NGC 6791

The beautiful Pleiades climbs high across the sky on November evenings. It’s in good view in the east by a couple of hours after sunset. The star cluster forms a tiny dipper.

Because the stars of the Pleiades are spread out, it’s known as an open cluster. Like most of its peers, it’s young, because open clusters normally get torn apart soon after their birth. But there’s a star cluster in the western sky that’s broken all the rules: It has survived for eight billion years.

NGC 6791 is more than 13,000 light-years away, in the constellation Lyra. It’s one of the oldest open clusters in the galaxy.

No one knows the secrets to its longevity. As soon as an open star cluster is born, it faces a hostile galaxy that tries to rip it apart. Massive clouds of gas and dust roam interstellar space, for example, and their gravity pulls stars away from the cluster. Other stars that pass near the cluster also lure away its stars. And so does the overall gravitational pull of the entire galaxy.

Part of the secret of NGC 6791 may be that it was born more massive than most clusters. That extra mass could have given it strong enough gravity to hold itself together. And its orbit carries it thousands of light-years outside the galaxy’s disk, safely away from the clouds of gas and dust that try to pull the cluster apart.

Whatever the reasons, NGC 6791 has managed to cheat death for longer than most other clusters in the galaxy.

Script by Ken Croswell