# Messier 21

> Open cluster

**Wikidata**: [Q11383](https://www.wikidata.org/wiki/Q11383)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Messier_21)  
**Source**: https://4ort.xyz/entity/messier-21


## References

1. SIMBAD
2. VizieR
3. On Structural Properties of Open Galactic Clusters and their Spatial Distribution. Catalog of Open Galactic Clusters
4. A Catalogue of Star Clusters shown on Franklin-Adams Chart Plates
5. Freebase Data Dumps. 2013
6. [Revised NGC Data for NGC 6531](http://spider.seds.org/ngc/revngcic.cgi?NGC6531)
7. A RAVE investigation on Galactic open clusters
8. Astrophysical parameters of Galactic open clusters
9. The orbits of open clusters in the Galaxy
10. Proper motions of the optically visible open clusters based on the UCAC4 catalog