van den Ancker, M. E.; Thé, P. S.; de Winter, D., The central part of the young open cluster NGC 6383, Astronomy and Astrophysics, June 2000, 362: 580, Bibcode:2000A&A...362..580V, arXiv:astro-ph/0006283
Forbes, Douglas, Star Formation in NGC 6531-Evidence From the age Spread and Initial Mass Function, Astronomical Journal, September 1996, 112: 1073, Bibcode:1996AJ....112.1073F, doi:10.1086/118079.
van den Ancker, M. E.; Thé, P. S.; de Winter, D., The central part of the young open cluster NGC 6383, Astronomy and Astrophysics, June 2000, 362: 580, Bibcode:2000A&A...362..580V, arXiv:astro-ph/0006283
Forbes, Douglas, Star Formation in NGC 6531-Evidence From the age Spread and Initial Mass Function, Astronomical Journal, September 1996, 112: 1073, Bibcode:1996AJ....112.1073F, doi:10.1086/118079.
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messier.seds.org
Frommert, Hartmut; Kronberg, Christine, Messier 21, SEDS Messier pages (Students for the Exploration and Development of Space), [2018-11-12], (原始内容存档于2015-03-16).
Frommert, Hartmut; Kronberg, Christine, Messier 21, SEDS Messier pages (Students for the Exploration and Development of Space), [2018-11-12], (原始内容存档于2015-03-16).