Radius of Betelgeuse

Betelgeuse is the canonical red supergiant: roughly 764 times the Sun's radius, or 3.6 au — centred on the Sun its surface would lie beyond the orbit of Mars, deep into the asteroid belt. The honest statement carries wide error bars, +116/−62 R⊙, because a red supergiant barely has an edge: an extended, convulsing atmosphere of enormous convection cells, boiling so vigorously that single plumes distort the whole star's shape, leaves "the radius" partly a matter of definition, and its distance is uncertain too. Betelgeuse was the first star ever resolved to a disc — Michelson and Pease, interferometry on Mount Wilson, 1920 — and modern imaging shows a surface mottled, lopsided and different every year.

The star is fusing its way through its final elements and will explode as a supernova within the next hundred thousand years or so. The Great Dimming of 2019–20, when it faded by two thirds, briefly looked like the prelude; it proved to be a dust cloud belched from that seething surface — a reminder, like this constant's error bars, that supergiants are weather, not geometry.