neufer wrote: ↑
Thu Aug 30, 2018 6:41 pm
BDanielMayfield wrote: ↑
Thu Aug 30, 2018 5:53 pm
No comments yet? How sad for the color red.
Ann finds the color red "intolerable!"
Eh. No, not intolerable in this context, particularly not when the color red is nicely contrasted with the color blue.
(Some shades of green
Anyway. I'm late to the party.
What I really loved about the NGC 6914 APOD is that the caption pointed out that the red background of the blue reflection nebulas is due to the enormous ionizing power of the nearby (well, relatively nearby) huge stellar association Cygnus OB2. According to the link provided by the caption, there are over 50 O-type stars in Cygnus OB2. I mean wowzers! That's a lot!
Compare that with Orion. The O-type stars I can think of there are Alnitak in the Belt, Iota Orionis in the Sword, Sigma Orionis A which ionizes the red background of the Horsehead Nebula, Theta 1C Orionis which does most of the ionizing of the Orion Nebula, and Lambda Orionis at the "top" of the Orion figure. That's five O-type stars. There are ten times more O-type stars in Cygnus OB2!
Anyway, I have long wondered why there is a red background behind NGC 6914. The blue color of NGC 6914 itself is far more straightforward. That is simple reflection nebulosity forming in a dusty environment around stars probably of spectral class B2 and later. These stars are not hot enough to ionize a red emission nebula, but they are bright and blue enough to light up background dust clouds which reflect the blue color of the stars our way. Easy, yes. But that much red emission nebulosity... we are talking a major ionizing source here! Huge amounts of energy! Probably some hot, hot stars! In fact, very many hot stars! And indeed...Cygnus OB2!
I found a brilliant annotated picture of the Gamma Cygni and NGC 6914 region, but unfortunately I can't post it here. It is too big, 753 KB. But I'll give you a link here
. In the picture by Michele Vietri, you can see Gamma Cygni (Sadr) at dead center, NGC 6910 (a cluster possibly associated with Cyg OB2) to the left of Sadr, and tiny little NGC 6914 even further to the left. And what about Cyg OB2 itself? We can't see it at all, because it is so heavily reddened. Even infrared photography can hardly see the stars, but Chandra's X-ray sensors pick them out.
So, all in all, that's a nice APOD and very interesting background information!