Cygnus OB2 or VI Cygni (abbreviated to VI Cyg) is an OB association that is home to some of the most massive and most luminous stars known, including suspected Luminous blue variable Cyg OB2 #12. It also includes one of the largest known stars, NML Cygni. The region is embedded within a wider one of star formation known as Cygnus X, which is one of the most luminous objects in the sky at radio wavelengths. The region is approximately 1,570 parsecs from Earth in the constellation of Cygnus.
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. Today, however, it is considered a massive, low-density stellar association.
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 to 100 of spectral type O and its total mass having been calculated as (4–10) or 3 solar masses according to other investigations.
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.
+ Prominent stars Schulte number MT number Other name/CPR number Spectral type Luminosity() Mass() HD 195213O7 180,300 43.3 WR 144WC4 158,500 9.9 WR 145WN7o/CE+O7V((f)) 371,000 18.3 WR 146WC6+O8III 115,000 8?+?WR 147WN8h+B0.5V2,000,00051 NML CygM4.5–M7.9Ia–III 229,000 50 V1827 Cyg/B17Ofpe 242,100 57 BD+40°4210http://www.sciops.esa.int/SD/ESACFACULTY/docs/seminars/160212_Comeron.pdf In and around the rich association Cygnus OB2B1III:e 630,000 54 BD+40 4223B0Ia 539,500 48.3 #1 59 O8.5V 120,000 26 #2 83 B1I 40,000 14 #3 O6IV+O9III 346,000 >17 + >8 #4 217 O7III((f)) 158,000 29 #5 V729 O7I+O6I+O9V 1,584,000 31 + 27 + 9 #6 317 O8V 109,000 25 #7 457 O3If 426,000 47 #8A 465 O6If+O5.5III(f) 501,000 44 + 37 #8B 462 O6.5III(f) 301,000 35 #8C 483 O5If 371,000 42 #8D 473 O8.5V 48,000 20 #9 431 O5–5.5I+O3–4III 707,000 >34 + >30 #10 632 O9.5I 478,000 37 #11 734 O5If 537,000 44 #12 304 B3-4Ia+ 1,230,000 110 #14 227O9V 45,000 19 #15 258O8V 61,000 22 #16 299O8V 83,000 23 #17 339O8.5V 61,000 21 #18 556 B1Ib 338,000 29 #19 601B0Iab 186,000 26 #20 145O9III 26,000 17 #21 259B0.5V 15,000 13 #22 417 O3If+O6V(f) 660,000 50 #23 470O9.5V 26,000 17 #24 480O7.5V 104,000 25 #25 531O8.5V 97,000 24 #26 642B1III 69,000 16 #27 696O9.5V+B0V 30,000 17 #29 745O7V 87,000 25 #30 793B1.5III 33,000 13 #37 358B3V 3,000 7 #41 378B0V 44,000 18 #51 425B0V 27,000 16 #54 395B1V 7,000 10 #64 488B2Ve 23,000 12 #66 515B1V 10,000 11 #70 588B0V 48,000 18 #71 646B1.5V 6,000 9 #73O8III+O8III 41,000 20 #74 555O8V 109,000 25 #75 736O9V 31,000 18 138O8I 88,700 26 267 A11O7.5III 323,000 35 448O6V 107,000 29 516 O5.5V((f)) 707,000 52 716O9V 28,000 18 771 O7V 151,000 29 A12B0Ia 373,300 36 A15O7I 263,000 32 A20O8II 380,000 35 A23B0.7Ib 263,000 26 A24O6.5III 154,000 30 A25O8III 88,700 28 A27B0Ia 426,000 35 A29O9.7Iab 167,500 29.1 A32O9.5IV 106,700 26.1 A36B0Ib+B0III 173,000 26 A37O5V((f)) 66,100 34.8 A46O7V((f)) 46,600 25.6 E47B0Ia 676,000 42 IRAS 20321+4009O9 278,000 32 TYC 3156-998-1OC9.7Ia 516,000 34.2
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.
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. A 12th star (Cyg OB2 #12) was added in 1954, and eight more shortly after. Schulte himself maintained the already-published numbers and added many more when studying the association which he called VI Cygni.
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.
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