Local Arm

One of the joys of a dark night sky is a chance to see the Milky Way — the subtle band of light that outlines the disk of our home galaxy. It’s in fine view over the next few nights because there’s no moonlight to spoil the view. It arches high across the east in early evening, and directly overhead a couple of hours later.

If we could view the Milky Way from outside the disk, we’d see beautiful spiral arms encircling the galaxy’s center. The arms are outlined by hot young stars that are the galaxy’s most brilliant.

Since we’re on the inside looking out, though, there’s some uncertainty on exactly what the spiral looks like. There’s a combination of major arms, minor arms, and short “spurs.” But astronomers have gone back and forth on the details.

There’s agreement that we reside in a structure called the Local Arm. It’s between the Sagittarius and Perseus arms, which are major arms — they wrap most of the way around the galaxy. The Local Arm has been considered a “spur,” which wrap