Betelgeuse


Betelgeuse is a red supergiant star in the equatorial constellation of Orion. It is usually the tenth-brightest star in the night sky and, after Rigel, the second brightest in its constellation. It is a distinctly reddish, semiregular variable star whose apparent magnitude, varying between +0.0 and +1.6, with a main period near 400 days, has the widest range displayed by any first-magnitude star. Betelgeuse is the brightest star in the night sky at near-infrared wavelengths. Its Bayer designation is ', Latinised to Alpha Orionis and abbreviated Alpha Ori or '.
With a radius between 640 and 764 times that of the Sun, if it were at the center of the Solar System, its surface would lie beyond the asteroid belt and it would engulf the orbits of Mercury, Venus, Earth, and Mars. 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 of the order of 400–600 light-years from the Suna comparatively wide uncertainty for a relatively nearby star. Its absolute magnitude is about −6. With an age of less than 10 million years, 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. When Betelgeuse explodes, it will shine as bright as the half-Moon for more than three months; life on Earth will be unharmed. 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.
Betelgeuse became the first extrasolar star whose photosphere's angular size was measured in 1920, and subsequent studies have reported an angular diameter 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. It then returned to a more normal brightness range, reaching a peak of 0.0 visual and 0.1 V-band magnitude in April 2023. Infrared observations found no significant change in luminosity over the last 50 years, suggesting that the dimming was due to a change in extinction around the star rather than a more fundamental change. A study using the Hubble Space Telescope suggests that occluding dust was created by a surface mass ejection; this material was cast millions of miles from the star, and then cooled to form the dust that caused the dimming.
Though unconfirmed, there is evidence that Betelgeuse may be a binary star. The companion star, named Betelgeuse B or Siwarha, would be much smaller and fainter than the red supergiant and is believed to orbit at a distance only a few times greater than the size of Betelgeuse.

Nomenclature

The star's Bayer designation is , given by Johann Bayer in 1603.
The traditional name Betelgeuse was derived from the Arabic يد الجوزاء "the hand of al-Jawzā’ ". An error in the 13th-century reading of the Arabic initial yā’ as bā’ 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 or :
In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog and standardize proper names for stars. The WGSN's first bulletin, issued 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.
The discoverers of the candidate companion star Betelgeuse B proposed the name Siwarha, which means her bracelet in Arabic. The name Siwarha has been officially recognized by the WGSN since 22 September 2025.

Observational history

Betelgeuse and its red coloration have been noted since antiquity; the classical astronomer Ptolemy described its color as , a term later described by a translator of Ulugh Beg's Zij-i Sultani as, Latin for 'ruddiness'. In the 19th century, before modern systems of stellar classification, Angelo Secchi included Betelgeuse as one of the prototypes for his Class III stars. Three centuries before Ptolemy, in contrast, Chinese astronomers observed Betelgeuse as yellow; such an observation, if accurate, could suggest the star was in a yellow supergiant phase around this time, a credible possibility, given current research into these stars' complex circumstellar environment.

Nascent discoveries

Aboriginal groups in South Australia have shared oral tales of the variable brightness of Betelgeuse for an unknown period.
The variation in Betelgeuse's brightness was described in 1836 by Sir John Herschel 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 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. 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.
In 1920, Albert A. Michelson and Francis G. Pease mounted a six-meter interferometer on the front of the 2.5-meter telescope at Mount Wilson Observatory, helped by John August Anderson. The trio measured the angular diameter of Betelgeuse at 0.047″, a figure that resulted in a diameter of based on the parallax value of. But limb darkening and measurement errors resulted in uncertainty about the accuracy of these measurements.
The 1950s and 1960s saw two developments that affected 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

Astronomers saw some major advances in astronomical imaging technology in the 1970s, 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. However, 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 J.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 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.
This image, like earlier pictures, 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 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. Images released by the European Southern Observatory in July 2009, taken by the ground-based Very Large Telescope Interferometer, 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.