Open cluster

An open cluster is a loosely bound group of stars formed from the same molecular cloud. Most are found in a galaxy’s disk and disperse over time.

Molecular cloud: A cold, dense concentration of interstellar gas and dust where stars can form. An open cluster begins when stars form together within one of these clouds.
Pleiades: A nearby, young open cluster in the constellation Taurus, also known as the Seven Sisters. Its bright stars make it a prominent example of a young open cluster.
Color–magnitude diagram: A plot comparing stars’ colors with their brightnesses to reveal stellar populations and evolutionary stages. A cluster’s coeval stars form a pattern that constrains its age and distance.
Globular cluster: A dense, gravitationally bound spherical collection of stars, usually much older than open clusters. Its compact structure and ancient population contrast with the loose, often young open cluster.
Star formation: The process by which dense regions of gas and dust collapse to form stars. It creates the cluster’s stars from a common reservoir of gas.
Hyades: The nearest major open cluster to the Sun, located in the constellation Taurus. Its nearby stars provide a well-studied example of cluster membership and dispersal.
Main-sequence fitting: A method for estimating stellar distances by matching a cluster’s main sequence to a calibrated reference. Open clusters supply groups of stars whose shared distance makes the method useful.
Stellar association: A loose grouping of young stars that share an origin but are not gravitationally bound. Associations resemble open clusters in origin but lack lasting gravitational cohesion.
Gravitational binding energy: The energy required to separate a gravitationally bound system into widely separated parts. It helps distinguish clusters that remain bound from stellar groups that disperse quickly.
Praesepe: An open cluster in the constellation Cancer, also called the Beehive Cluster. Its rich population offers a contrast to the sparse appearance of nearby clusters.

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