to convection in the star’s atmosphere. The images were part of a study conducted by a team of astronomers at the Mullard Radio Astronomy Observatory in Cambridge and the Palomar Observatory in California.

Interferometric observations from the Royal Greenwich Observatory and the Mullard Radio Astronomy Observatory carried out over eight weeks between November 1994 and January 1995 showed complex asymmetries in the star’s brightness distribution and at least three bright spots changing positions during the observation period, indicating convective surface hotspots. The observations also revealed the presence of a dust halo at least 0.3 arcseconds across.

A study published in 1997, conducted in 1995 by a team at the Mullard Radio Astronomy Observatory and the lnstitute of Astronomy in Cambridge, did not find evidence of the brightness asymmetries, only a strongly limb-darkened, but symmetric disc.  (Limb darkening is a gradual decrease in the brightness of a stellar disk seen from the centre to the limb or edge.)

In 1995, the Hubble Space Telescope captured the first direct-image of the star’s disk. The ultraviolet image had a significantly better resolution than those taken by ground-based telescopes. The image confirmed the presence of a bright patch in the southwest quadrant, strongly suggesting that there was a region about 2,000 K hotter than the star’s surface.

This is the first direct image of a star other than the Sun. Called Alpha Orionis, or Betelgeuse, the star is a red super giant, a Sun-like star nearing the end of its life. The Hubble picture reveals a huge ultraviolet atmosphere with a mysterious hot spot on the stellar behemoth’s surface. The enormous bright spot, twice the diameter of the Earth’s orbit, is at least 2,000 degrees Kelvin hotter than the star’s surface. Image: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA

Ultraviolet spectra obtained with the Goddard High Resolution Spectrograph on board the Hubble Space Telescope indicated that the region was located at one of the star’s poles of rotation. If this is accurate, the rotational axis is inclined about 20° to our line of sight. The observations also yielded a rotation period of 17 years, with a rotational speed of 1.2 × 10-8 rad s-1.

Betelgeuse is surrounded by an intricate envelope about 250 times the star’s size, composed of material lost from the star. The vast dust and gas shells extend almost 20,000 astronomical units away from the star. The star’s extended atmosphere and radial pulsations make it difficult to measure the star’s diameter with any degree of certainty.

Image showing the red supergiant Betelgeuse that pulsates, swells and shrinks asymmetrically, credit: Andrea Dupree, Ronald Gilliland, CfA, STScI, NASA, ESA

The known components of the star’s atmosphere are the photosphere (the outer shell or visible surface), the MOLsphere (the molecular region stretching for 2-3 astronomical units above the photosphere, between the photosphere and the expanding shell), an envelope of gas, the chromosphere (the gaseous layer above the photosphere whose spectrum is dominated by emission lines), a region of dust and two carbon monoxide shells. Betelgeuse’s MOLsphere is known to be composed of carbon monoxide and water vapour, first detected in the 1960s.

Radio image of Betelgeuse’s atmosphere – propelled by convective forces, the star’s atmosphere can expand beyond the orbit of Saturn. Image: NRAO/AUI and J. Lim, C. Carilli, S.M. White, A.J. Beasley, and R.G. Marson

The gaseous envelope extends for about 10 to 40 astronomical units from the photosphere and is asymmetrical.

Radio-telescope observations made in 1998 confirmed that Betelgeuse has a dense and very complex atmosphere which is many times larger than the star itself. They also showed that most of the gas in the star’s atmosphere has about the same temperature as the gas on the surface and constitutes the largest portion of the atmosphere. The lower-temperature gas cools as it is propelled from the surface into the atmosphere.

Betelgeuse’s chromosphere was imaged in near-ultraviolet wavelengths by the Hubble Space Telescope in 2002. The images revealed that it was accelerating outward. The images also showed warm chromospheric plasma at least an arcsecond away from the star, which overlaps and co-exists with the cool gas in the star’s circumstellar dust envelope.

The dust envelope was given a radius estimate between 0.5 and 1 arcseconds in the 1990s, but as it undergoes constant significant changes, even over shorter periods, different studies have produced very different results.

The size of the two outer carbon monoxide shells is also uncertain, but observations in 2009 detected one shell 1.5 arcseconds west of the star, extending for about 4 arcseconds, while the other is extending to about 7 arcseconds.

Betelgeuse may be one of the most studied stars in the sky, but its birthplace is still a mystery. Projecting Betelgeuse’s motion back in time would put its birthplace at a location without a star forming region, indicating that the star has changed course at some point. One theory suggests that the star is a runaway member of the Orion OB1 association, a group of several dozen O and B-class stars that includes Alnitak, Alnilam, Mintaka and the stars in M42 (the Orion Nebula) and M43 (De Mairan’s Nebula). A study published in March 2008 proposed that the star was most likely a member of a multiple star system within the Orion Ob1a subgroup. The subgroup is located to the northwest of Orion’s Belt stars and has an estimated age of 12 million years.

However, a study published in 2015 uncovered a previously unknown group, the Taurion OB association, and proposed that, based on its velocity and proximity to the group, Betelgeuse may belong to this association instead. The Taurion OB association occupies an area of about 20 by 15 degrees between the constellations Orion and Taurus, and includes 36 B stars with an average estimated distance of only 150 parsecs (about 490 light years).

Observations in 2011 showed that Betelgeuse is creating a bow shock as it moves through the interstellar medium at 30 km/s. The bow shock is produced by the star’s strong stellar wind as it collides with interstellar matter in the star’s way and ejects gas into the interstellar medium at 17 km/s. The bow shock is about three light years in diameter and the mass of the bow shock shell is believed to be low, indicating that the bow shock was created relatively recently (in the past 30,000 years) and that Betelgeuse has not been a red supergiant for very long.

Betelgeuse is one of the largest visible stars. Its estimated radius (887 R) does not come near those of Mu Cephei (mag. 4.08, 1,260 – 1,650 R), VV Cephei A (mag.  4.80, 1,050 – 1,900 R) or the mostly invisible VY Canis Majoris (mag. 6.5 – 9.6, 1,420 R), but it is by far the brightest enormous star in the sky. For comparison, the current size record holder, the red supergiant UY Scuti, has an estimated radius about 1,708 times that of the Sun, which makes it almost twice as large as Betelgeuse.

Betelgeuse has up to nine dim line-of-sight companions at a separation of 1 to 4 minutes of arc. All the stars are fainter than 10th magnitude.  Betelgeuse itself is believed to be a single star, one not associated with any companions, clusters or molecular clouds. It was once suspected to have two spectroscopic companions, which were indicated by the polarization data from 1968 to 1983, but later observations, including high-resolution interferometry, found no evidence of close companions.

Betelgeuse is often used or referenced in works of fiction. Its many appearances in literature include Jean Louis De Esque’s Betelguese, a trip through hell (1908), a poetic work in which the star embodies hell, H. P. Lovecraft’s The Cthulhu Mythos (1921- ), M. A. R. Barker’s Tékumel novels and games, Philip K. Dick’s short stories “Tony and the Beetles” (1953) and “Shell Game” (1954), Kurt Vonnegut, Jr.’s novel The Sirens of Titan (1959), Pierre Boulle’s Planet of the Apes (1963), Frank Herbert’s Dune series (1965-), Gérard Klein’s Starmasters’ Gambit (Le Gambit des Étoiles, 1958), Douglas Adams’ The Hitchhiker’s Guide to the Galaxy (1978–1993) novels, and Jeffrey Carver’s From a Changeling Star (1989).

In film, Betelgeuse was memorably referenced in Tim Burton’s Beetlejuice (1988). In Ridley Scott’s Blade Runner (1982), it may have been the star the dying replicant Roy Batty alluded to as “the shoulder of Orion” in his “Tears in the rain” speech. The other candidate is the blue-white Bellatrix, the left shoulder of Orion.

Procyon (upper left), Sirius (botton centre) and Betelgeuse (upper right), image: Hubble/European Space Agency, credit: Akira Fujii

Name

The name Betelgeuse (English pronunciation: /ˈbɛtəldʒuːz/, /ˈbɛtəldʒuːs/, /ˈbiːtəldʒuːz/ or /ˈbiːtəldʒuːs/) was officially approved for the star Alpha Orionis Aa by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on June 30, 2016.

Betelgeuse is the star’s traditional name. The exact origin of the name is disputed, but it is taken either from the Arabic phrase Ibṭ al-Jauzā’, which means “the armpit of the central one” or from the phrase Yad al-Jauzā’, meaning “the hand of the central one.” Al-jauza is translated as “the central one” or sometimes as “the giant,” while the first part of the phrase refers to the star’s position in the constellation.  The name may also have come from the phrase bait al-jauza, which means “the house of the central one.” Jauza was an old Arabic name for a female constellation figure that encompassed the stars of Orion and the neighbouring constellation Gemini, but the identity of the figure is unknown.

The Chinese know Betelgeuse as 参宿四 (Shēnxiùsì), or the Fourth Star of the Constellation of Three Stars. The constellation originally included only the three stars of Orion’s Belt (Alnitak, Alnilam and Mintaka), but was later expanded to include other bright stars of Orion. However, the constellation’s original name stuck.

The Inuit call Betelgeuse Ulluriajjuaq, which means “large star.” To observers in Arctic latitudes, Betelgeuse may appear more prominent than Rigel because it rises higher above the horizon. The Inuit called Betelgeuse and Bellatrix Akuttujuuk, meaning “those (two) placed far apart.” The stars’ appearance over the southern horizon signalled the coming of spring.

Betelgeuse has also been known as Bašn (“the Arm”) in Persian, Klaria (“an Armlet”) in Coptic, and Bahu in Sanskrit, referring to the Hindu conception of Orion as a running stag or antelope.

Another Sanskrit name for the star was ārdrā, or “the moist one.” Rudra, the god of storms, presided over the star.

In Hawaii, the star’s name was Kaulua-koko, meaning “brilliant red star.” The Lacandon people in Mexico also had a name that reflected the star’s colour. They called it chäk tulix, which means “red butterfly.”

Mythology and culture

Like other exceptionally bright stars, Betelgeuse is part of the folklore and history of many different cultures.

In Japan, Betelgeuse was chosen as the symbol of the Taira clan, one of the four important clans during the Heian period (794-1185). They called the star Heike-boshi (平家星). The rivalling Minamoto clan chose Rigel as their symbol. The two clans fought the legendary Genpei War and the two stars were seen as facing off against each other, with only the three stars of Orion’s Belt keeping them apart.

The Batak in North Sumatra, Indonesia celebrated the New Year with the first new Moon after Orion’s Belt disappeared below the horizon, when Betelgeuse lingered “like the tail of a rooster” and Antares rose in the east, at the opposite end of the sky. They saw Orion and Scorpius as a pair of scorpions facing toward one another.

Tahitians knew Betelgeuse as Anâ-varu and considered it one of the pillars holding the sky. They also called it Ta’urua-nui-o-Mere, which means “great festivity in parental yearnings.”

Aboriginal Australians in the Great Victoria Desert incorporated the star’s variability into a legend. In the tale, Orion represents Nyeeruna, who creates fire-magic in his right hand, represented by Betelgeuse, to reach the Yugarilya sisters (the Pleiades). The eldest sister, Kambugudha, symbolized by the Hyades cluster, kicks sand in his face, dispelling his magic, and stops him from gaining access to the sisters. The process is described as cyclic, with Betelgeuse brightening and fading over time.

The Wardaman people in the Northern Territory of Australia called Betelgeuse Ya-jungin, or “Owl Eyes Flicking.” The star’s variability was associated with the watching of ceremonies led by Unumburrgu (Rigel), the Red Kangaroo Leader.

The Pemon people in Brazil called Orion Zililkawai and the constellation represented a man whose wife cut off his leg. Betelgeuse’s variable brightness was associated with the cutting of the limb. In North America, the Lakora people linked the star with a similar legend, one about a chief whose arm was cut off.

In South African lore, the star is associated with a lion watching three zebras, symbolized by the stars of Orion’s Belt.

In Macedonian folklore, Betelgeuse was linked with Orach, “the ploughman,” and the constellation Orion represented a plough with oxen. Betelgeuse rising before the dawn in late summer signalled the time for the people to get up and go to the fields to plough.

Location

Betelgeuse is very easy to find because it is one of the brightest stars in the sky and because it is a part of one of the most recognizable constellations. It marks the right shoulder of Orion (Orion’s right shoulder, or the left one from our perspective) and outlines the constellation’s familiar hourglass shape with the bright stars Bellatrix, Rigel and Saiph, while Alnitak, Alnilam and Mintaka form Orion’s Belt, and the fainter Meissa marks the Hunter’s head.

Orion stars, image: Wikisky

Betelgeuse marks the upper right vertex of the Winter Triangle, a prominent asterism that is easy to spot in the sky during the northern hemisphere winter. The other two vertices of the triangle are marked by Sirius, the brightest star in the sky and the luminary of the constellation Canis Major, and Procyon, the brightest star in Canis Minor and the 8th brightest star in the sky. Also known as the Great Southern Triangle, the Winter Triangle dominates the evening sky from December to March.

The Winter Triangle and the Winter Hexagon, image: Wikisky

While it is not a part of the Winter Circle (Winter Hexagon), a larger winter asterism that contains the Winter Triangle, Betelgeuse is the brightest star within its borders and can be seen near its centre. The asterism is formed by the bright stars Rigel, Sirius, Procyon, Pollux, Capella and Aldebaran.

Procyon, Betelgeuse and the Rosette Nebula, image: Wikisky

Betelgeuse is located in the vicinity of several notable deep sky objects. The Rosette Nebula – a large H II region encompassing New General Catalogue designations NGC 2237, NGC 2238, NGC 2239, NGC 2244 and NGC 2246 – is located in the neighbouring constellation Monoceros, the Unicorn, and can be found in the region of the sky between Betelgeuse and Procyon in Canis Minor. The Cone Nebula with the Christmas Tree Cluster and the smaller Dreyer’s Nebula (IC 2169) are located in the same area.

Betelgeuse, the Rosette Nebula, the Cone Nebula, Dreyer’s Nebula, and Messier 78, image: Wikisky

The Lambda Orionis Ring (Angelfish Nebula) and the Lambda Orionis Cluster (Collinder 69) are centred on the star Meissa, Lambda Orionis, which marks Orion’s head and forms a triangle with Betelgeuse and Bellatrix. The planetary nebula NGC 2022, visible in medium-sized telescopes, can be found about two thirds of the way from Betelgeuse to Meissa.

Constellation

Betelgeuse is located in the constellation Orion, the Hunter. Associated with a hunter, warrior or giant in many mythologies, the constellation is home to two of the 10 brightest stars in the sky – Rigel and Betelgeuse – and contains a good number of popular targets for backyard telescopes.

Orion constellation, image: Roberto Mura

The best known of these are the Orion Nebula (Messier 42), the nearest massive star forming region to Earth, containing the young Trapezium Cluster, the neighbouring De Mairan’s Nebula (Messier 43), the reflection nebula Messier 78, the emission nebulae Barnard’s Loop and the Flame Nebula (NGC 2024), and the dark Horsehead Nebula.

Orion also contains one of the best known asterisms in the sky, Orion’s Belt, formed by the supergiant stars Alnitak and Alnilam and the bright giant Mintaka.

The best time of year to observe the stars of Orion is northern hemisphere winter, when the constellation dominates the evening sky. Betelgeuse is visible from everywhere except south of the latitude 82°S (Antarctica) from mid-September to mid-March. In June and July, it can only be seen with a telescope in daylight, except in Antarctic regions between the latitudes 70°S and 80°S, when the Sun is below the horizon around noon.

The 10 brightest stars in Orion are Rigel (Beta Ori, mag. 0.05 – 0.18), Betelgeuse (Alpha Ori, mag. 0.0 – 1.3), Bellatrix (Gamma Ori, mag. 1.59 to 1.64), Alnilam (Epsilon Ori, mag. 1.64 – 1.74), Alnitak A (Zeta Ori A, mag. 2.00), Saiph (Kappa Ori, mag. 2.09), Mintaka AB (Delta Ori AB, mag. 2.23), Hatysa (Iota Ori, mag. 2.77), Tabit (Pi3 Ori, mag. 3.16), and Eta Orionis (mag. 3.31 – 3.6).

Betelgeuse – Alpha Orionis  

Spectral classM1–M2 Ia–ab
Variable typeSemiregular (SRc)
U-B colour index+2.06
B-V colour index+1.85
Apparent magnitudeV: 0.50 (0.0–1.3)
J: -3.00
K: -4.05
Absolute magnitude-5.85
Distance724 light years (222 parsecs), or 643 ± 146 light years (197 ± 45 parsecs)
Parallax4.51 ± 0.80 mas, or 5.07 ± 1.10 mas
Radial velocity+21.91 km/s
Proper motionRA:  26.42 ± 0.25 mas/yr
Dec.: 9.60 ± 0.12
Mass11.6 M(7.7 to 16.6 M)
Luminosity90,000 – 150,000 L
Radius887 ± 203 R, or 955 ± 217 R
Temperature3,590 K
Age8 – 8.5 million years
Rotational velocity5 km/s
Surface gravity-0.5 cgs
Metallicity+0.05 dex
ConstellationOrion
Right ascension05h 55m 10.30536s
Declination+07° 24′ 25.4304”
DesignationsBetelgeuse, Alpha Orionis, α Ori, 58 Orionis, HD 39801, HR 2061, SAO 113271, HIP 27989, GC 7451, FK5 224, BD+07°1055, AAVSO 0549+07, CCDM J05552+0724AP, GCRV 3679, PPM 149643, IRAS 05524+0723, 2MASS J05551028+0724255, TYC 129-1873-1