Distance to Betelgeuse

Betelgeuse's distance is embarrassingly uncertain for so famous a star, and the reason is instructive: its parallax, about 6 milliarcseconds, is smaller than the star's own 42-milliarcsecond disc. The photocentre a telescope tracks wanders across that bloated, spotted surface as convection cells brighten and fade, swamping the parallax signal; Hipparcos and radio measurements disagreed by tens of per cent, and Betelgeuse is too bright for Gaia to handle cleanly. The value adopted here, 168 (+27/−15) pc, folds asteroseismic constraints in with the astrometry — and every property scaled from the distance, the radius and luminosity above all, inherits its error bars.

The uncertainty is a preview of a general problem: the same photocentre jitter afflicts every red supergiant, exactly the stars whose distances calibrate their use as extragalactic beacons. When Betelgeuse does explode, the light show at this distance will rival the full Moon — spectacular, and comfortably outside the roughly 50-light-year radius of danger.