Having looked at the Vulpecula OB1 Association in my last image, it's time to turn to the Cygnus Stellar Associations. The study of Cygnus Associations is difficult. The area, referred to as Cygnus X (or Cygnus Star Cloud), is a massive star-formation region and is partially (sometimes largely) obscured by interstellar clouds of cosmic dust and gas called the Great Rift (or locally, in Cygnus alone, as the Cygnus Rift).

Traditionally, there are nine identified OB Associations in Cygnus, numbered as you would expect, OB1 through OB9. Without good data indicating stellar age, direction and distance, and the further issue of many of the stars being obscured or blocked completely by dust, classifying the Associations was problematic at best. Recently, using newer technology and methodology, compelling research by Quintana & Wright (2021) has cast further doubt on those historic designations. With this caveat in mind, let's begin today's journey.

This is an image of the Cygnus Wall, a well-known part of NGC 7000 (Sh2-117, W80, North America Nebula). This nebula, along with the nearby IC 5070 and 5067 (Pelican Nebula) are considered one diffuse nebula. The area is well-evolved, showing none of the characteristic signs of active massive star formation, like pillars and Bok globules, although the presence of Be  and T Tauri stars indicate continuing, low mass star formation.   

Astronomers long wondered about the ionizing star of this HII region. Hubble (1922) proposed Deneb as the excitation source, but Deneb is too distant and clearly of the wrong spectral class (A2). It was Herbig (1958) who first suggested it was a star behind L935 (the dark nebula informally referred to as the "Gulf of Mexico"). Since then, numerous suggestions have been proposed, but ultimately it was Comerón and Pasquali (2004) who finally proposed the now-accepted answer to this long-standing question. 

Comerón and Pasquali identified J205551.3+435225 (2MASS  J205551.25+435224.6, on SIMBAD as [SL2008c] 8), a massive O5V star, cloaked behind the dense dust of L935 as the ionizer. Subsequent research has revised the classification of the star to an O3.5III. Huge, hot and luminous are all understatements in describing this giant. 

These blue giants do not exist in isolation; they were born and have moved out of their original cluster into these looser stellar associations. That leaves us with the question: what OB Association does this star belong to? Reminding you of the caveat above, traditionally the star would have been classified as a member of (probably) OB6 or (maybe OB7) (depending on the decade and researcher). I have yet to go through all of the data by Quintana and Wright to see if they have clarified the matter for us.

As much as I'd like to know for sure (and tell you), I don't. It's too bad, because these OB Associations are important in understanding galactic evolution and there simply hasn't been a lot of research done in this regard. You can be sure, though, that I will continue reading the research and reporting what I find through my images.

As for this star and the surrounding HII region, stop for a moment and really look at the picture. Imagine this hidden star (marked in Revision B) causing all of this ionization and the ionization found in the broader North America and Pelican nebulae! How one star can light up such an area is truly awesome. Already, on the far side of the star you can see the dust and gas thinning from the star's power. Too, the dark, thick swath of cosmic dust and gas (L935) that is blocking our view to the star is likewise incredible. Dense and cold, resisting the ionization of the star...but for how long?

These processes may be beyond our ability to comprehend, but we can do our best to image them and understand them. In so doing we can become the eyes of the universe.

Thanks for stopping by and taking the time to read and comment. I hope you enjoyed this journey.


Sources:
Comerón and Pasquali, 2005, Centro Astronómico Hispano-Alemán Newsletter.
Herbig, G. H. 1958, ApJ, 128, 259.
Hubble, E. P. 1922, ApJ, 56, 400.
Quintana and Wright, 2021, MNRAS 000, 1–15.
Wright N. J., 2020, New Astron. Rev., 90, 101549.