Auriga is a fascinating and beautiful constellation with a lot of clusters and nebulae. Today's APOD brings out several of these nebulae and a few of the clusters beautifully.
However, for me as a color freak, it is obvious that the blue color of IC 405 and the blue color of IC 410 means different things. In IC 405, the Flaming Star nebula, the blue color is ordinary reflection nebulosity, not really different from the blue reflection nebulosity of the Pleiades. But this kind of reflection nebulosity is rarely seen close to O-type stars. Why is that?
The reason is that the dust that does the reflection is dispersed or destroyed by the hot O-type stars. Stars, including O-type stars, are born out of clouds of gas and dust. The O-type stars are incredibly energetic right from the start. When they are first born, they are often still cocooned in thick gas clouds that are made to glow red, as is the case of the O-type stars of
M17. Here the stars haven't broken through their stellar cocoon, and we can only see the red nebulosity. Or else we may actually see the stars, but their intense ultraviolet light makes both the surrounding hydrogen and oxygen strongly ionized, so that they emit red and green light, as is the case with the
Trapezium Cluster. There is little blue reflection nebulosity to be seen here.
So why is O-type star AE Aurigae "swimming in blue-reflecting dust"" in the middle of the red Flaming Star nebula? It is because AE Aurigae is just passing through this cloud of gas and dust. AE Aurigae was not born here, so it never had to break out of a thick dusty cocoon in the middle of this red cloud. And precisely because AE Aurigae is just passing through, it hasn't had time to blow away the dust in its vicinity or "cook" the dust grains to a smaller size. It hasn't had time to blow a rarefied cavity where oxygen can be ionized and glow aqua-green. So just because AE Aurigae is such a temporary visitor, it is dressed in blue swirls of reflecting dust.
What about the blue light that can be seen in IC 410, the nebula to the right of the Flaming Star nebula in this image? Is this nebula also full of dust that is glowing blue from reflection? No, I find that quite unlikely. IC 410 is a very different nebula from IC 405. The hot stars were born here and have blown a cavity in the center of their birth cloud. There shouldn't be be much dust here to reflect the light of the hot blue stars inside. Of course it is still possible to "weight" the color balance in this kind of image to underscore the fact that the red IC 410 nebula is made to glow red by the blue stars inside.
I'm fascinated by "The Spider and The Fly" nebulae in this image. The Fly, in particular, looks very compact, bright and relatively small. I'm actually reminded of the Orion Nebula and what it would look like if it was very far away.
The cluster NGC 1907 at bottom center right is interesting. According to Wikipedia it is probably at least 500 million years old. You can actually tell from its appearance that it is moderately old, since most of the stars in it appear to be pretty much the same brightness, and the cluster doesn't stand out in a really remarkable way from its surroundings. So there are no extremely bright young stars here.
Finally, look at the group of stars located between the two conspicuous red nebulae here, IC 405 and IC 410.The group of stars are an asterism, which means that they don't belong together, but are a line-of-sight coincidence. The asterism is called "The Leaping Minnow". I like the name! And some of the stars are interesting. The bottom blue star in this image is an A-type giant, probably more than 3,000 times as bright as the Sun. The middle blue star is a "magnetic silicon star", classified as spectral class A0p (peculiar). It is the hottest and the bluest of all stars classified as A-type stars, with a temperature of about 17,000 Kelvin, typical of a B4 star.
Ann