Star cluster

Lore & Background

Star clusters are classified into two main types: globular clusters, which are dense, spherical groupings of ten thousand to millions of old stars, and open clusters, looser groups of typically a few hundred stars. Open clusters are confined to the galactic plane and spiral arms, are generally young (up to a few tens of millions of years), and form in H II regions such as the Orion Nebula. They become disrupted over time by giant molecular clouds, and their members may continue as stellar associations. Globular clusters are distributed spherically in the galactic halo, orbit the center in highly elliptical orbits, and contain very old Population II stars.

Reader's Guide

Star clusters are important in many areas of astronomy because almost all stars in old clusters were born at roughly the same time, so properties of stars in a cluster are a function only of mass. This makes them crucial for testing stellar evolution theories, especially for old globular clusters. Open clusters allow calibration of the period-luminosity relationship of Cepheid variable stars, which are used as standard candles to establish distances to remote galaxies and the expansion rate of the Universe (Hubble constant). Embedded clusters are groups of very young stars encased in interstellar dust or gas, forming in molecular clouds; most young embedded clusters disperse shortly after star formation ends, and nearly all freely floating stars, including the Sun, were originally born into such clusters that disintegrated. Super star clusters are very large regions of recent star formation thought to be precursors of globular clusters.

Did You Know?

• Open clusters become disrupted by giant molecular clouds and may continue as stellar associations moving in the same direction.
• The globular cluster Omega Centauri was observed in antiquity and catalogued as a star before the telescopic age.
• Embedded clusters form in molecular clouds and often contain protostars and pre-main-sequence stars.