Stars are usually born in groups. A star cluster is a family of stars that formed from the same cloud at about the same time, so they share a similar age and chemical makeup. This makes clusters ideal laboratories for studying how stars evolve.
Open clusters
- Size: from a few dozen to a few thousand stars, loosely grouped.
- Age: usually young, from a few million to a few billion years.
- Location: in the disk of the Milky Way, along the spiral arms.
- Fate: they drift apart over time as gravity from passing clouds and the galaxy pulls them apart.
Famous examples: the Pleiades (the Seven Sisters), visible to the naked eye in winter, and the Hyades in Taurus. The Beehive Cluster (M44) is a fine sight in binoculars.
Globular clusters
- Size: tens of thousands to millions of stars packed into a tight ball.
- Age: very old, often more than 10 billion years, among the oldest objects in the galaxy.
- Location: in the halo surrounding the Milky Way. The galaxy has about 150 to 160 known globular clusters.
- Stars: mostly old, low-mass, red stars, poor in heavy elements.
Examples: Omega Centauri, the largest in our galaxy, visible from the Southern Hemisphere, and M13 in Hercules for Northern Hemisphere observers.
Stellar associations
Associations are very loose groups of young, often hot and bright stars that are no longer gravitationally bound. The bright stars of Orion’s belt region belong to such groups.
Why clusters matter
Because the stars in a cluster are the same age and distance, differences between them come down mainly to mass. Plotting them on a temperature and brightness diagram shows where the most massive stars have already left the main sequence. That “turnoff point” gives the cluster’s age, see how stars form and evolve. Globular cluster ages also set a minimum age for the universe, see how astronomers know the age of the universe.
Many clusters are wrapped in the gas they formed from, see the different types of nebulae.
