Messier 21
Open cluster in the constellation Sagittarius
title: "Messier 21" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["open-clusters", "carina–sagittarius-arm", "sagittarius-(constellation)", "messier-objects", "ngc-objects", "astronomical-objects-discovered-in-1764", "discoveries-by-charles-messier"] description: "Open cluster in the constellation Sagittarius" topic_path: "general/open-clusters" source: "https://en.wikipedia.org/wiki/Messier_21" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0
::summary Open cluster in the constellation Sagittarius ::
::data[format=table title="Infobox open cluster"]
| Field | Value |
|---|---|
| name | Messier 21 |
| image | Trifid and Lagoon Nebulae Excerpt- Messier 21 (noirlab2521ag).jpg |
| caption | Messier 21 imaged by the Vera C. Rubin Observatory |
| epoch | J2000.0 |
| ra | |
| dec | |
| dist_ly | 1205 pc |
| appmag_v | 6.5 |
| size_v | |
| mass_msol | 783.4 |
| radius_ly | 3.6 pc |
| age | |
| names | M21, NGC 6531, Cr 363, OCl 26.0 |
| :: |
| name = Messier 21 | image = Trifid and Lagoon Nebulae Excerpt- Messier 21 (noirlab2521ag).jpg | caption = Messier 21 imaged by the Vera C. Rubin Observatory | credit = | epoch = J2000.0 | ra = | dec = | dist_ly = 1205 pc | dist_pc = | appmag_v = 6.5 | size_v = | mass_msol = 783.4 | radius_ly = 3.6 pc | v_hb = | age = | notes = | names = M21, NGC 6531, Cr 363, OCl 26.0
Messier 21 or M21, also designated NGC 6531 or Webb's Cross, is an open cluster of stars located to the north-east of Sagittarius in the night sky, close to the Messier objects M20 to M25 (except M24). It was discovered and catalogued by Charles Messier on June 5, 1764. This cluster is relatively young and tightly packed. A few blue giant stars have been identified in the cluster, but Messier 21 is composed mainly of small dim stars. With a magnitude of 6.5, M21 is not visible to the naked eye; however, with the smallest binoculars it can be easily spotted on a dark night. The cluster is positioned near the Trifid Nebula (NGC 6514), but is not associated with that nebulosity. It forms part of the Sagittarius OB1 association.
This cluster is located away from Earth with an extinction of 0.87. Messier 21 is around 6.6 million years old with a mass of . It has a tidal radius of 11.7 pc, with a nucleus radius of and a coronal radius of . There are at least members within the coronal radius down to visual magnitude 15.5, including many early B-type stars. An estimated 40–60 of the observed low-mass members are expected to be pre-main-sequence stars, with 26 candidates identified based upon hydrogen alpha emission and the presence of lithium in the spectrum. The stars in the cluster do not show a significant spread in ages, suggesting that the star formation was triggered all at once.
As of January 2022, Messier 21 is one of the few remaining objects within the Messier Catalog to not have been photographed by the Hubble Space Telescope.
Gallery
File:Messier 21.jpg|Open cluster Messier 21 in Sagittarius imaged by John Saunders File:M21map.png|Map showing the location of M21
References
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| last1=Kharchenko | first1=N. V. | last2=Piskunov | first2=A. E. | last3=Röser | first3=S. | last4=Schilbach | first4=E. | last5=Scholz | first5=R.-D. | display-authors=1 | title=Astrophysical parameters of Galactic open clusters | journal=Astronomy and Astrophysics | volume=438 | issue=3 | pages=1163–1173 | year=2005 | bibcode=2005A&A...438.1163K | arxiv=astro-ph/0501674 | doi=10.1051/0004-6361:20042523 | s2cid=9079873 | postscript=. }}
| last1=Morales | first1=Esteban F. E. | last2=Wyrowski | first2=Friedrich | last3=Schuller | first3=Frederic | last4=Menten | first4=Karl M. | title=Stellar clusters in the inner Galaxy and their correlation with cold dust emission | journal=Astronomy & Astrophysics | display-authors=1 | volume=560 | pages=A76 | year=2013 | doi=10.1051/0004-6361/201321626 | postscript=. | bibcode=2013A&A...560A..76M | arxiv=1310.2612 | s2cid=118422539 }}
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| title=The Galactic Open Cluster NGC 6531 (M21) | last1=Park | first1=Byeong-Gon | last2=Sung | first2=Hwankyung | last3=Kang | first3=Yong Hee | display-authors=1 | postscript=. | journal=Journal of the Korean Astronomical Society | volume=34 | issue=3 | pages=149–155 | date=December 2001 | doi=10.5303/JKAS.2001.34.3.149 | bibcode=2001JKAS...34..149P | doi-broken-date=12 July 2025 }}
| title=Timescale of mass accretion in pre-main-sequence stars | last1=Fedele | first1=D. | last2=van den Ancker | first2=M. E. | last3=Henning | first3=Th. | last4=Jayawardhana | first4=R. | last5=Oliveira | first5=J. M. | display-authors=1 | journal=Astronomy and Astrophysics | volume=510 | id=A72 | pages=7 | date=February 2010 | doi=10.1051/0004-6361/200912810 | postscript=. | bibcode=2010A&A...510A..72F | arxiv=0911.3320 | s2cid=118520073 }}
| title=The central part of the young open cluster NGC 6383 | journal=Astronomy and Astrophysics | volume=362 | pages=580 | last1=van den Ancker | first1=M. E. | last2=Thé | first2=P. S. | last3=de Winter | first3=D. | date=June 2000 | bibcode=2000A&A...362..580V | arxiv=astro-ph/0006283 }}
| title=Imaging the Messier Objects Remotely from Your Laptop | series=The Patrick Moore Practical Astronomy Series | first1=Len | last1=Adam | publisher=Springer | year=2018 | isbn=978-3319653853 | pages=128–130 | postscript=. | url=https://books.google.com/books?id=7nNUDwAAQBAJ&pg=PA128 | bibcode=2018imor.book.....A
References
- "Messier 21".
- (28 August 2017). "Explore - the Night Sky | Hubble's Messier Catalog".
- "M 21".
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