- M 20 (The Trifid Nebula) and M 21
This is a wide-field view of these two remarkable objects in the constellation Sagittarius. M 20 (NGC 6514) combines a vibrant pink emission nebula with an adjacent relection nebula lit by the young blue-white giant stars formed from the condensed gas of the nebula. The open cluster M 21 (NGC 6531) sits to the side of M20, blazing with young blue-white supergiant stars formed by the same processes occurring in M 20. Several blue giants seem to link the two objects but, in reality, M 20 is considerably closer (2660 LY) than M 21 (3930 LY). Most of the stars in M 21 have an age of around 7.5 to 8 million years, while the young stars within M 20 are only around 400,000 years old. The brighest of the cluster stars in M 20, HD164494, is a double star that can be seen close to the confluence of the dark dust lanes that divide the appearance of the nebula into three (hence, "trifid") sections.
M 20 was discovered on 5 June 1764 by Charles Messier, although it is apparent from his notes that he only saw the cluster stars in it and not the nebulosity. M 21 was described by Messier on the same date. The nebulosity of M 20 was first described by the English astronomer William Herschel. The name "Trifid Nebula" comes from his son's (John Herschel) observation, describing the object as "trifid, three nebulae with a vacuity in the midst."
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