Betelgeuse

**Betelgeuse** 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**.

Classified as a red supergiant of spectral type M1-2, Betelgeuse is one of the largest stars visible to the naked eye. 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 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 Sun – a 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 Association – which includes the stars in Orion's Belt – this runaway star has been observed moving through the interstellar medium at a speed of 30 km/s, 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. Infrared observations found no significant change in brightness over the last 50 years, suggesting that the dimming is due to a change in extinction rather than an underlying change in the luminosity of the star. Further studies suggested that occluding "large-grain circumstellar dust" may be the most likely explanation for the dimming of the star.

## Nomenclature

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

The traditional name *Betelgeuse* is derived from the Arabic يد الجوزاء '**Yad al-Jauzā’'** "the hand of *al-Jauzā’* [i.e. Orion]". 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":

* /ˈbɛtəldʒuːz/
* /ˈbiːtəldʒuːz/
* /ˈbɛtəldʒuːs/
* /ˈbiːtəldʒuːs/

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 the beginning of the Christian era, a possibility given current research into the complex circumstellar environment of these stars.