M21
Webb's Cross Cluster
物性
位置と識別子
可視性
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M21について
説明
M21 is a small, young open cluster in Sagittarius, about 4,250 light-years from Earth. It contains roughly 60 stars spanning about 14 light-years, with an age of only 4.6 million years — making it one of the youngest Messier clusters. Its brightest members are hot blue B-type stars.
観測のコツ
Located just 0.7 degrees northeast of the Trifid Nebula (M20), the two objects share the same low-power field of view. The cluster appears as a small, bright grouping in binoculars. A telescope at 50-80x shows about 20 stars in a compact arrangement. Not spectacular on its own, but the pairing with M20 makes it a worthwhile stop. Best observed from June through August.
歴史
Discovered by Charles Messier on June 5, 1764, the same night he cataloged M20. Due to its proximity to the much more famous Trifid Nebula, M21 is often overlooked.
豆知識
At only 4.6 million years old, M21 is so young that its stars have barely begun their main-sequence lives. If our Sun were in this cluster, the dinosaurs would have been around for millions of years before the cluster even formed.
コミュニティ写真 (1)
Credit: RubinObs/NOIRLab/SLAC/NSF/DOE/AURA. License: CC BY 4.0. (Wikimedia Commons)
Skybred Feb 28, 2026