The young cluster is one of the largest known and the largest in the northern hemisphere with some authors formerly classifying it as a young
globular cluster similar to those in the
Large Magellanic Cloud.[5] Today, however, it is considered a massive, low-density stellar association.[6]
Although it is over ten times more massive than the
Orion Nebula, which is easily seen with the naked eye, Cygnus OB2 is hidden behind a massive dust cloud known as the
Cygnus Rift, which obscures many of the stars in it. This means that despite its large size, it is hard to determine its actual properties. The estimated number of massive stars range from 50[7] to 100[5] of
spectral type O and its total mass having been calculated as (4–10)×104[5] or 3×104solar masses according to other investigations.[7]
Despite this, recent surveys ranging from radio to X-ray wavelengths have observed the region to great depths to gain a better understanding of how the processes of star and planet formation occur on such a large scale. These studies include observations with the
Chandra X-ray Observatory,
Spitzer Space Telescope, the
Herschel Space Observatory and the
Gran Telescopio Canarias. As for recent observations, the final stages of the process of photoablation is taking place, where the biggest stars formed and cleared the ambient material from the region.[8]
The progenitor of
BD+43°3654 might have been a member of Cygnus OB2. Two stars from two binaries would have collided and merged forming BD+43°3654, which would have then been ejected from the stellar association along with the two remaining stars.[25]
Prominent members of the association are often referred to by their Schulte numbers: for example Schulte 12, VI Cygni 12, or Cygnus OB2 #12. The numbers were first used in the 1953 discovery paper where 11 "blue giants" were numbered.[26] A 12th star (
Cyg OB2 #12) was added in 1954,[27] and eight more shortly after.[28] Schulte himself maintained the already-published numbers and added many more when studying the association which he called VI Cygni.[9]
Cygnus OB2 contains embedded star clusters as well as two
open clusters located in the center of Cygnus OB2. The open clusters are called
Bica 1 and
Bica 2. Both Bica 1 and Bica 2 contain several OB-stars, such as
Cygnus OB2 #8A and
Cygnus OB2 #22.[29]
^Schuster, M. T.; Marengo, M.; Hora, J. L.; Fazio, G. G.; Humphreys, R. M.; Gehrz, R. D.; Hinz, P. M.; Kenworthy, M. A.; Hoffmann, W. F. (2009). "Imaging the Cool Hypergiant NML Cygni's Dusty Circumstellar Envelope with Adaptive Optics". The Astrophysical Journal. 699 (2): 1423–1432.
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^Wright, Nicholas J.; Parker, Richard J.; Goodwin, Simon P.; Drake, Jeremy J. (2014). "Constraints on massive star formation: Cygnus OB2 was always an association". Monthly Notices of the Royal Astronomical Society. 438 (1): 639–646.
arXiv:1311.4537.
Bibcode:
2014MNRAS.438..639W.
doi:
10.1093/mnras/stt2232.
S2CID16957082.
^Sota, A.; Maíz Apellániz, J.; Morrell, N. I.; Barbá, R. H.; Walborn, N. R.; Gamen, R. C.; Arias, J. I.; Alfaro, E. J.; Oskinova, L. M. (2019). "The Galactic WN stars revisited. Impact of Gaia distances on fundamental stellar parameters". Astronomy & Astrophysics. A57: 625.
arXiv:1904.04687.
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^
abKiminki, D. C.; Kobulnicky, H. A.; Ewing, I.; Bagley Kiminki, M. M.; Lundquist, M.; Alexander, M.; Vargas-Alvarez, C.; Choi, H.; Henderson, C. B. (2012). "Additional Massive Binaries in the Cygnus OB2 Association". The Astrophysical Journal. 747 (1): 41.
arXiv:1112.3383.
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