Open star clusters occur near the galactic plane. Now more than 1200 open clusters are known, of which 500 have been studied in detail. The most famous among them are the Pleiades and Hyades in the constellation Taurus. The total number of open clusters in the Galaxy may be as high as one hundred thousand. Open clusters are composed of hundreds or thousands of stars. Their mass is small (100-1000 M), and the gravitational field cannot contain them for a long time in a small volume of space, therefore, over billions of years, open clusters disintegrate. There are many more young stars among open star clusters than old ones. All the stars in the cluster share a common motion. The diameter of globular clusters ranges from 20 to 100 pc, and the mass is 104-106 M. Now more than 150 clusters are known in our Galaxy. Globular clusters are the oldest formations in our Galaxy, their age is from 10 to 15 billion years and is comparable to the age of the Universe. The poor chemical composition and the elongated orbits along which they move in the Galaxy suggest that globular clusters were formed during the formation of the Galaxy itself.
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