Double Cluster

The Double Cluster is a pair of bright open star clusters, NGC 869 and NGC 884, in Perseus. The clusters lie close together in the sky and are visible to the unaided eye under dark skies.

Open cluster: A gravitationally associated group of stars formed from the same molecular cloud. NGC 869 and NGC 884 are separate open clusters that appear close together.
Star formation: The process by which dense regions of molecular clouds collapse to form stars. Both clusters formed from star-forming material in the Perseus region.
Binocular astronomy: Astronomical observation using binoculars to gather light and provide a wider apparent field of view. Binoculars often resolve the pair into two distinct concentrations of stars.
Hipparchus: An ancient Greek astronomer and mathematician whose surviving work includes a star catalog. The Double Cluster is often linked to an ancient catalog entry attributed to Hipparchus.
NGC 869: An open star cluster in Perseus, also known as h Persei and one half of the Double Cluster. It is the western member of the celebrated pair.
Molecular cloud: A cold, dense interstellar cloud composed primarily of molecular hydrogen and dust. Molecular clouds provide the material from which open clusters form.
Messier 34: An open cluster in Perseus cataloged by Charles Messier in 1764. This nearby cluster offers a useful comparison with the Double Cluster's paired appearance.
Perseus Arm: A major spiral arm of the Milky Way extending through the direction of Perseus. The clusters lie along a rich, star-forming section of the Milky Way's Perseus direction.
NGC 884: An open star cluster in Perseus, also known as χ Persei and one half of the Double Cluster. It is the eastern member of the celebrated pair.
Stellar association: A loose grouping of young stars that share an origin but are not tightly gravitationally bound. The two clusters are associated with a larger population of young Perseus stars.

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