You might think that astronomers would know just about everything there is to know about the brighter stars in the night sky. That’s not the case, though. In fact, some of those stars can be especially vexing.

An example is Antares, the orange supergiant that marks the heart of Scorpius, the scorpion. The star huddles quite close to the Moon at nightfall, with the gap growing smaller as the night goes on.

Antares is the 15th-brightest star in the night sky. And astronomers do know quite a bit about it. It’s roughly a dozen times as massive as the Sun, perhaps a thousand times wider, and tens of thousands of times brighter.

The star’s brightness isn’t constant, though. It appears to vary by a few percent. But just how much it varies, and how often, are poorly understood. Estimates for the period of the changes range from about three years to six years. The star may be pulsing in and out like a beating heart. Or bubbles of hot gas bigger than the Sun may occasionally rise to the surface, then sink back into Antares, changing the star’s brightness.

The American Association of Variable Star Observers has been keeping its eyes on Antares since 1945. And it’s been using electronic instruments in that effort since 1981. Even with those decades of observations, though, astronomers still don’t have a good understanding of how and why this massive star changes brightness.

Tomorrow: summertime.

Script by Damond Benningfield