Field guide · Deep sky
Open clusterOpen cluster
A family of young stars born from the same cloud, still travelling together — for now. The Pleiades are the most famous.
- Stars
- Dozens to a few thousand
- Age
- Usually under a few hundred million years
- Where
- Along the Milky Way, in the galaxy’s disc
- Best seen with
- The eye and binoculars
Stars are born in litters. When an emission nebula collapses it makes dozens to thousands of stars at once, and for a while they stay together as a loose, sparkling group.
Only for a while: an open cluster is too lightly held to survive the tug of passing clouds and the galaxy’s tides, and over a few hundred million years it drifts apart. The Sun was almost certainly born in one; its siblings are scattered across the galaxy now. Most of the bright stars of the Big Dipper are members of one such family, still moving through space together.
Open clusters are the best deep-sky objects for binoculars. Because their stars are all the same age and distance, astronomers use them as a laboratory for how stars age.
See it in the sky
Each one opens the sky map pointed at it, for tonight, from where you are.