APOD Robot wrote:
When stars form, pandemonium reigns.
The resulting morass can be as beautiful as it is complex.
I'll say! This region is one of the most beautiful parts of the sky, at least photographically. O so I think anyway.
But while it is very beautiful indeed, it is also complex and hard to understand. Let's start with the bright red arc at two o'clock. How was that thing formed? Please note that the cluster "inside" the red arc is wrapped in a cocoon of blue light. That blue light strongly suggests a reflection nebula to me. But if this little cluster is sitting in a reflection nebula, then it can contain no O-star, since O-stars are found inside emission nebulae, not reflection nebulae. But if there is no O-star here, how can this cluster power that fantastic red arc???
Here is my take on it. Note the rather bright blue star to the lower left of the red arc. I believe that this star, HD 162951, is the central powerhouse of the Simeis 188 region. If you check out the M8 link
of today's APOD, you can see how the Simeis 188/ NGC 6559 region is "attached" to the Lagoon Nebula, but even so the nebulosity surrounding HD 162951 forms a center of its own.
What kind of a star is HD 162951? My software quotes sources which define this star as belonging alternatively to spectral classes O8, B0 and B6!! Apparently not too much is known about this star. I'd say, judging from the "visual appearance of the nebula", that B0 seems like a good guess. If HD 162951 belonged to spectral class O8, I think it would be "too powerful" and blow many of the rather delicate structures away. Also I think that the red nebulosity of Simeis 188 would be generally brighter. A spectral class of B6 is out of the question, at least as far as I'm concerned, since that would mean that the star would be unable to do any ionization at all.
So let's say, tentatively, that HD 162951 belongs to spectral class B0. The star is able to provide most of the faint red emission nebulosity of Simeis 188. But the star also produces a stellar wind that must be taken into account.
Now look at that red arc at two o'clock and the small cluster inside it. As I said, we can be sure that the small cluster contains no O star. We can also be sure that the cluster is very young. Note a blackish "pillar of dust" that seems to "drop down" into the cluster in today's APOD. Closer to the newborn stars the black pillar turns into a "black bow shock", blown by the stellar winds of the newborn stars.
So the stars in the small cluster are very young and still surrounded by much of their natal birth cloud. The stars also produce stellar winds, which blows the "black pillar of dust" into a "black bow shock".
Their stellar winds are also trying to blow away the dust that lies between them and HD 162951. Yes, but HD 162951 is clearly more powerful than the newborn stars in the cluster: it is brighter and hotter, and it blows harder. So when the newborn stars in the small cluster are blowing the dust in one direction, HD 162951 is blowing it in the opposite direction, so that the dust is being "slammed together" from two directions. The result is a bright arc, which is the most highly ionized part of the entire Simeis 188 region. Check out the M8 link
again, to see how the red arc stands out.
And when you are looking at that M8 link, note how the Lagoon Nebula itself is surrounded by red arcs, which where produced when the stellar winds from the many O-stars in the Lagoon slammed into the surrounding medium.
Check out today's APOD again. Note that HD 162951 has produced red arcs on the other side of itself, too, not just on the side that faces the small cluster. But on the other side the "surrounding medium" was not so thick, the collision between the stellar wind and the surrounding medium was not so powerful, and the resulting level of ionization was nowhere near as high.