This one was a significant challenge. Between imaging a faint narrowband target from what is now a white zone (thanks, Nashville!) and trying to cover it as a mosaic, looooooooooots of processing wharrgarbl.
I am, to be frank, not terribly happy with the outcome, though the opinion of other more talented individuals seems to be less harsh…not sure how I feel about that, frankly.
Sh2-91, often called “the other Veil” in Cygnus, is a ~ 30k yr old supernova remnant. As the shockwave from the supernova hurtles through space at 30,000km/s, it excites Hydrogen and Oxygen (among others) molecules on its way by, kicking electrons to higher orbits than they’re especially comfortable with.
Once those little electrons gather their wits about them, and return to their lower, less stressful orbits, they emit a photon of light in celebration…red for Hydrogen (Hydrogen Alpha) or blue-green for Oxygen (Doubly ionized Oxygen, or OIII)
That light reveals the supernova’s shockwave to us in beautiful faint tendrils of light.