What Is an Open Cluster? Stars Born Together and Slowly Drifting Apart

An open cluster contains from a few dozen to a few thousand stars occupying a region a handful of light years across, held loosely by their mutual gravity and sharing a common motion through space. They form when a giant molecular cloud collapses and fragments, producing many stars from the same material within a short interval, so the members share an age and a chemical composition to a good approximation. They lie in the disc of the galaxy, which is where the gas that forms stars is found, and they are correspondingly concentrated along the band of the Milky Way. Several are visible without a telescope, including a famous group in Taurus and a scattered one nearby, and a small instrument shows many more, which makes them among the most rewarding objects for an observer with modest equipment.

The shared origin makes a cluster a natural experiment:

• •All members are at essentially the same distance, so differences in apparent brightness reflect real differences in output
• •All members have the same age, which allows the effect of mass on stellar evolution to be isolated
• •All members have the same composition, removing another variable
• •Plotting brightness against colour for a cluster produces a curve whose shape depends only on age
• •The point where that curve turns off the main sequence gives the age directly, since the most massive stars leave first
• •Distances can be established for nearby clusters by geometry, which then calibrates methods used further away

These groups are not permanent and most disperse within a few hundred million years, which is brief on any astronomical scale. The gravitational binding is weak, since the total mass is small and the stars are spread out, so several processes pull them apart. The gas left over after star formation is blown away by the radiation and winds of the most massive stars, and losing that mass reduces the gravity holding the group together, which unbinds many clusters within a few million years of forming. Encounters with giant molecular clouds disrupt them. The tidal pull of the galaxy stretches them. And internal encounters between members gradually eject stars one at a time. The result is that the stars end up spread around the galaxy in orbits that still carry a trace of their shared origin, detectable as a common motion long after any visible grouping has gone.

Several are among the best objects available to an observer without much equipment. The brightest, a compact group in Taurus, is visible as a small knot of stars to the unaided eye and resolves into dozens in binoculars, and it has been recorded in art and text for thousands of years across many cultures. A larger and closer group nearby forms the V-shaped face of the same constellation. A double cluster in Perseus presents two rich groups in one field. A conspicuous group in Cancer was known to ancient observers as a hazy patch and was resolved by the first telescopes. Many more require a small telescope and reward a wide field of view, since the whole point of such a group is the grouping, and high magnification that shows only part of it misses the effect entirely. They are also among the easiest targets for anyone starting out.

Globular clusters are the other category and the differences are instructive. They contain hundreds of thousands to millions of stars in a dense spherical arrangement, they are strongly bound and survive for billions of years, they orbit in a halo around the galaxy rather than sitting in the disc, and their stars are among the oldest known and are poor in heavy elements, indicating formation very early before successive generations of stars had enriched the gas. Open clusters by contrast are young, small, loose, disc-dwelling and comparatively rich in heavy elements. The two therefore sample different epochs and different parts of the galaxy, and studying both is how the history of star formation is reconstructed. The origin of globular clusters remains less settled than that of open ones, which is a reminder that the older something is the harder the question tends to be.

A few dozen to a few thousand stars from one collapsing cloud, sharing age, composition and distance, which isolates the effect of mass and makes a cluster a natural experiment. The point where its brightness and colour curve turns off gives the age. Losing the leftover gas unbinds most of them quickly, and few last more than a few hundred million years.