Star cluster

A group of stars associated by gravity or common origin and moving through space as a related system. Clusters range from compact, gravitationally bound systems to loose groups dispersing into the galaxy.

Molecular cloud: A cold, dense interstellar cloud of gas and dust where stars commonly form. Cluster stars often begin in the same molecular cloud.
Open cluster: A relatively loose, often young group of stars bound by gravity in a galaxy’s disk. Open clusters are the less densely packed major class of star cluster.
Virial theorem: A relation connecting the average kinetic and potential energies of a stable, bound system. It helps determine whether a cluster is dynamically bound.
Color–magnitude diagram: A plot comparing stars’ brightness with color, which serves as a proxy for surface temperature. A cluster’s diagram reveals its age and stellar evolutionary stages.
Pleiades: A nearby young open cluster in the constellation Taurus, visible to the naked eye. Its bright stars make it a classic example of a young open cluster.
Star formation: The process by which dense regions of gas and dust collapse to form stars. It produces the groups of young stars that become clusters.
Globular cluster: A dense, roughly spherical system of old stars orbiting a galaxy. Globular clusters contrast with open clusters in age, density, and location.
Two-body relaxation: The gradual change in stellar orbits caused by repeated gravitational encounters between stars. It redistributes energy and gradually reshapes a cluster.
Main-sequence turnoff: The point on a cluster’s color–magnitude diagram where stars begin leaving the main sequence. Its position provides a standard estimate of cluster age.
Hyades: A nearby open cluster in Taurus whose stars form a distinctive V-shaped pattern. Its proximity makes it a key benchmark for stellar distance and age.

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