Backyard Astronomy: Big Dog, Little Dog (2004-01-10)

By Cheryl Haimann

The last two stars in the Winter Hexagon asterism, Sirius and Procyon, are two of the brightest stars in the night sky. Their constellations, however, are real dogs. Specifically, they are Canis Major and Canis Minor, the Big and Little Dogs.

Orion has pointed to the other stars in the Hexagon, and these two are no exception. The two shoulder stars point toward Procyon, in Canis Minor (KAH-niss or KAY-niss), about 30 degrees to the left. The belt stars point to Sirius, 20 degrees below and to the left in Canis Major. Sirius, in fact, appears to be waiting at Orion's feet, and is also called the Dog Star. Procyon, Sirius, and Orion's shoulder Betelgeuse form an almost-equilateral triangle, sometimes called the Winter Triangle.

Sirius (SEAR-ee-us) may be called a dog, but at -1.4 magnitude, it is the brightest star visible in the night sky. Its name means "scorching." The phrase "dog days of summer" came about because Sirius rises with the sun in summer. It was believed that the star's brightness amplified the sun's heat. Procyon (PRO-see-on) is no slouch, either. At magnitude 0.4, it is between Rigel and Betelgeuse in brightness. Because it rises about an hour before Sirius, it bears a name meaning "before the dog."

Both of the stars, coincidentally, have white dwarf companion stars. The companion of Sirius is called the Pup. It would be a respectable star on its own, but Sirius outshines it so greatly that the Pup can only be seen with a 10 inch or larger telescope.

From a dark location, you may be able to see that the constellations sit on opposite sides of the winter Milky Way. (The Milky Way is dimmer in winter than in summer because Earth is facing the outer edge of our galaxy, which is less densely populated than the center.) Arab legends explain this by telling how two companions or two sisters became separated when one was able to cross a river and the other was not. Procyon, the dimmer of the two stars, is usually considered to be the weaker one who was left behind.

In Greco-Roman mythology, the two constellations are often referred to as Orion's hunting dogs, but there are many more legends about them. One says that they belonged to a mortal who made the mistake of watching Diana, goddess of the hunt, skinnydipping. Furious, Diana sicced the dogs on their master. Another story says that the dogs are under the table of twins Castor and Pollux, waiting to gobble up the crumbs they drop. The stars of the Milky Way represent those crumbs.

Apparently the Big Dog got himself some of those crumbs. Just below Sirius is an open star cluster. M41, The Little Beehive, is about as wide as the moon, and can be seen in binoculars.