Star cluster

A star cluster is a collection of stars born together at about the same time, bound to one another by their own gravity. There are two main kinds: globular clusters, which are dense, spherical groups containing tens of thousands to millions of old stars held tightly together; and open clusters, which are looser assemblies of typically fewer than a few hundred stars. When a cluster is hidden in visible light by interstellar dust but can be seen in infrared, it is called an infrared cluster.

As open clusters travel through a galaxy, they are gradually pulled apart by the gravity of giant molecular clouds, so the ones we see are usually young. Even after they are no longer gravitationally bound, their stars continue moving in roughly the same direction and are then known as stellar associations or moving groups. Globular clusters, being more massive and tightly packed, survive for much longer, and those we observe are often billions of years old. In the Milky Way, naked-eye examples include the Pleiades and Hyades open clusters and the globular cluster 47 Tucanae.

Open clusters stay close to the galactic plane, almost always within spiral arms. They are mostly young—up to a few tens of millions of years old—though a few, like Messier 67, are billions of years old. They form in H II regions such as the Orion Nebula. Typically a few hundred members across a region up to 30 light-years wide, they are loosely bound and easily disrupted by giant molecular clouds and other clusters. Close encounters between stars can also eject members, a process called evaporation. The Pleiades and Hyades in Taurus are the most prominent, and the Double Cluster in Perseus is also visible under dark skies. Open clusters often contain hot, young blue stars, which die within tens of millions of years, but the clusters themselves usually break up before those stars expire. Some open clusters form binary or aggregate groups; new research suggests Messier 25 may be part of a ternary cluster with NGC 6716 and Collinder 394. Precise distances to open clusters help calibrate the period-luminosity relation of Cepheid variable stars, which are used as standard candles to measure distances to remote galaxies and the expansion rate of the universe. The open cluster NGC 7790, for example, contains three classical Cepheids critical for this work.

Embedded clusters are very young groups still partially or fully wrapped in interstellar dust and gas, often invisible in optical light. They form in molecular clouds as the clouds collapse and produce stars. Ongoing star formation means they may contain protostars and pre-main-sequence stars. An example is the Trapezium Cluster in the Orion Nebula, and another lies in the ρ Ophiuchi cloud core. This embedded phase lasts a few million years, after which the gas is depleted by star formation or dispersed by radiation, stellar winds, outflows, or supernovae. Typically less than 30% of the cloud’s mass turns into stars before dispersal, and the loss of mass often disrupts the cluster. Most young embedded clusters break up soon after star formation ends. The open clusters we see today are former embedded clusters that survived this early stage, but nearly all freely floating stars, including the Sun, originally formed in embedded clusters that later disintegrated.

Globular clusters are roughly spherical, packed with 10,000 to several million stars within 10 to 30 light-years. They consist mainly of very old Population II stars—only a few hundred million years younger than the universe—mostly yellow and red, with masses under two solar masses. Hotter, more massive stars have already exploded as supernovae or ended as white dwarfs. A few rare blue stars, called blue stragglers, are thought to form from stellar mergers in the dense cores. In the Milky Way, globular clusters are spread spherically in the galactic halo, orbiting the center in highly elliptical paths. In 1917, Harlow Shapley used their distribution to make the first reliable estimate of the Sun’s distance from the Galactic Center. Until the mid-1990s, globular clusters posed a puzzle: theories gave their oldest stars an age greater than the estimated age of the universe. Improved distance measurements from the Hipparcos satellite and better measurements of the Hubble constant resolved this, setting the universe’s age at about 13 billion years and the clusters’ ages accordingly.