M21 (疏散星團)

梅西耶21
	在人馬座的疏散星團M21 ; Credit: 約翰·桑德斯（John Saunders）
觀測資料 (J2000.0 曆元)
赤經	18h 04m 13.0s
赤緯	−22° 29′ 24″
距離	3,930 ly（1,205 pc）
視星等 (V)	6.5
視大小 (V)	14.0′
物理特徵
質量	783.4 M☉
估計年齡	6.6×106 年
其他特徵	M21、NGC 6531、Cr 363、 OCl 26.0
	相關條目：疏散星团、NGC天體表

梅西耶21或M21，也稱為NGC 6531，是在人馬座的一個疏散星團。它於1764年6月5日被查爾斯·梅西耶發現，並編入目錄中^[7]。這個星團中的恆星排列緊密，顯示它相對年輕。在星團中發現了一些藍巨星，但M21主要由小且暗的恆星組成^[3]。M21的視星等為6.5等，但肉眼看不見；然而，用最小的雙筒望遠鏡，很容易在夜空中發現它。這個星團位於三裂星雲（NGC6514）附近，但與該星雲沒有關係^[8]，它是人馬座OB1星協的一部分^[9]。

這個星團距離地球7019371824148566796♠1,205 pc^[2]，其消光為0.87^[10]。M21的年齡大約為660萬年，質量為 7033155783006999999♠783.4 M_☉^[5]。它的潮汐半徑為 11.7秒差距^[5]，核半徑為7016493708413034750♠1.6±0.1 pc和冕半徑為7017111084392932818♠3.6±0.2 pc。在冕半徑內視星等比15.5等亮員至少有7002105000000000000♠105±11個成員^[11]，並且包括許多早期型的B型星^[8]。在觀測到的低質量恆星中，估計有40-60顆是前主序星^[8]，根據Hα發射譜線和光譜中存在鋰的譜線，確定了26個候選者^[10]。星團中的恆星沒有險租明顯的年齡分布，表明這些恆星是在一次觸發下形成的。

圖集

顯示M21位置的星圖。

參考資料

^ ^1.0 ^1.1 Wu, Zhen-Yu; et al, The orbits of open clusters in the Galaxy, Monthly Notices of the Royal Astronomical Society, November 2009, 399 (4): 2146–2164, Bibcode:2009MNRAS.399.2146W, S2CID 6066790, arXiv:0909.3737 , doi:10.1111/j.1365-2966.2009.15416.x.
^ ^2.0 ^2.1 Kharchenko, N. V.; et al, Astrophysical parameters of Galactic open clusters, Astronomy and Astrophysics, 2005, 438 (3): 1163–1173, Bibcode:2005A&A...438.1163K, S2CID 9079873, arXiv:astro-ph/0501674 , doi:10.1051/0004-6361:20042523.
^ ^3.0 ^3.1 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）.
^ Morales, Esteban F. E.; et al, Stellar clusters in the inner Galaxy and their correlation with cold dust emission, Astronomy & Astrophysics, 2013, 560: A76, Bibcode:2013A&A...560A..76M, S2CID 118422539, arXiv:1310.2612 , doi:10.1051/0004-6361/201321626.
^ ^5.0 ^5.1 ^5.2 ^5.3 Piskunov, A. E.; et al, Tidal radii and masses of open clusters, Astronomy and Astrophysics, January 2008, 477 (1): 165–172, Bibcode:2008A&A...477..165P, doi:10.1051/0004-6361:20078525.
^ M 21. SIMBAD. 斯特拉斯堡天文資料中心.
^ Adam, Len, Imaging the Messier Objects Remotely from Your Laptop, The Patrick Moore Practical Astronomy Series, Springer: 128–130, 2018, Bibcode:2018imor.book.....A, ISBN 978-3319653853.
^ ^8.0 ^8.1 ^8.2 Park, Byeong-Gon; et al, The Galactic Open Cluster NGC 6531 (M21), Journal of the Korean Astronomical Society, December 2001, 34 (3): 149–155, Bibcode:2001JKAS...34..149P, doi:10.5303/JKAS.2001.34.3.149 (不活跃 2020-09-10).
^ 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 
^ ^10.0 ^10.1 Fedele, D.; et al, Timescale of mass accretion in pre-main-sequence stars, Astronomy and Astrophysics, February 2010, 510: 7, Bibcode:2010A&A...510A..72F, S2CID 118520073, arXiv:0911.3320 , doi:10.1051/0004-6361/200912810, A72.
^ 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.

外部連結

Messier 20 and 21, Astronomy Picture of the Day, August 28, 2014 [2018-11-12], （原始内容存档于2020-11-11）. </ref>
WikiSky上关于M21 (疏散星團)的内容：DSS2, SDSS, GALEX, IRAS, 氢α, X射线, 天文照片, 天图, 文章和图片

天球赤道座標： 18^h 04^m 36^s，−22° 30′ 00″

[Wu2009-1] 1.0 ^1.1 Wu, Zhen-Yu; et al, The orbits of open clusters in the Galaxy, Monthly Notices of the Royal Astronomical Society, November 2009, 399 (4): 2146–2164, Bibcode:2009MNRAS.399.2146W, S2CID 6066790, arXiv:0909.3737 , doi:10.1111/j.1365-2966.2009.15416.x.

[kharchenko2005-2] 2.0 ^2.1 Kharchenko, N. V.; et al, Astrophysical parameters of Galactic open clusters, Astronomy and Astrophysics, 2005, 438 (3): 1163–1173, Bibcode:2005A&A...438.1163K, S2CID 9079873, arXiv:astro-ph/0501674 , doi:10.1051/0004-6361:20042523.

[SEDS-3] 3.0 ^3.1 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）.

[Morales-4] Morales, Esteban F. E.; et al, Stellar clusters in the inner Galaxy and their correlation with cold dust emission, Astronomy & Astrophysics, 2013, 560: A76, Bibcode:2013A&A...560A..76M, S2CID 118422539, arXiv:1310.2612 , doi:10.1051/0004-6361/201321626.

[Piskunov2008-5] 5.0 ^5.1 ^5.2 ^5.3 Piskunov, A. E.; et al, Tidal radii and masses of open clusters, Astronomy and Astrophysics, January 2008, 477 (1): 165–172, Bibcode:2008A&A...477..165P, doi:10.1051/0004-6361:20078525.

[SIMBAD-6] M 21. SIMBAD. 斯特拉斯堡天文資料中心.

[Adam2018-7] Adam, Len, Imaging the Messier Objects Remotely from Your Laptop, The Patrick Moore Practical Astronomy Series, Springer: 128–130, 2018, Bibcode:2018imor.book.....A, ISBN 978-3319653853.

[Park2001-8] 8.0 ^8.1 ^8.2 Park, Byeong-Gon; et al, The Galactic Open Cluster NGC 6531 (M21), Journal of the Korean Astronomical Society, December 2001, 34 (3): 149–155, Bibcode:2001JKAS...34..149P, doi:10.5303/JKAS.2001.34.3.149 (不活跃 2020-09-10).

[Ancker2000-9] 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 

[Fedele2010-10] 10.0 ^10.1 Fedele, D.; et al, Timescale of mass accretion in pre-main-sequence stars, Astronomy and Astrophysics, February 2010, 510: 7, Bibcode:2010A&A...510A..72F, S2CID 118520073, arXiv:0911.3320 , doi:10.1051/0004-6361/200912810, A72.

[Forbes1996-11] 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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