Sunday, February 11, 2007
Summer has its triangle, spring has its diamond, autumn has its great square, and winter has its circle.
High overhead on cold winter nights is a ring of seven of the brightest stars in the sky, an asterism known as the Winter Circle. Although not an official constellation, the pattern is so striking that it tends to jump right out at you.
The seven stars that make up the Winter Circle are among the 20 brightest stars in the sky. Having so many bright stars concentrated in such a small area is part of what makes the winter sky so wonderfully beautiful.
You can locate the first star in the Winter Circle by extending a line through the familiar three stars of Orion's Belt to the lower left. This flashy star is Sirius, nicknamed the "Dog Star" because it represents the nose of Orion's big hunting dog, Canis Major. Sirius is the brightest and closest of the seven stars in the Winter Circle at a distance of only nine light years.
From Sirius, shoot a line to the upper right and find the bright blue star Rigel, which marks the foot of Orion the Hunter. Rigel looks nearly as bright as Sirius, but it is located 100 times farther away at a distance of 900 light years. If Rigel were viewed at the same distance as Sirius, it would shine like a second sun in our sky, being visible in broad daylight.
To the upper right of Rigel is the orange giant star Aldebaran, marking the eye of Taurus the Bull. Aldebaran is 68 light years away. It's name means "The Follower" because it closely follows the Pleiades star cluster as it marches across the sky.
Continue from Aldebaran by shooting a line nearly overhead to the second brightest star in the Winter Circle, Capella, a yellow-giant star in Auriga the Charioteer. Capella is a mere 45 light years away. Although it looks like a single star to the naked eye, astronomers have discovered that it is composed of the light from four different stars.
Dropping down from Capella toward the eastern horizon will lead you to a pair of bright stars - the Gemini Twins, Castor and Pollux. Castor is 52 light years away and is not only the fainter of the twins, but also is the faintest star in the Winter Circle. Pollux is considerably closer to us at 35 light years away.
The circle now continues to the southeast of the Gemini Twins to the star Procyon. This star's name means "Before the Dog," and it is aptly named because it rises in our sky just before Sirius appears. Procyon, like Sirius, is a nearby star, only 11 light years away.
Close the Winter Circle by shooting a line from Procyon back to Sirius, where we started.
As if having seven of the sky's brightest stars in one spot weren't enough, we can easily add an eighth. Almost smack dab in the middle of the Winter Circle is the red supergiant star named Betelgeuse, which marks the shoulder of Orion. Betelgeuse is a long way off, at a distance of 520 light years, yet it is one of the closest candidates in our sky of stars that could explode as supernovas. Astronomers think Betelgeuse could explode at any time - any time in the next one million years, that is.
That's our Winter Circle and your opportunity to learn eight bright stars in one night.