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The concentration of stars in m21 cluster toward the center is quite strong.Therefore, woldemar g ö tz classifies its Trumpler level into i3r type (strong concentration towards the center, large brightness range, that is, bright and dim stars, and a large number of stars, rich), while according to Kenneth glyn Jones, Trumpler himself classifies it into i3p type (that is, a small number of stars, poor, or less than 50 stars).
According to Burnham, s.n. svolopoulos has identified 57 member stars in 1953 (making their trumpler level to i 3 m), of which the brightest star is a giant star of spectral type b0.This means that this is a very young star cluster: sky catalog 2000 has an estimated age of 4.6 million years, and claims that this star cluster is part of the Sagittarius OB1 star association.
Because it's very closeTrifid Nebula M20 (its outer nebula appears on the upper left edge of our image), and the photo showing the lagoon Trifid sky area does contain m21, such as the large dssm image of this sky area.
Data sources with different distances of this cluster give different results: Mallas/kreimer gives 3000 light years, Burnham 2200, and Kenneth glyn Jones and sky catalog 2000 both 4250 light years.Interestingly, all data sources give different distances to the Trifid Nebula m20, so it is uncertain which cluster m21 or Trifid Nebula is closer to us.
Open clusterM21 was discovered by Messier, who catalogued it on June 5, 1764.
