Open cluster
An open cluster is a relatively loose grouping of stars that share a common origin— they were born from the same molecular cloud, at roughly the same time. Unlike globular clusters, they are young, low-density, and will gradually disperse under the influence of external gravitational perturbations.
Formation and Early Life
Open clusters form in active star-forming regions, such as the spiral arms of galaxies. All of their stars form over a period of a few million years from the same molecular cloud. They therefore share the same age, the same initial chemical composition, and the same distance from us—making them ideal laboratories for studying éstellar evolution.
A Limited Lifespan
Open clusters are not as tightly bound as globular clusters. Over time, the lightest stars are ejected due to gravitational interactions with other stars and galactic gas clouds. Most open clusters disperse within a few hundred million years. The oldest known ones, such as M67, have persisted for several billion years.
Famous examples
The Pleiades (M45), 444 light-years away, are the best-known open cluster— a group of young blue stars visible to the naked eye in the constellation Taurus. The Hyades, only 153 light-years away, are the open cluster closest to the Sun. The Double Cluster in Perseus (NGC 869 and NGC 884) is another spectacular example, visible through binoculars.
Did you know?
The Sun itself was likely born in an open cluster 4.6 billion years ago. That cluster has long since dispersed throughout the Milky Way. Some astronomers have attempted to identify possible sister stars of the Sun based on their chemical composition and galactic trajectory.
Frequently Asked Questions
How many stars are in an open cluster?
Generally, between a few dozen and a few thousand stars. The most populous ones, such as M11 (the Wild Duck Cluster), can contain several thousand. The sparsest contain only a few dozen.
Are the Pleiades visible to the naked eye?
Yes, easily. In a dark sky, you can see six to seven stars with the naked eye (hence the nickname “The Seven Sisters”). With binoculars or a telescope, hundreds more stars become visible.
What is the difference between an open cluster and a stellar association?
A stellar association is even more diffuse and less gravitationally bound than an open cluster. OB associations, for example, consist of massive stars (O and B types) that were recently born in the same region but are dispersing even more rapidly.