Objects in the night sky aren’t always what they seem. Take Messier 71, a star cluster in Sagitta, the arrow. Originally, it was classified as an open cluster. In recent decades, though, it’s been reclassified as a globular cluster. Both types of clusters are big groups of stars that were born together, from the same cloud of gas and dust. But that’s where the similarities end. Most globular clusters contain a hundred thousand stars or more. The stars clump together in a tight ball. The clusters move well beyond the galaxy’s wide, flat disk. And their stars are among the oldest in the entire Milky Way — typically 10 billion years or older. Open clusters, on the other hand, generally contain a few hundred to a few thousand stars, they’re concentrated in the disk, and they tend to be young. The combined gravity of their stars isn’t enough to hold a cluster together, so most open clusters vanish within a couple of billion years. M71 has many of the traits of an open cluster: It’s in the g
