Open clusters are groups of up to several thousand very young stars that have formed in the same molecular cloud. In other words, all their stars are very similar in age. The stars in an open cluster are gravitationally bound to each other, but they end up dispersing as they undergo close encounters with other clusters, the interstellar medium, and even by interactions between their own stars.
When a molecular cloud begins to collapse on itself, the process causes a cloud to form that is increasingly dense (and smaller). Eventually, part of the giant molecular cloud ends up being of sufficient size and density to cause the birth of several thousand stars. In the Milky Way, it is estimated that new open clusters form every few thousand years.
Several open clusters can arise from the same molecular cloud. In the Large Magellanic Cloud, for example, two open clusters (Hodge 301 and R136) are forming from the Tarantula Nebula. In our own Milky Way, analyzing the movement they follow, it seems that the Hyades and Manger clusters formed in the same molecular cloud about 600 million years ago.
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