page_title: Star cluster
section_title: Astronomical significance of  star clusters
hierarchical_section_title: Star cluster / Astronomical significance of  star clusters
caption_attribution_description: English: This artist's impression shows one of the three newly discovered planets in the star cluster Messier 67. In this cluster the stars are all about the same age and composition as the Sun. This makes it a perfect laboratory to study how many planets form in such a crowded environment. Very few planets in clusters are known and this one has the additional distinction of orbiting a solar twin — a star that is almost identical to the Sun in all respects.
context_page_description: Star clusters are large groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds to millions of old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain fewer than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the Galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.
  Star clusters visible to the naked eye include the Pleiades, Hyades, and the Beehive Cluster.
context_section_description: Stellar clusters are important in many areas of astronomy. Because the stars were all born at roughly the same time, the different properties of all the stars in a cluster are a function only of mass, and so stellar evolution theories rely on observations of open and globular clusters.
  Clusters are also a crucial step in determining the distance scale of the universe. A few of the nearest clusters are close enough for their distances to be measured using parallax. A Hertzsprung–Russell diagram can be plotted for these clusters which has absolute values known on the luminosity axis. Then, when similar diagram is plotted for a cluster whose distance is not known, the position of the main sequence can be compared to that of the first cluster and the distance estimated. This process is known as main-sequence fitting. Reddening and stellar populations must be accounted for when using this method.
  Nearly all stars in the Galactic field, including the Sun, were initially born in regions with embedded clusters that disintegrated. This means that properties of stars and planetary systems may have been affected by early clustered environments. This appears to be the case for our own Solar System, in which chemical abundances point to the effects of a supernova from a nearby star early in our Solar System's history.
caption: Artist's impression of an exoplanet orbiting a star in the cluster Messier 67[12]
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