star cluster

Most stars do not form alone. They are born in batches, hundreds or thousands at a time, out of the same collapsing cloud of gas — like siblings born together. A star cluster is one of these family groups, a bound or loosely gathered swarm of stars that share a common birthplace and birth time. For astronomers, a cluster is a precious gift: a set of stars all the same age, the same distance, and the same starting chemistry.

There are two main kinds. Open clusters are loose, irregular groups of a few dozen to a few thousand relatively young stars, scattered through the disk of a galaxy; the Pleiades and the Hyades are familiar naked-eye examples. They are loosely bound and slowly disperse over hundreds of millions of years. Globular clusters are utterly different: dense, spherical swarms of hundreds of thousands to millions of very old, metal-poor stars, packed so tightly near the center that their night sky would blaze with thousands of brilliant stars. Globulars orbit in the halo and are among the oldest objects in the galaxy, often 12 to 13 billion years old.

Star clusters matter because they are nature's controlled experiments. Since every star in a cluster shares an age and distance, plotting the cluster on a Hertzsprung-Russell diagram shows clean structure rather than a smear, letting astronomers test stellar models with precision. Above all, the point where a cluster's main sequence bends off (the main-sequence turnoff) acts as a clock that dates the entire cluster, which is how we know that globular clusters, and thus the galaxy, are billions of years old.