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open cluster

or galactic cluster

noun, Astronomy
a comparatively young, irregularly shaped group of stars, often numbering up to several hundred, and held together by mutual gravitation; usually found along the central plane of the Milky Way and other galaxies. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Contemporary definitions for open cluster

a loose grouping of stars

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astronomy's 21st Century Lexicon
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open cluster in Science
open cluster  
A loose, irregular grouping of stars that originated from a single nebula in the arms of a spiral galaxy. Compared to globular clusters, open clusters generally contain younger and fewer (from a hundred to several thousand) stars and are confined to the disk of the galaxy. Because they are young, open clusters are sometimes still surrounded by the leftover gas and dust from which they formed. Visible from Earth with just a pair of binoculars and containing over 3,000 stars, the Pleiades is the best known open cluster. Also called galactic cluster. Compare globular cluster.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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