Cygnus OB9: NGC 6888 (Sh2-105, The Crescent Nebula) & WR 136

Cygnus OB9 is an historical designation for a wide swath of OB stars that are not considered part of other Cygnus OB Associations. Since it is not an association with unified and coherent kinematics (Quintana and Wright 2021), I am not going to spend time discussing the association. However, there is one prominent and unique DSO that lies within the bounds of OB9 that is worth a serious look: NGC 6888 (Sh2-105, the Crescent Nebula). 

NGC 6888 (Sh2-105) is an odd structure caused by star HD 192163, also known as Wolf-Rayet 136. Wolf-Rayet stars are now widely believed to have evolved from massive OB stars and represent their evolutionary end. We can, therefore, look at WR 136 through a different lens to understand better understand the lives of massive OB stars. 

First, some data on the star itself. It carries the stellar classification WN6(h)-s C: The WN6 is a reference to the number and type of Nitrogen emission lines, the h means there is lingering (weak -s) hydrogen emission (unusual for WR stars) and the C indicates Carbon emission lines are present. With a mass of 21 M☉, radius of 5.10 R☉, and luminosity of 600,000 L☉, it is a very large, heavy and particularly luminous star. It is nearing the end of its life, too. At an estimated age of around 4.7 million years, the "young" age of this star underscores just how short-lived the massive OB stars are. (Recall that our Sol is ~4.5 billion years old. Billion.)

Visually this is an interesting object, too. Massive OB stars shed their outer layers as they age. This is the point at which they enter the Red (Super) Giant phase of their lives. This is relatively short and if sufficient mass remains during the shedding process, the star evolves into a WR star (if not, it likely will supernova during the Red Giant phase). Having moved into the WR phase, the star ejects even more of its outer layers as well as heavier elements that are brought up from the core through convection. After the WR phase the star's life likely ends in a supernova (the type depends on the mass of the star). The evolutionary path can be shown schematically as:

Massive OB --> Red (super) Giant --> WR phase --> Supernova

What we have in this image is the interaction of the older star skin (red) with the new, high velocity ionizing stellar winds of the WR phase (blue). I have long puzzled about the "bumpy" nature of the old skin and thought it was just peculiar ionization patterns reminiscent of the pillars and Bok globules we see in HII regions affected by other OB stars, but came to the conclusion that this was not entirely correct.

As it turns out, there is growing evidence that WR 136 is a binary system. (Incidentally, if you've followed my writings, you might have noticed that these massive OB stars often--very often--occur in pairs, just another oddity that characterizes this star type.) WR 136 is also a weak X-ray source which is possible evidence for that second body being a lurking neutron star or even black hole. If WR 136 really is a binary system (especially if a neutron star or black hole), the strong stellar winds would no doubt be affected by the orbiting partner, affecting the surrounding nebulosity in peculiar ways. This may account for both the elliptical and bumpy appearance of the nebula. I am following the literature closely, hoping the real scientists will give us an answer!

I hope this brief look at WR 136 has given you new insight in the lives of OB stars.

Incidentally, in the lower left corner you can see the recently discovered (2008) and rarely imaged PN G75.5+1.7 (aka PN Ju-1 after its discoverer, Dave Jurasevich, hence, JU). This discovery can give us all hope that one day we, too, can make a bona fide contribution to the world of astronomy! I will leave this interesting object to someone else to image and write about.

Finally, I am thinking about starting an AB group that focuses a bit more on the science in our images. Image submission would be moderated (with quite a low bar) and descriptions would need to contain some meaningful reflection on the objects in the image, their context and perhaps broader importance. If you would be interested in posting to a group like this, let me know. 

Some sources:

Groh, Jose H.; Meynet, Georges; Georgy, Cyril; Ekström, Sylvia. "Fundamental properties of core-collapse supernova and GRB progenitors: Predicting the look of massive stars before death". Astronomy & Astrophysics. 558: A131 (2013). https://arxiv.org/abs/1308.4681

Quintana, A.; Wright, N. "Revisiting the Cygnus OB associations". MNRAS 0000, 1-15 (2021).
https://arxiv.org/abs/2109.07499

Rustamov, D. N. & Cherepashchuk, A. M. "The Wolf–Rayet star HD 192163 as a possible evolutionary progenitor of a low-mass X-ray binary". Astronomy Reports 55, 347–358 (2011). https://ui.adsabs.harvard.edu/abs/2011ARep...55..347R/abstract