Betelgeuse

Betelgeuse is Alpha Orionis, a red supergiant of spectral type M1-M2 Ia-ab, at roughly 640 light years, with an apparent magnitude that varies irregularly around 0.5.

It is enormous and its size is genuinely uncertain: estimates of its radius range widely, but it is of the order of several hundred to a thousand times the Sun's, large enough to swallow the orbit of Mars and possibly Jupiter if placed at the centre of the solar system. Its outline is not sharp, the star has no well-defined surface, being surrounded by an extended, irregular, convecting atmosphere with enormous convection cells. It was the first star other than the Sun whose disc was resolved, by Michelson and Pease in 1920 using an interferometer, and the first to be imaged with visible surface detail.

In late 2019 and early 2020 Betelgeuse dimmed dramatically, dropping to around magnitude 1.6, its faintest in a century of records, and prompting widespread speculation that it was about to explode. It recovered. The explanation now generally accepted is a dust cloud: the star ejected material which condensed into obscuring dust in front of it, combined with a cool patch on the surface.

Betelgeuse will end as a supernova, and when it does it will be bright enough to be visible in daylight. The timescale is of order a hundred thousand years, which is imminent for a star and unhelpfully long for anyone waiting.

Despite the Alpha designation it is usually fainter than Rigel, Beta Orionis, Bayer's letters do not strictly follow brightness, and Betelgeuse's variability may have contributed.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy

Ptolemy gives Betelgeuse the nature of Mars and Mercury, and the Mars half is a direct reading of the colour. This is the same move Ptolemy makes with Aldebaran and Antares: the visibly red stars are martial.

That reasoning deserves an assessment rather than either endorsement or dismissal. It is signature reasoning, of exactly the kind found in the plant material, the appearance is read as a statement of character. But it differs in one respect worth noting: the colour Ptolemy recorded is physically real and correctly observed. Betelgeuse is red because it is a cool supergiant, and its spectral type confirms what the eye reports. Ptolemy's observation is sound; only the inference from it is arbitrary. Compare Sirius, which Ptolemy also describes as red and which is not, and where the observation itself fails.

The Mercury half supplies the inventiveness and quickness in Robson's delineation, and it tempers what would otherwise be a purely martial reading. Betelgeuse is generally read as fortunate (martial honour, preferment, wealth, enduring vitality), which distinguishes it from most Mars-flavoured stars.

Betelgeuse is not among Agrippa's Behenian fifteen, despite being one of the brightest stars in the sky. Neither is Rigel. The Behenian list is not a list of the brightest stars and was not selected on that basis; it is a talismanic working set, and its selection criteria are not stated by Agrippa, who attributes the whole scheme to Hermes Trismegistus, an attribution that means the true origin is unknown.

The name's derivation is uncertain, and the common gloss the hand of the giant rests on a misreading of the Arabic that entered European usage early and stuck.