M21 (天体)
1. Messier 21Messier 21 or M21 (also designated NGC 6531) is an open cluster of stars in the constellation of Sagittarius. It was discovered and catalogued by Charles Messier on June 5, 1764. M21 is a relatively young cluster of a mere 4.6 million years of age. It is tightly packed but contains about 57 stars. A few blue giant stars have been identified in the cluster, but Messier 21 is composed mainly of small dim stars.
Read “Messier 21” on English Wikipedia
Read “M21 (天体)” on Japanese Wikipedia
Read “Messier 21” on DBpedia
Read “Messier 21” on English Wikipedia
Read “M21 (天体)” on Japanese Wikipedia
Read “Messier 21” on DBpedia
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