Open Clusters

Open clusters are loose groups of stars that formed together from the same molecular cloud. Unlike the ancient, tightly packed globular clusters, open clusters are young, sparse, and relatively short-lived. They typically contain a few dozen to a few thousand stars spread across 10 to 30 light-years of space.

The most famous open cluster is the Pleiades, also known as the Seven Sisters or M45. It’s visible to the naked eye as a small dipper-shaped group of bright stars in Taurus. Sharp-eyed observers can count six or seven stars without optical aid, but the cluster actually contains over 1,000 members. The Pleiades formed about 100 million years ago - young by astronomical standards - and the brightest stars are hot, blue, and massive.

Open clusters form in the spiral arms of galaxies where gas and dust are compressed by density waves. A molecular cloud collapses under gravity, fragmenting into hundreds or thousands of protostars. These stars ignite within a few million years of each other, making them essentially the same age. The cluster emerges from its birth cloud as stellar winds and radiation from hot young stars blow away the remaining gas.

The Hyades is another naked-eye open cluster in Taurus, appearing as a V-shaped group forming the bull’s face. At 153 light-years away, it’s the nearest open cluster to Earth. The Hyades is much older than the Pleiades - about 625 million years - and its stars have begun to drift apart. This is the fate of all open clusters.

Here’s why open clusters don’t last: they’re not dense enough to remain gravitationally bound indefinitely. Over hundreds of millions of years, gravitational interactions with passing molecular clouds and the tidal forces of the galaxy gradually pull clusters apart. Stars drift away one by one. After a few hundred million to a billion years, most open clusters have dissolved completely, their stars scattered throughout the galactic disk.

The age of open clusters can be determined from their color-magnitude diagrams. Young clusters like the Pleiades contain many hot, blue, massive stars on the main sequence. As clusters age, these massive stars evolve off the main sequence and die as supernovae. The point where stars begin to leave the main sequence - called the turnoff point - directly indicates the cluster’s age.

BTW open clusters are excellent laboratories for studying stellar evolution. All the stars in a cluster have the same age and initial chemical composition, and they’re all at roughly the same distance. This eliminates major variables that complicate studies of field stars. Differences between cluster members reflect only their initial masses and evolutionary stages.

The Beehive Cluster, M44 in Cancer, is another famous open cluster visible to the naked eye under dark skies. Ancient astronomers used it as a weather predictor - if the cluster appeared hazy on an otherwise clear night, rain was supposedly coming. The haze was actually moisture in the atmosphere scattering the cluster’s light.