Star Clusters
Groups of stars born from the same cloud.
Overview
A star cluster is a group of stars that formed together from the same molecular cloud. Open clusters are young, loosely bound and found in the Galactic disc; globular clusters are ancient, densely packed and distributed through the halo.
- Because cluster members share an age and composition, clusters are the primary laboratory for testing stellar evolution.
- Open clusters disperse over hundreds of millions of years; globular clusters have survived for over ten billion.
- Globular clusters contain some of the oldest stars known, and their ages constrain the age of the universe.
- The Sun almost certainly formed in a cluster that has long since dispersed.
Open clusters
Open clusters contain from a few dozen to a few thousand stars, are found in the Galactic disc where star formation is ongoing, and are loosely bound. Tidal forces from the Galaxy and encounters with molecular clouds gradually strip members away, so most disperse within a few hundred million years.
The Pleiades and the Hyades are the nearest bright examples, both visible without equipment. The Pleiades is young, around 100 million years old, so it still contains hot blue-white B-type stars, and those will be gone within a few hundred million years. The Hyades is several times older — roughly 600 to 800 million years — and its most massive stars have already evolved away: its main-sequence turn-off now sits among the A stars, and several prominent orange giants mark where earlier generations left the main sequence.
Globular clusters
Globular clusters are dense, roughly spherical systems containing hundreds of thousands to millions of stars, orbiting through the Galactic halo rather than the disc. Around 150 are known in the Milky Way, and they are among the oldest structures in it, with ages exceeding ten billion years.
Their stars are metal-poor, reflecting formation from gas that had been enriched by relatively few previous stellar generations. Because a cluster's stars all formed at once, fitting its observed colour–magnitude diagram against stellar models gives an age — and those ages have historically provided an independent lower bound on the age of the universe, a constraint that cosmology had to satisfy.
Why clusters are so scientifically useful
In a cluster, the stars share age, initial composition and distance. That removes three of the largest confounding variables in stellar astrophysics simultaneously, so differences between members must be due to mass.
Plotting a cluster on a colour–magnitude diagram therefore produces a clean sequence with a distinct turn-off point where the most massive surviving stars are leaving the main sequence. The location of that turn-off dates the cluster directly. This is the single most important application of cluster observation, and it is why clusters anchor much of the calibration of stellar-evolution models.
Complications in the standard picture
The classical assumption that a globular cluster is a single stellar population has not survived detailed spectroscopy. Most well-studied globulars show multiple populations with distinct chemical signatures, indicating more than one epoch or channel of star formation within the cluster's early history.
There are also intermediate cases that resist the open/globular dichotomy, and some globular clusters — Omega Centauri being the standard example — may be the stripped cores of dwarf galaxies captured by the Milky Way rather than clusters in the usual sense.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- What is the difference between an open and a globular cluster?
- Age, density and location. Open clusters are young, loosely bound groups of up to a few thousand stars in the Galactic disc, and they disperse within a few hundred million years. Globular clusters are ancient, tightly bound systems of hundreds of thousands to millions of stars orbiting in the halo, and they have survived for over ten billion years.
- How do astronomers determine a cluster's age?
- From the main-sequence turn-off. All the cluster's stars formed at the same time, and more massive stars exhaust their core hydrogen faster, so the point on the colour–magnitude diagram where stars are leaving the main sequence directly indicates how much time has passed. It is one of the most reliable age determinations in astronomy.
- Did the Sun form in a cluster?
- Almost certainly. Most stars form in groups, and isotopic evidence in meteorites indicates the early Solar System was exposed to material from a nearby massive star, which implies a cluster environment. That birth cluster dispersed billions of years ago and its members are now spread throughout the Galaxy; identifying the Sun's siblings is an active area of research.