Ralph Paonessa wrote: ↑Tue Dec 01, 2020 6:26 pm
Beautiful picture. But I’m puzzled by this in the description : “Therefore, in this 18-hour exposure image, blue areas are hotter than red.”
I realize that blue stars are hotter than red ones. And O III emission is blue and so higher energy than Red H II. But does this tell us anything about the temperature of those regions?? It seems like it really tells us about the distribution of oxygen and hydrogen in these areas.
Ralph (curious amateur)
These mapped color images!!!
Here's the deal. In mapped color images, blue usually means ionized oxygen emission (of circa 501 nm). Green often means ionized hydrogen emission (of circa 656 nm), and sometimes red means ionized hydrogen emission. But often red means ionized sulfur emission (of circa 658 nm). (Sigh.)
Let's cut to the chase. It takes a lot more energy to ionize oxygen and make emit photons of 501 nm than it takes takes to ionize hydrogen and make it emit photons of 656 nm. In narrowband imagery, blue color typically means that a very energetic phenomenon has ionized oxygen atoms into OIII ions and made them emit their photons of 501 nm. But the reason why they have done this is that a very energetic source has affected them.
Take a look at these two pictures of emission nebulas IC 405 and IC 410. In optical light, IC 405 is a reddish-pink nebula that also contains some blue light that has been scattered from the hot star passing through this nebula and ionizing it. IC 410 is more brick red in color than IC 405 overall, because IC 410 is located much farther away from us, and is reddened by intervening dust.
In mapped color, IC 410 becomes blue, because it contains a cluster of hot stars that emit a lot of ultraviolet light, which in turn ionizes oxygen and makes it glow green. IC 405, by contrast, contains a lot less ultraviolet light, because it is ionized by a single passing star. Not much oxygen is being ionized there.
And that's the best I can do when it comes to explaining why mapped color looks the way it looks.