Betelgeuse

Betelgeuse is a red supergiant of spectral type M1-2 and one of the largest stars visible to the naked eye. It is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orion. It is a distinctly reddish, semiregular variable star whose apparent magnitude, varying between +0.0 and +1.6, has the widest range displayed by any first-magnitude star. At near-infrared wavelengths, Betelgeuse is the brightest star in the night sky. Its Bayer designation is α Orionis, Latinised to Alpha Orionis and abbreviated Alpha Ori or α Ori.

If it were at the center of our Solar System, its surface would lie beyond the asteroid belt and it would engulf the orbits of Mercury, Venus, Earth, and Mars. Nevertheless, there are several even larger stars in the Milky Way, including supergiants like Mu Cephei and the peculiar hypergiant, VY Canis Majoris. Calculations of Betelgeuse's mass range from slightly under ten to a little over twenty times that of the Sun. For various reasons, its distance has been quite difficult to measure; current best estimates are on the order of 500–600 light-years from the Suna comparatively wide uncertainty for a relatively nearby star. Its absolute magnitude is about −6. Less than 10 million years old, Betelgeuse has evolved rapidly because of its large mass and is expected to end its evolution with a supernova explosion, most likely within 100,000 years. Having been ejected from its birthplace in the Orion OB1 associationwhich includes the stars in Orion's Beltthis runaway star has been observed to be moving through the interstellar medium at a speed of , creating a bow shock over four light-years wide.

In 1920, Betelgeuse became the first extrasolar star whose photosphere's angular size was measured. Subsequent studies have reported an angular diameter (i.e., apparent size) ranging from 0.042 to 0.056 arcseconds; that range of determinations is ascribed to non-sphericity, limb darkening, pulsations and varying appearance at different wavelengths. It is also surrounded by a complex, asymmetric envelope, roughly 250 times the size of the star, caused by mass loss from the star itself. The Earth-observed angular diameter of Betelgeuse is exceeded only by those of R Doradus and the Sun.

Starting in October 2019, Betelgeuse began to dim noticeably, and by mid-February 2020 its brightness had dropped by a factor of approximately 3, from magnitude 0.5 to 1.7. By 22 February 2020, Betelgeuse stopped dimming and started to brighten again; and, as reported on 25 February 2022, has remained in a more normal brightness range. Infrared observations found no significant change in brightness over the last 50 years, suggesting that the dimming was due to a change in extinction or "large-grain circumstellar dust", rather than an underlying change in the luminosity of the star. A 2022 study using the Hubble Space Telescope suggests that occluding dust was created by a surface mass ejection. This surface mass ejection cast material millions of miles from the star that then cooled to form the dust that caused the star's dimming.

Nomenclature

The star's designation is ''α Orionis'' (Latinised to ''Alpha Orionis''), given by Johann Bayer in 1603.

The traditional name ''Betelgeuse'' has been derived from the Arabic ' "the hand of ''al-Jauzā’'' .e. Orion. An error, in the 13th century AD, reading the Arabic ''ya'' as ''ba'' led to the European name. In English, there are four common pronunciations of this name, depending on whether the first ''e'' is pronounced short or long and whether the ''s'' is pronounced "s" or "z":
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The last pronunciation has been popularized for sounding like " beetle juice".

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)

to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016

included a table of the first two batches of names approved by the WGSN, which included ''Betelgeuse'' for this star. It is now so entered in the IAU Catalog of Star Names.

Observational history

Betelgeuse and its red coloration have been noted since antiquity; the classical astronomer Ptolemy described its color as ὑπόκιρρος (''hypókirrhos'' = more or less orange-tawny), a term that was later described by a translator of Ulugh Beg's ''Zij-i Sultani'' as ''rubedo'', Latin for "ruddiness". In the nineteenth century, before modern systems of stellar classification, Angelo Secchi included Betelgeuse as one of the prototypes for his Class III (orange to red) stars. By contrast, three centuries before Ptolemy, Chinese astronomers observed Betelgeuse as having a ''yellow'' color; if accurate, such an observation could suggest the star was in a yellow supergiant phase around this time,

a possibility given current research into the complex circumstellar environment of these stars.

Nascent discoveries

Aboriginal groups in South Australia have shared oral tales of the variable brightness of Betelgeuse for at least 1,000 years.

The variation in Betelgeuse's brightness was 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. 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.

This variability in brightness may explain why Johann Bayer, with the publication of his ''Uranometria'' in 1603, designated the star ''alpha'' as it probably rivaled the usually brighter Rigel (''beta'').

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".

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 () based on the parallax value of .

However, limb darkening and measurement errors resulted in uncertainty about the accuracy of these measurements.

The 1950s and 1960s saw two developments that would affect 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.

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.

Imaging breakthroughs

In the 1970s, astronomers 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.

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,

causing them to suspect the presence of huge gas bubbles resulting from convection.

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.

The technique contributed some of the most accurate measurements of Betelgeuse while revealing bright spots on the star's photosphere.

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.

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.

Because ultraviolet light is absorbed by the Earth's atmosphere, observations at these wavelengths are best performed by space telescopes.

Like earlier pictures, this image contained a bright patch indicating a region in the southwestern quadrant hotter than the stellar surface. Subsequent ultraviolet spectra taken with the Goddard High Resolution Spectrograph suggested that the hot spot was one of Betelgeuse's poles of rotation. This would give the rotational axis an inclination of about 20° to the direction of Earth, and a position angle from celestial North of about 55°.

2000s studies

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 – a figure entirely consistent with Michelson's findings eighty years earlier.

At the time of its publication, the estimated parallax from the Hipparcos mission was , yielding an estimated radius for Betelgeuse of . However, an infrared interferometric study published in 2009 announced that the star had shrunk by 15% since 1993 at an increasing rate without a significant diminution in magnitude.

Subsequent observations suggest that the apparent contraction may be due to shell activity in the star's extended atmosphere.

In addition to 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 process by which mass is lost remains a mystery.

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 extending from the star into the surrounding atmosphere.

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.

2019–20 fading

A pulsating semiregular variable star, Betelgeuse is subject to multiple cycles of increasing and decreasing brightness due to changes in its size and temperature. The astronomers who first noted the dimming of Betelgeuse, Villanova University astronomers Richard Wasatonic and Edward Guinan, and amateur Thomas Calderwood, theorize that a coincidence of a normal 5.9-year light-cycle minimum and a deeper-than-normal 425-day period are the driving factors. Other possible causes hypothesized by late-2019 were an eruption of gas or dust, or fluctuations in the star's surface brightness.

By August 2020, long-term and extensive studies of Betelgeuse, primarily using ultraviolet observations by the Hubble Space Telescope, suggest that the unexpected dimming was probably caused by an immense amount of superhot material ejected into space. The material cooled and formed a dust cloud that blocked the starlight coming from about a quarter of Betelgeuse's surface. Hubble captured signs of dense, heated material moving through the star's atmosphere in September, October, and November before multiple telescopes observing the more marked dimming in December and the first several months of 2020.

By January 2020, Betelgeuse had dimmed by a factor of approximately 2.5 from magnitude 0.5 to 1.5, and reported still fainter in February in '' The Astronomer's Telegram'' at a record minimum of +1.614, noting that the star is currently the "least luminous and coolest" in the 25 years of their studies and also calculating a decrease in radius. ''Astronomy'' magazine described it as a "bizarre dimming", and popular speculation inferred that this might indicate an imminent supernova. This dropped Betelgeuse from one of the top 10 brightest stars in the sky to outside the top 20, noticeably dimmer than its near neighbor Aldebaran. Mainstream media reports discussed speculation that Betelgeuse might be about to explode as a supernova, but astronomers note that the supernova is expected to occur within approximately the next 100,000 years and is thus unlikely to be imminent.

By 17 February 2020, Betelgeuse's brightness had remained constant for about 10 days, and the star showed signs of rebrightening. On 22 February 2020, Betelgeuse may have stopped dimming altogether, all but ending the dimming episode. On 24 February 2020, no significant change in the infrared over the last 50 years was detected; this seemed unrelated to the recent visual fading, and suggested that an impending core collapse may be unlikely. Also on 24 February 2020, further studies suggested that occluding "large-grain circumstellar dust" may be the most likely explanation for the dimming of the star. A study that uses observations at submillimetre wavelengths rules out significant contributions from dust absorption. Instead, large starspots appear to be the cause for the dimming. Followup studies, reported on 31 March 2020 in ''The Astronomer's Telegram'', found a rapid rise in the brightness of Betelgeuse.

Betelgeuse is almost unobservable from the ground between May and August because it is too close to the Sun. Before entering its 2020 conjunction with the Sun, Betelgeuse had reached a brightness of +0.4 . Observations with the STEREO-A spacecraft made in June and July 2020 showed that the star had dimmed by 0.5 since the last ground-based observation in April. This is surprising, because a maximum was expected for August/September 2020, and the next minimum should occur around April 2021. However, Betelgeuse's brightness is known to vary irregularly, making predictions difficult. The fading could indicate that another dimming event might occur much earlier than expected. On 30 August 2020, astronomers reported the detection of a second dust cloud emitted from Betelgeuse, and associated with recent substantial dimming (a secondary minimum on 3 August) in luminosity of the star.

In June 2021, the dust has been explained as possibly caused by a cool patch on its photosphere and in August a second independent group confirmed these results. The dust is thought to have resulted from the cooling of gas ejected from the star. An August 2022 study using the Hubble Space Telescope confirmed previous research and suggested the dust could have been created by a surface mass ejection. It conjectured as well the dimming could came from short-term minimum coinciding with a long-term minimum producing a grand minimum, a 416-day cycle and 2010-day cycle respectively, a mechanism first suggested by astronomer Leo Goldberg.

Observation

As a result of its distinctive orange-red color and position within Orion, Betelgeuse is easy to find with the naked eye. It is one of three stars that make up the Winter Triangle asterism, and it marks the center of the Winter Hexagon. At the beginning of January of each year, it can be seen rising in the east just after sunset. Between mid-September to mid-March (best in mid-December), it is visible to virtually every inhabited region of the globe, except 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), except around midday in Antarctic regions between 70° and 80° south latitude (during polar night, when the Sun is below the horizon).

Betelgeuse is a variable star whose visual magnitude ranges between 0.0 and +1.6 . There are periods during which it surpasses Rigel to become the sixth brightest star, and occasionally it will become even brighter than Capella. At its faintest, Betelgeuse can fall behind Deneb and Beta Crucis, themselves both slightly variable, to be the twentieth-brightest star.

Betelgeuse has a B–V color index of 1.85 – a figure which points to its pronounced "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 (rather than ionized). This extended gaseous atmosphere has been observed moving toward and away from Betelgeuse, depending on fluctuations in the photosphere. Betelgeuse is the brightest near-infrared source in the sky with a J band magnitude of −2.99; only about 13% of the star's radiant energy is emitted as visible light. If human eyes were sensitive to radiation at all wavelengths, Betelgeuse would appear as the brightest star in the night sky.

Catalogues list up to nine faint visual companions to Betelgeuse. They are at distances of about one to four arc-minutes and all are fainter than 10th magnitude.

Star system

Betelgeuse is generally considered to be a single isolated star and a runaway star, not currently associated with any cluster or star-forming region, although its birthplace is unclear.

Two spectroscopic companions to Betelgeuse have been proposed. Analysis of polarization data from 1968 through 1983 indicated a close companion with a periodic orbit of about 2.1 years, and by using speckle interferometry, the team concluded that the closer of the two companions was located at (≈9 AU) from the main star with a position angle of 273°, an orbit that would potentially place it within the star's chromosphere. The more distant companion was at (≈77 AU) with a position angle of 278°. Further studies have found no evidence for these companions or have actively refuted their existence, but the possibility of a close companion contributing to the overall flux has never been fully ruled out. High-resolution interferometry of Betelgeuse and its vicinity, far beyond the technology of the 1980s and 1990s, has not detected any companions.

Distance measurements

Parallax is the apparent change of the position of an object, measured in seconds of arc, caused by the change of position of the observer of that object. As the Earth orbits the Sun, every star is seen to shift by a fraction of an arc second, which measure, combined with the baseline provided by the Earth's orbit gives the distance to that star. 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.

In 1920, when the first interferometric studies were performed on the star's diameter, the assumed parallax was . This equated to a distance of or roughly , 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 or about .

Before the publication of the Hipparcos Catalogue (1997), there were two slightly conflicting parallax measurements for Betelgeuse. The first, in 1991, gave a parallax of , yielding a distance of roughly or .

The second was the Hipparcos Input Catalogue (1993) with a trigonometric parallax of , a distance of or .

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

The results from the Hipparcos mission were released in 1997. The measured parallax of Betelgeuse was , which equated to a distance of roughly or , and had a smaller reported error than previous measurements.

However, later evaluation of the Hipparcos parallax measurements for variable stars like Betelgeuse found that the uncertainty of these measurements had been underestimated.
In 2007, an improved figure of was calculated, hence a much tighter error factor yielding a distance of roughly or .

In 2008, using the Very Large Array (VLA), produced a radio solution of , equaling a distance of or . As the researcher, Harper, points out: "The revised Hipparcos parallax leads to a larger distance () 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. An updated result from further observations with ALMA and e-Merlin gives a parallax of mas and a distance of pc or ly.

In 2020, new observational data from the space-based ''Solar Mass Ejection Imager'' aboard the Coriolis satellite and three different modeling techniques produced a refined parallax of mas, a radius of , and a distance of pc or ly, which, if accurate, would mean Betelgeuse is nearly 25% smaller and 25% closer to Earth than previously thought.

Although the European Space Agency's current Gaia mission was not expected to produce good results for stars brighter than the approximately V=6 saturation limit of the mission's instruments, actual operation has shown good performance on objects to about magnitude +3. Forced observations of brighter stars mean that final results should be available for all bright stars and a parallax for Betelgeuse will be published an order of magnitude more accurate than currently available. There is no data on Betelgeuse in Gaia Data Release 2.

Variability

Betelgeuse is classified as a semiregular variable star, indicating that some periodicity is noticeable in the brightness changes, but amplitudes may vary, cycles may have different lengths, and there may be standstills or periods of irregularity. It is placed in subgroup SRc; these are pulsating red supergiants with amplitudes around one magnitude and periods from tens to hundreds of days.

Betelgeuse typically shows only small brightness changes near to magnitude +0.5, although at its extremes it can become as bright as magnitude 0.0 or as faint as magnitude +1.6. Betelgeuse is listed in the General Catalogue of Variable Stars with a possible period of 2,335 days. More detailed analyses have shown a main period near 400 days, a short period of 185 days, and a longer secondary period around 2,100 days. The lowest reliably-recorded V-band magnitude of +1.614 was reported in February 2020.

Radial pulsations of red supergiants are well-modelled and show that periods of a few hundred days are typically due to fundamental and first overtone pulsation. Lines in the spectrum of Betelgeuse show doppler shifts indicating radial velocity changes corresponding, very roughly, to the brightness changes. This demonstrates the nature of the pulsations in size, although corresponding temperature and spectral variations are not clearly seen. Variations in the diameter of Betelgeuse have also been measured directly. First overtone pulsations of 185 days have been observed, and the ratio of the fundamental to overtone periods gives valuable information about the internal structure of the star and its age.

The source of the long secondary periods is unknown, but they cannot be explained by radial pulsations. Interferometric observations of Betelgeuse have shown hotspots that are thought to be created by massive convection cells, a significant fraction of the diameter of the star and each emitting 5–10% of the total light of the star. One theory to explain long secondary periods is that they are caused by the evolution of such cells combined with the rotation of the star. Other theories include close binary interactions, chromospheric magnetic activity influencing mass loss, or non-radial pulsations such as g-modes.

In addition to the discrete dominant periods, small-amplitude stochastic variations are seen. It is proposed that this is due to granulation, similar to the same effect on the sun but on a much larger scale.

Diameter

On 13 December 1920, Betelgeuse became the first star outside the Solar System to have the angular size of its photosphere measured. 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 , 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". Since then, other studies have produced angular diameters that range from 0.042 to . Combining these data with historical distance estimates of 180 to yields a projected radius of the stellar disk of anywhere from 1.2 to . Using the Solar System for comparison, the orbit of Mars is about , Ceres in the asteroid belt , Jupiter