The long constellation Hydra, the water snake, is beginning its annual crawl across the southern evening sky. As night falls, its head is in the east-southeast, about a third of the way up the sky. The head is outlined by a pentagon of meek stars.

The first star below the head is a bit brighter. In fact, Zeta Hydrae is the third-brightest star in the whole constellation.

The star is only about 400 million years old — less than a tenth the age of the Sun. But it’s nearing the end of its “normal” lifetime. That’s because it’s more than four times as massive as the Sun. Heavier stars live bright but short lives.

One distinction for Zeta Hydrae is that astronomers have a direct measurement of its size. They estimate the sizes of most stars. They measure a star’s temperature, brightness, distance, and other parameters. They then plug the numbers into models of how stars work. The models then calculate the star’s size.

But they used interferometry to get a true measure of the size of Zeta Hydrae. Interferometry uses computers to combine the observations from two or more telescopes. That produces a view that’s as sharp as that of a single giant telescope.

Combined with the star’s distance of about 165 light-years, the technique reveals that Zeta Hydrae is about 18 times the diameter of the Sun. That’s a good match to what the models predict — showing that astronomers have a pretty good grasp of how stars work.

Script by Damond Benningfield