Cygnus OB2, Part I: Cygnus Rift, Cygnus X and Extinction

We all recognize Cygnus as one end of a dense part of the Milky Way, of course, but if I ask you which spiral arm of our galaxy you're looking at and where it is in relation to Sol, could you answer? This is typically where we lose our geographic (galactic) bearings. Here is a simple answer. From the galactic perspective, we live in the Orion spur (arm) of the galaxy, also called the local arm. From a bird's eye view, looking to one side of us is the nearby Orion molecular cloud complex. Looking 180deg to the other side--along the full length of the Orion Arm some 1500 parsecs distant is the Cygnus X region (not the same as Cygnus X-1!), another star-forming region (and significant radio source) in our local arm. Other targets, like the Crab and Rosette Nebulae on one hand and the Trifid, Lagoon and Eagle nebulae on the other, are actually in completely different arms of our galaxy.

The Orion Complex and Cygnus X happen to be our closest star-forming regions. We have a fine view of the Orion Molecular Cloud (at the right time of year), but large parts of Cygnus X are actually hidden behind the extreme end of a dark band of interstellar clouds and dust that sits between our Orion Arm and the Sagittarius Arm (where the Trifid, Lagoon and Eagle are) and partially extends into (bisects) our own local arm near Cygnus X. This is called the "Great Rift" and the one end of it--near Cygnus--is called the Cygnus Rift. The Cygnus Rift is particularly obvious around Deneb and the North American Nebula--the so-called "Gulf of Mexico" of the latter is actually a very dense part of the rift.

These details are important because today's target, Cygnus OB2, is partially obscured by that rift. How, then, do we image Cyg OB2? Optically, it's difficult. Most of the ionizing activity of the type we've studied so far is obscured by the rift...and the stars, well, many of them are hidden, too...but not all. Those stars that are visible and partially visible are what's on my imaging mind. 

Cygnus OB2, the largest of the Cygnus OB Associations is roughly situated between Gamma Cygni (Sadr) and Alpha Cygni (Deneb), the center lying much closer to Sadr. Specifically, its center is at galactic coordinates l =79.8 and b =+0.8 with an approximate radius of 1-2 degrees from that point. The most recent best census of the association lists the number of Type O stars at 70 and Type B stars at upwards of 150. 

In this image I've chosen to feature five of the association stars (annotated in Revision B). However, because of the intervening Cygnus Rift, these and many other association stars are affected by interstellar extinction, a phenomenon that causes the absorption and scattering of light from an object due to the presence of dust and gas. Stars affected by extinction (obviously) appear dimmer than they actually are. Further, stars affected by extinction also suffer from interstellar reddening: the dust and gas in the interstellar medium (ISM) of the Cygnus Rift absorbs shorter wavelength photons (blue, UV), allowing only longer wavelengths to pass (such as red and IR). That is, our normally blue Type O and Type B stars appear red, the amount depending on the amount of both extinction and reddening. Due to extinction and reddening, I processed this image differently. I minimized or eliminated all but the brightest stars and was quite careful about maintaining star color, since this is the central focus of the image.

Without further delay, here are today's stars. 

V* V1827 Cyg  O6Iaf+O9Ia
Weighing in at a massive 57 M☉ with a luminosity of 240000 L☉ this is an eclipsing binary system (Algol type) composed of two Type O Supergiants.

Schulte 8A    O6If+O5.5III(f) 
This pair of giants (I and III) make the previous pair appear as a dimming candle, shining at 500000 L☉ and weighiing in at ~80 M☉. The pair is located within or near a small cluster of stars that at one time were thought to be a single, massive star.

Schulte 12    B3-4Ia+ 
This one is special. A Type B Hypergiant, this Lumnious Blue Variable (LBV) may be the brightest star in our galaxy   with a calculated lumniosity of nearly 1.7 *million* L☉ and a mass of 110 M☉. This star's **absolute magnitude** (magnitude measured at 10 parsecs) would be -10.6. Compare that to our Sun (4.8) or Sirius (1.4). How large is this star? 246 R☉. That's larger than the orbit of Saturn! It is a real behemoth in our galaxy, and not much to look at in this image.

Schulte 22    O3If*+O6V((f))
Weighing in at a massive 50 M☉ this spectroscopic binary of a Type O Giant and Type O Main Sequence has a luminosity of 660000 L☉. 

Schulte 64    B1Ve-B3Ve
With a lumniosity of 23000 L☉ and mass of 12 M☉, this star is the least luminous and least massive of the featured stars. That makes it no less interesting. Classed as a non supergiant Be star, this gem is surrounded by ionized gas, solar winds and a circumstellar disk which may be forming a shell around the star. 

The association is aged around 3-4 million years, so these stars are all young, hot, luminous and massive. Most of the surrounding HII has already been burned off (ablation), so you'll see neither pillars nor Bok globules (sorry, no eye candy). Many of these stars are quite unstable and all are likely to end their days as supernovae. They all appear highly reddened in the image due to the obstructing gas and dust from the intervening Cygnus Rift. Optical studies of Cygnus OB2 are, therefore, quite difficult, but the Association is nonetheless a focus of very active research using space-based telescopes and non-optical means (X-ray, IR, Radio). 

There is much more to say about Cygnus OB2, but I've gone on long enough for now! If you'd like to know a bit more about Cygnus OB2, let me know in the comments below. I have sufficient thoughts for a "Part II". Thank you--as always--for taking the time to look, read, and comment and to be a part of this great journey along with me.

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In addition to the sources already cited in my other Cygnus OB images, here are others that are of interest and bear on this discussion:

Berlanas, S. R.; Wright, N. J.; Herrero, A.; Drew, J. E.; Lennon, D. J. (2019). "Disentangling the spatial substructure of Cygnus OB2 from Gaia DR2". Monthly Notices of the Royal Astronomical Society. 484 (2): 1838. (https://ui.adsabs.harvard.edu/abs/2019MNRAS.484.1838B/abstract)

Comerón, F.; Pasquali, A. (2012). "New members of the massive stellar population in Cygnus". Astronomy & Astrophysics. 110: 2715. (https://ui.adsabs.harvard.edu/abs/2012A%26A...543A.101C/abstract)

Negueruela, I.; Marco, A.; Herrero, A.; Clark, J. S. (2008). "New very massive stars in Cygnus OB2". Astronomy and Astrophysics. 487 (2): 575. (https://ui.adsabs.harvard.edu/abs/2008A%26A...487..575N/abstract)

Pigulski, A.; Kołaczkowski, Z. (1998). "Mapping the ß Cephei instability strip: photometric variability of stars in the central part of the Cygnus OB2 association". Monthly Notices of the Royal Astronomical Society. 298: 753–762.
(https://ui.adsabs.harvard.edu/abs/1998MNRAS.298..753P/abstract)

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. (https://ui.adsabs.harvard.edu/abs/2014MNRAS.438..639W/abstract)