Northern Crown

A pretty little semicircle of stars crowns the sky on spring and summer nights: Corona Borealis, the northern crown. It’s in the east as night falls right now, but 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 its center: Alphecca, “the bright one.”

Alphecca’s actually 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, cooler, 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. They 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 eclipse the brighter one. 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 detect a disk of debris around the stars. It extends billions of miles out into space. The disk consists mainly of small grains of dust — material left over from the formation of Alphecca itself.

Tomorrow, we’ll talk about another pair of stars in Corona Borealis that blew itself up.

Script by Damond Benningfield