Cygnus OB3, Part III: The Central Mass

In this picture, it would seem there is no object to focus on. To be sure, there may be a loose star cluster on the right. There are some dark structures near the bottom. There are many, many stars. So where is the focal point? What is this a picture of? What's going on here? 

If you're following my series, then you already know I'm writing about the Cygnus OB3 Association. To date, I have focused primarily on the Type O stars and their evolution. Associations, you will recall, are unstructured, gravitationally unbound, co-moving groups of young Type O and Type B stars (and certain fully evolved WR stars, I would also argue). This image features a dozen stars from the Association: six Type O stars (including two binary systems), five Type B stars (members of cluster NGC 6871) and one Wolf-Rayet binary. 

If you're reading along, flip over to Revision "B" and look at the broad distribution of the selected stars. The stars and their MK classifications are:

HD 227960    O8.5 
HD 191612    O8fpe (A spectroscopic binary with unknown partner)
BD+35 3976   OB-                   
HD 190864    O6.5III(f)
HD 190967    O9.5V + B1Ib (A spectroscopic binary)

BD+35 3955   B0.7Iab  (A blue supergiant)
HD 227634    B0.2II
BD+35 3956   B0.5Vne
HD 190919    B0.7Ib

WR 133       WN5 + O9I  (A spectroscopic binary, aka HD 190918 or V* V1676 Cyg)

Of these interesting stars, HD 191612 and WR 133 are perhaps the most intriguing. HD 191612 shows variable luminosity, ranging from O6 to O8. At first researchers thought it was a variable, but it turns out that the star not only has peculiar emission spectra, but also has a strong magnetic field (Type O stars rarely, if ever, have magnetic fields), making HD 191612 quite unique. In fact, in is one of only three known stars in our galaxy with this peculiar combination of emission spectra.

This Wolf-Rayet is certainly among the brightest of the known WR stars, but its luminosity is overshadowed by its partner, a massive Type O supergiant. There are faint traces of blue in the image surrounding the entire cluster, which may be a developing bubble or lost mass from the Wolf-Rayet. As most WR stars, WR 133 likely evolved from an earlier Type O star and ultimately may be headed for a supernova.

In any case, all of these hot, large and luminous stars, along with those featured in Parts I and II (as well as the nearly 100 additional Type B stars not mentioned and spread across the three images) were born in the same star-forming region at approximately the same time. The interstellar gas and dust that gave rise to these stars is, at the same time, being destroyed by these very same stars, ionized by a process known as Radiation Driven Implosion. 

This process is responsible for the pillars and other details seen in LBN 179 and B147 at the bottom right of the image and in other areas of the image. One way to think about this picture is to visualize the association stars as inside a dark cavern with the surrounding HII regions being the walls. In such a visualization the stars themselves are sculpting the walls of the cavern. Once you know which stars are foreground (or background) and which are the actors on the stage, the image can take on real meaning. Context matters. 

...

Last time I promised to keep it short, and so I will end it here today. I hope this three part series on Cygnus OB3 has been interesting and that you've learned something. I certainly have! We've seen the distribution of Type O stars in the association, as well as highlighted three WR stars that likely evolved from earlier Type O stars. Two loose, open clusters (NGC 6883 and NGC 6871) are part of the broader Cygnus OB3 Association, and we've seen how all of these stars ionize their surrounds and, at their deaths, become supernovae or can collapse to a microquasar, as in Cygnus X-1. 

I hope you enjoyed the narrative and got something from the image and, of course, I thank you for the time you've given me.

Other Parts in the Series:
Cygnus OB3, Part I: The Wolf-Rayet Stars
Cygnus OB3, Part II: Of Stellar Classification, Bow Shockwaves and Microquasars

Additional Sources:
Howarth, Ian D. et al. “Towards an understanding of the Of?p star HD 191612: optical spectroscopy.” Monthly Notices of the Royal Astronomical Society 381 (2007): 433-446.

Massey, P.; Degioia-Eastwood, K.; Waterhouse, E. "The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. II. Results from 12 Galactic Clusters and OB Associations". The Astronomical Journal. 121 (2001): 1050–1070.