Uncertain Giant

One of the biggest stars in the entire galaxy is climbing into prime-time viewing hours. Betelgeuse, the bright orange shoulder of Orion, the hunter, is in good view in the east by around 9 o’clock.

Astronomers know that Betelgeuse is a supergiant — a star that’s far bigger than the Sun. But its exact size remains uncertain. Figuring out how big it is depends on accurate measurements of its distance and its angular size — how “wide” it appears from Earth. But both measurements are tough to make.

Take distance. The most accurate technique for measuring stellar distances is parallax: measure the star’s position when Earth is on opposite sides of the Sun, creating a tiny shift compared to the background of more-distant stars. Measuring that angle reveals the star’s distance.

Parallax gives a distance of about 640 light-years. But the uncertainties are fairly large. What’s more, other techniques give distances that are a good bit closer.

Betelgeuse’s angular diameter is also difficult