On the Edge

Our solar system sits on the edge. It’s at the inner edge of a spiral arm — a curving blade of stars and gas that wraps around the center of the Milky Way Galaxy. It’s known as the Orion Arm because it includes many of Orion’s brilliant stars.

The stars and gas clouds of the Milky Way form a wide, flat disk. Hot young stars, which are born in big clouds of gas and dust, outline curving spiral arms. So from afar, the galaxy would look like a cosmic pinwheel spinning through space.

We can’t see the spiral structure directly, though, because we’re inside the disk. But astronomers can get a rough outline of the spiral arms by using radio telescopes, which map giant clouds of gas throughout the disk.

Those observations suggest that the Milky Way has two major spiral arms, which wrap all the way around the galaxy, plus a few smaller ones.

That includes the Orion Arm. Until recently, it wasn’t really considered an “arm” at all. Instead, it was called a “spur” — a fairly short streamer between two larger arms. But a study a few years ago found that it’s about 20,000 light-years long — long enough to be a spiral arm. Our solar system sits near the arm’s inner edge.

And Orion is entering prime time now. The constellation is in good view by about 7 o’clock. Look for a short line of three stars pointing up from the east-southeastern horizon, flanked by a couple of much brighter stars — some of the more prominent members of the Orion Arm.

Script by Damond Benningfield