It is always great to see an RGB image of the central parts of NGC 7023 that brings out the blue color of it!
What I like best about today's APOD (apart from the the fact that it shows the blue color of the Iris Nebula) is that it dramatically demonstrates the dark dust right next to the flames of blue. Indeed, that is where stars are born; where there is a lot of concentrated dark dust. Often the dust is shaped like a long dark tube, with star formation at one end of it. NGC 6726/6727
and the Cocoon Nebula
are good examples. The Iris Nebula, too, glows blue at one end of a tube of brownish darkness.
The Iris Nebula. Photo: Tony Hallas.
APOD Robot wrote:
of the reflection nebula glow with a faint reddish photoluminesence as some dust grains effectively convert the star's invisible ultraviolet radiation to visible red light.
Yes, central filaments of the Iris Nebula are
reddish due to photoluminescence - ionization of atoms leading to the release of visible-light photons - but neither today's APOD nor the link provided by the caption shows the reddish color of the central filaments. Instead, the link takes us to a site discussing the red color of the solid dust particles in the northwestern part of NGC 7023. I can't believe that reddish dust (which may be Mars-colored
) would give central filaments of a bright blue reflection nebula a rose-colored tinge. Or, rather, it might be just possible. This Adam Block image shows Pleiades member Merope
and its two nebulas - one small bright one very near the star, and one large, curved, and partly somewhat reddish - and the large one almost certainly owes its reddish color to the color of the dust particles. Merope is not a very hot star as B-type stars go - it is spectral type B6 - and it is quite unable to ionize appreciable amounts of hydrogen.
As I said, today's APOD doesn't show the reddish central filaments, but they can be seen in the image of the Iris Nebula at right by tony Hallas. Brian Lula's APOD from 2001
also shows them. I believe that the color is probably caused by the ionization of hydrogen due to the ultraviolet light from the central star. The star, HD 200775, is of spectral class B2V. It is only just hot enough to ionize some hydrogen, which glows rose-colored. But its bright blue light is reflected by dust much farther from the star than the reddish filaments.