I shot this on the third night of GSSP 2010.
This is 1h 20m total exposure, four 20m subframes, shot with Cassie/350D. Scope was pointed West at the time.
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I shot this on the third night of GSSP 2010.
This is 1h 20m total exposure, four 20m subframes, shot with Cassie/350D. Scope was pointed West at the time.
So…talk to me about the colors
The Trifid Nebula is a mixture of “emission” nebula, “reflection” nebula, and dark dust lanes.
The “emission” portion is easiest to explain. The nebula is a cloud that is primarily composed of hydrogen. When hydrogen ions are excited (“singly ionized”), they glow at 656.3nm (the “Hydrogen-alpha” emission line), which human eyes see deep in the red part of the visible light spectrum. That’s what’s responsible for the “pink” color of most of the nebula. It turns out that doubly-ionized oxygen (OIII) is probably also present, since a purely H-alpha nebula would be much redder than this. OIII emits light on two strong emission lines near 501nm, right in the middle of the visible light spectrum (our eyes detect this as “green”, but also as a much brighter emission than H-alpha). The mix of “dim red” and “bright green” light makes the emission portion of the nebula more “pink” and less “blood red”.
What’s exciting the molecular cloud? Nebulae are the birthplaces of stars, and it’s the solar radiation from these new stars that makes their “mother” nebula glow. I haven’t checked, but those two really bright stars in the center of the pink area (which look like one long star because of my processing) are probably among those stars responsible for the pink glow.
The dark lanes are composed of dust, which blocks most visible light, so they look dark against the glowing nebula. If you take a look at my shot of B72, you can see what a dust lane looks like without an emission nebula behind it.
The Trifid Nebula is interesting, because there is also a nearby cloud of particles (perhaps dust) that is “front-lit”. It’s not being ionized by the stars, but their light is being reflected (and scattered) off of the cloud. For the same reason that the sky looks blue (blue light scatters across the sky), the “reflection” nebula also appears blue. The Pleiades, a gorgeous autumn cluster, is also associated with blue reflection nebulosity.
If you look closely at the stars in the photo, you can see that they come in different shades of yellow, orange, and blue. They are also somewhat smeared, because my telescope’s tracking was not as precise as I’d have like it; oh well. In any case, the star colors are based on the stars’ composition and temperature, a discussion of which is beyond the scope of this response. But it’s neat to see different star colors in the photos.
You might notice that the color difference between the stars is much more pronounced in the images shot through Cassie (a “reflecting” telescope that uses a mirror to focus light) than in the ones shot through Pumpkin (a “refracting” telescope that uses a lens to focus light) because mirrors do an inherently better job at bringing all colors of light to focus at the same point than lenses do. So (generally speaking) you get more brilliant colors in a reflector than in a refractor. Getting a reflector to produce the wide fields of view that I get from Pumpkin’s short (400mm) focal length is nearly impossible, however, so there’s a tradeoff between being able to encompass a large object (like IC1805) in a single frame, vs higher magnification (and tighter tracking tolerances).