What Is a Star Cluster? Stars Born Together and Studied Together

Open clusters contain tens to a few thousand stars in a loose irregular grouping, lie in the disc of the galaxy, and are young by astronomical standards, typically tens to hundreds of millions of years. They are loosely bound and gradually dissolve as gravitational encounters with passing clouds and the galaxy's own tidal forces strip members away, so most survive only a few hundred million years and the Sun's own birth cluster dispersed long ago. Globular clusters are entirely different: hundreds of thousands to millions of stars in a dense spherical concentration, orbiting in a halo around the galaxy rather than sitting in its disc, tightly bound and extremely old at over ten billion years in most cases. Their stars are correspondingly poor in heavy elements, because they formed before generations of supernovae had enriched the gas, which makes them direct evidence about conditions in the early universe. The Milky Way contains roughly one hundred and fifty known globular clusters and thousands of open ones.

Almost every measurement in stellar astronomy is easier in a cluster, for reasons that follow from what the members have in common:

• •All members lie at effectively the same distance, so differences in apparent brightness are real differences in luminosity rather than distance effects
• •All members formed at the same time, so differences between them reflect mass alone, which isolates the single variable that controls stellar evolution
• •Plotting brightness against colour for a cluster produces a diagram whose shape depends on age, since more massive stars evolve away from the main sequence first
• •The point at which stars are leaving the main sequence, called the turn-off, dates the cluster directly, and this is the primary method for measuring stellar ages
• •Globular cluster ages set a lower bound on the age of the universe, which was a genuine crisis when early estimates made clusters older than the cosmos
• •Clusters supply calibration points for the cosmic distance ladder, since a cluster with a measurable geometric distance anchors the luminosity scale for its stars

Several clusters are prominent enough to have been recorded across cultures and to remain important observationally. The Pleiades is a young open cluster visible to the naked eye as a compact knot of stars, referenced in texts and oral traditions across the world, and its distance was the subject of a well-publicised disagreement when a space astrometry mission returned a value inconsistent with other methods, later resolved in favour of the other methods. The Hyades is closer still and forms the head of Taurus, and its proximity allowed a geometric distance that underpinned the distance scale for decades. Omega Centauri is the brightest globular cluster in the sky and is unusual in containing stars of differing composition, which suggests it may be the stripped core of a small galaxy captured by the Milky Way rather than a true cluster. The Jewel Box and the Double Cluster in Perseus are prominent southern and northern open clusters respectively, and the globular cluster in Hercules is the standard northern target for amateur telescopes.

The simple picture of a cluster as a single population formed in one event has broken down for globular clusters, where precise photometry and spectroscopy have revealed multiple populations with different chemical compositions within the same cluster, implying more than one episode of star formation or pollution by an earlier generation. Explaining that has proved difficult, since the cluster would need to retain gas long enough for a second generation while losing most of the first generation's mass, and no proposed mechanism accounts for the observations comfortably. The origin of globular clusters generally remains open, with candidate explanations including formation in the earliest galaxies, in massive gas clouds at high redshift, and as the remnants of accreted dwarf galaxies, and the answer probably involves more than one route. Open clusters raise their own questions about how quickly they dissolve and how much of the galaxy's disc population originated in them, which matters because it bears on the history of the Sun and on whether its siblings could be identified.

Members share an age, a composition and a distance, which removes three variables and makes clusters the standard laboratory for stellar physics. Open clusters are young, loose and dissolve within a few hundred million years, while globulars hold hundreds of thousands of very old metal-poor stars in a halo. The point where stars leave the main sequence dates a cluster directly, and globular ages once appeared older than the universe.