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Chasing Dogs

September 25, 2010

A pair of "dog stars" chases across autumn's pre-dawn sky. But which dog is the leader of the pack depends on your location.

The brighter star is Sirius, in Canis Major, the big dog. The other is Procyon, of Canis Minor, the little dog.

Sirius is the brightest star in the night sky. In part, that's because it really is bright -- about 20 times brighter than the Sun. But the main reason it looks so bright is that it's one of our closest neighbors, at a distance of less than nine light-years.

Sirius is actually a double star. Its companion is a white dwarf -- the hot, dense core of a star that was once like Sirius itself. At the end of its life, the star blew its outer layers into space, leaving only the core. The same thing will happen to Sirius when it reaches the end of its life.

Procyon is a little farther than Sirius, and it doesn't produce quite as much light. Even so, it's the eighth-brightest star system in the night sky. And like Sirius, it also has a white-dwarf companion.

The name Procyon means "before the dog." The name comes from the fact that, from north of about 30 degrees latitude, the star rises before Sirius does. From Denver, for example, it precedes Sirius by about 20 minutes. But the viewing angle is different from more southerly latitudes, so Sirius rises first. Look for it rising in the east-southeast in the wee hours of the morning, with Procyon well to its left. Both stars are high in the sky at first light.


Script by Damond Benningfield, Copyright 2010


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