Moon and Antares

• ‹ Previous program
• 221 of 1996
• Next program ›

In mythology, Orion, the hunter, was killed by the sting of a scorpion. To keep these two enemies apart, the gods placed them on opposite sides of the sky. Orion's in view in winter, while Scorpius is best seen in summer. In fact, the scorpion is in good view tonight. Its brightest star, Antares, is just to the left of the Moon at nightfall.

But it turns out that the leading stars of both constellations are part of the same neighborhood of our Milky Way galaxy -- a band of stars known as the Orion Spiral Arm. Our solar system is in the same arm -- smack between the two rivals.

The Milky Way is a giant spiral galaxy. Several sweeping spiral arms encircle a fat bar of stars in the galaxy's middle. But the galaxy also includes several partial arms, which are known as "spurs." The Orion arm is one of these spurs. It's several hundred light-years wide, but it wraps only a small way around the galaxy.

The arm is named for Orion because its stars are the arm's most prominent features. They're away from the center of the galaxy. Antares is part of a big group that marks the arm's inner edge -- toward the galaxy's center.

Spiral arms form when a "wave" sweeps through the galaxy. It squeezes clouds of gas and dust to give birth to new stars. Some of these stars are bright and heavy, like Antares, so they make the spiral arms shine brightly. But these stars die quickly, so the spiral arms soon begin to fade -- as new ones take shape.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.