Cygnus OB3, Part II: Of Stellar Classification, Bow Shockwaves and Microquasars

In my last image, I wrote about the northeasterly extreme of Cyg OB3 and the possible inclusion of Wolf-Rayet stars in OB Associations. Today we head to the Southwest to talk about the other extreme of Cyg OB3. Here we find three of the nine OB stars of the association. We also have a compact HII region to discuss (Sh2-101) and the microquasar-black hole known as Cygnus X-1.

The first of the Type O stars to discuss is HD 227245 (refer to Revision B to see the location). This is a main sequence Type O star. "Main sequence? I thought O stars were giants?" Main sequence O stars are not giants (although they likely will evolve to giants), but they do make our sun look like a tiny speck. Type O main sequence stars are "only" 8-12 R☉, but are also 15-90 M☉ and an astounding 40K-100K L☉. At the minimum, then, a main sequence Type O is 8 times larger, 15 times heavier and 40,000 times brighter than our little Sol!

HD 227245 is likely the primary ionizing source of the HII regions seen in the vicinity of the star. Here is the full spectral type of the star:

HD 227245     O7V((f))z C

That's a lot of information! Type O star. Effective temperature 7 (0-9 scale, 0=hottest, 9=coolest), V=main sequence (0 or I=hypergiant, VII=white dwarf), ((f))= strong He II absorption and weak N III emissions, z=zero age (i.e. very young) and finally, C=prominent ionized Carbon lines.

Do you need to know all of this? Probably not. You may wish to be able to understand at least the star type (O) and size (V).

Incidentally, did you know that this is called the Morgan-Keenan (MK) classification system? Did you know that the types of stars in this system (O,B,A,F,G,K,M) came from an earlier Harvard spectral system created by astronomer Annie Jump Canon? Ms. Canon was another one of the "Harvard Computers"--pioneering women astronomers from Harvard who played a critical role in cataloging and understanding the cosmos. Ms Cannon is rarely mentioned, sadly of course, for her prodigious output and innovative contributions to the field of astronomy.

Annie Cannon cataloged more than 300,000 (!) stars in her life. She could, they say, catalog at a rate of 200 stars an hour in her prime! Most of these stars found their way into the Henry Draper (HD) Catalog, of which this star is a member. Most likely Ms. Canon herself was the first to catalog HD 227245. Editorial comment: The HD Catalog contains approximately 350,000. Perhaps this star catalog should be renamed the AC (Annie Cannon) catalog.

Knowing the voluminous work Ms. Canon did during her life to classify so many objects that you manipulate in your images, why wouldn't you always want to do RGB stars?   I've zealously done RGB stars today in her honor.

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The second of our three Type O stars is HD 227018. Originally noted as a O6.5V((f))z C in the Simbad database, recent studies have reclassified it from a main sequence (V) to a O6.5III (Rao et al 2020). The III indicates that it is a giant (not a supergiant or hypergiant, just a normal giant). This is the primary ionizing star for Sh2-101 (aka the Tulip Nebula), which many of you have seen before or perhaps even imaged. This particular HII region, with the ionizing star nestled at its center, carries a lot of fine detail that allow you to visualize the power of the ionization process.  

HD 227018 has an unusually high velocity compared to others in the association, indicating that it is probably a runaway star, probably from NGC 6871 (just off the left edge of the image). Runaways are most usually caused by an interaction with a massive neighbor or as the result of being blown off course by a supernova. Both are possible here, although the supernova cause is the more likely. Most interesting to me--and additional proof of HD 227018 being a runaway--is the notable bow shock in the image. (See annotation in Revision B). There's a moral to the story here: beware of excess noise reduction and processing that eliminates details that tell a star's story!

The large star nearby to the lower left of HD 227018 is a K5 E star, HD 189688 mid temp (5) star, cooler than our sun. This star is much closer to earth (at about 600 parsecs) compared to the nebula, at about 2000 parsecs. This star--totally irrelevant to the nebula--is a great reminder of the 3D space we're viewing. 

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Our final star today is HD 226868. HD 226868 is a O9.7Iabpvar: Type O, less hot (9.5) intermediate luminous supergiant (Iab) with unspecified peculiar (p) properties, that is also a variable star (var). The (p) notation in this case is the marker of interest. HD 226868 is a high mass x-ray binary system. That x-ray source is from the binary partner called Cygnus X-1, considered to be the site of a microquasar surrounding a black hole (take a look at my AB avatar for a crude graphic representation of a quasar/microquasar). For a sense of perspective, this black hole is closer to the massive HD 226868 supergiant (about 25 R☉, 33.3 M☉) than Mercury is to Sol. The compact object (aka black hole) is very, very small. It has an event horizon measured at about 26km (16 miles) with a mass of approximately 33.3 M☉. Stop and think about this for a while. Really. Stop.

In their cosmic dance, the blue giant is shedding its outer layers which are being swept up by the black hole into a thin accretion disk. The gravitational potential energy of the matter in this disk is so strong, that the "compact object" is generating invisible jets (hyper accelerated ionized matter) perpendicular to the accretion disk (see my avatar again for a visual). These jets are not optically visible, but in images of the area you can see a nebula forming where one of the jets is interacting with a dense region of the interstellar medium. 

The origin of Cyg X-1 is what most fascinates me. We can't know for sure, of course, but these tiny black holes are often the remains of a star death. Without going into the complicated details of stellar evolution, our short-lived Type O stars typically end in a supernova (with or without passing through a Wolf-Rayet phase). In some cases, however, the core of the star can collapse immediately without an explosion, resulting in a black hole. Such appears to be the case here. Had Cygnus X-1 been the result of a supernova, then HD 226868 would no doubt have been affected. Instead, we find that the O stars in the association, and Cygnus X-1, are showing similar proper motions and velocities (except for HD 227018 as noted above). Cygnus X-1, in my opinion, is (was) part of the Association, perhaps being the first of the Type O stars in the association to live out its life and die.

That wraps it up for Part II of the Cygnus OB3. Obviously, I've merely skimmed the surface of what can be said about these Type O stars and their complicated surroundings. Hopefully you'll be interested enough to explore further!  Next time, in Part III, we'll take a look at NGC 6871 and the remaining Type O stars in the association. I will not write as much. 

As always, I'm grateful for you stopping by and spending time with my images and words. I'd love to hear what you think.


Sources:
Noriega-Crespo et al, 1997, The Astronomical Journal 113, 780-786. 
Quintana and Wright, 2021, MNRAS 000, 1–15.
Rao et al, 2020, MNRAS 000, 1-11.
Russell et al, 2007, MNRAS 376, Issue 3, 1341–1349.
Wright N. J., 2020, New Astron. Rev., 90, 101549.