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Messier 21

Open Cluster M21 (NGC 6531), type 'd', in Sagittarius

Right Ascension 18 : 04.6 (h:m)
Declination -22 : 30 (deg:m)
Distance 4.25 (kly)
Visual Brightness 6.5 (mag)
Apparent Dimension 13.0 (arc min)

Discovered by Charles Messier in 1764.

Messier 21 (M21, NGC 6531) is an open cluster which shows quite a strong concentration toward its center. Therefore, it is classified by Woldemar Götz as of Trumpler class I 3 r (strong concentration to the center, large range in brightness, i.e. bright and faint stars, and richly populated), while Trumpler, according to Kenneth Glyn Jones, classified it I 3 p (i.e., poor, or under 50 stars).

According to Burnham, S.N. Svolopoulos, in 1953, has demonstrated the membership of 57 stars (making it Trumpler type I 3 m), the brightest of which are giants of spectral type B0. This implies that this cluster is very young: the Sky Catalog 2000 gives an estimated 4.6 million years, and states that this cluster is part of the Sagittarius OB1 stellar association.

As it is situated close to the Trifid Nebula M20 (the outlayers of which show up in our image at the upper left edge), many images showing the Lagoon-Trifid region do also show M21, e.g. the bigger DSSM image of this region.

The distance of this cluster is discordantly given by the sources: Mallas/Kreimer give 3,000 light years, Burnham 2,200, while Kenneth Glyn Jones and the Sky Catalog 2000 have 4,250 light years. It is interesting that all sources have different distances for the Trifid Nebula M20, but discord which is closer to us, the cluster M21 or the Trifid nebula.

Open cluster M21 was discovered by Charles Messier, who cataloged it on June 5, 1764.

  • Historical Observations and Descriptions of M21
  • More images of M21
  • Amateur images of M21
  • More images of M20 and M21
  • Amateur images of M20 and M21
  • More images of the M8/M20/M21 region
  • Amateur images of the M8/M20/M21 region

  • WEBDA cluster page for M21
  • SIMBAD Data of M21
  • NED Data of M21
  • Publications on M21 (NASA ADS)
  • Observing Reports for M21 (IAAC Netastrocatalog)
  • NGC Online data for M21

    Hartmut Frommert
    Christine Kronberg

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    Last Modification: August 21, 2007