Moon and Antares

Antares is a supergiant. It’s hundreds of times the diameter of our own star, the Sun, and millions of times the Sun’s volume. But the star’s true size is still a puzzle. Estimates of its diameter vary by quite a bit.

To determine a star’s true size, astronomers first measure its apparent size — the angle it covers on the sky. They then compare that number to the star’s distance — the farther the bigger. In the case of Antares, though, it’s hard to lock down either number.

Consider its apparent size in the sky, known as angular diameter. The star’s outer layers are quite thin. And Antares blows a thick “wind” of hot gas out into space. That surrounds the star with a cloud of gas. So it’s hard to tell where the surface of the star ends and the surrounding cloud begins.

And Antares changes in both size and brightness — factors that make it even tougher to get a true measurement of its angular diameter.

Then there’s the matter of its distance. The best estimate says it’s about 550 light-years away. There’s a good bit of uncertainty in that number, though — for some of the same reasons as the uncertainty in its apparent size.

When you account for all of that, estimates of the star’s true diameter range from about 700 times that of the Sun, to almost 900 — a giant uncertainty for a supergiant star.

Look for Antares to the right or lower right of the Moon as night falls. The star marks the bright heart of the scorpion.

Script by Damond Benningfield