A pretty semicircle of stars crowns the sky on spring and summer nights: Corona Borealis, the northern crown. It’s in the east as night falls now, and stands high overhead a few hours later. In a couple of months, it’ll be overhead at nightfall.

Most of the semicircle isn’t very bright – you need pretty dark skies to see it. It stands out because of the tight pattern, with a fairly bright star at the center: Alphecca, “the bright one.”

Alphecca is really a binary – two stars locked in a gravitational embrace. The heavier of them is about three times as massive as the Sun, thousands of degrees hotter, and dozens of times brighter. Its companion is a little smaller and fainter than the Sun.

The stars are quite close together – an average of about half the distance between the Sun and its closest planet, Mercury. The stars orbit each other once every 17 and a half days.

And they’re lined up in such a way that we see the fainter star pass in front of the brighter one – an eclipse. When that happens, Alphecca dims by a few percent. That’s not enough for most of us to notice with the eye alone, but it’s an easy catch for astronomical instruments.

Instruments also see a disk of debris around the stars. It extends billions of miles into space. It consists mainly of small grains of dust – material left over from the formation of Alphecca itself.

We’ll talk about a pair of stars in Hercules tomorrow.

Script by Damond Benningfield