Sirius, the Dog Star, is in the southeast this evening. It’s the night sky’s brightest star, so it’s hard to miss. And it’s led into the sky by another bright light — Procyon, the little dog star. It’s to the left of Sirius and a little higher, so it precedes Sirius across the evening sky.

Procyon is among the dozen brightest stars in the night. That’s mainly because it’s just 11 and a half light-years away.

It’s actually a system of two stars. The main star is known as Procyon A. It’s about one and a half times as massive as the Sun, about twice as big, and about seven times as bright.

The star is nearing an important transition point. It’s converted most of the original hydrogen fuel in its core to helium. That’s made the core get smaller and hotter, which in turn has caused its outer layers to puff outward a bit.

Before long, all the hydrogen in the core will be gone, and the star will swell up to giant proportions. It’ll then start converting the helium to carbon and oxygen. Once that process is complete, the nuclear reactions in the core will shut down. Procyon A then will cast its outer layers into space, leaving behind only the dead core — a tiny ember known as a white dwarf.

That’s already happened to the second star in the system, Procyon B. It’s so faint and so close to Procyon A that it’s almost impossible to see, even through a big telescope. But it once was even bigger and brighter than Procyon A. More about that tomorrow.

Script by Damond Benningfield