Post
by Ann » Fri Jun 01, 2012 6:43 am
This is a beautiful portrait of a lovely region of star formation and nebulae in the sky.
Let's start by looking at the colors of the picture. The large Lagoon Nebula is strikingly pink in color. The filters used for this image is, apart from colorless luminance, Ha, OIII, R,G, and B. Ha emission is deeply red, but OIII emission is blue-green. The pink color of the Lagoon Nebula tells us that there is not only Ha emission here, but also substantial OIII emission, which proves that there is a lot of UV radiation inside the Lagoon. And no wonder: the cluster of stars that you can see inside the Lagoon Nebula, to the upper left of the brightest part of the nebula, contains many hot early B-type stars and probably one or two O-stars, too. Immediately to the upper left of the brightest patch of nebulosity are two stars, one of which is probably an O-type star, and one, 9 Sagittarii, which is definitely of spectral class O. The brightest patch of nebulosity itself also contains an O-type star. All in all, the Lagoon Nebula is a part of the sky which is the home of several O-stars and a huge amount of UV-radiation. Therefore the Lagoon Nebula itself is pink rather than deep red, because it emits both red Ha and blue-green OIII light. (It is possible that the OIII filter also detects Hβ light, which is almost the same color as the OIII emission, and which is definitely there all inside the Lagoon Nebula.)
But look at the large patch of red above and to the upper left of the Lagoon Nebula. Not only is it much fainter than the Lagoon nebula, but it is also redder and less pink. The reason must be that there is hydrogen gas here, but far less UV radiation than there is inside the Lagoon Nebula. The UV radiation is enough to ionize the hydrogen to emit a dull red glow, but the UV is insufficient to ionize the oxygen. Therefore this patch of faint nebulosity is red and not pink.
As the APOD Robot wrote, there is a dark dust lane that separates the Lagoon Nebula region from the NGC 6559 region region. Note, however, that there is another dust lane that binds these two regions of star formation together. This dust lane starts on the right side of the Lagoon Nebula in this picture, and then it extends upwards all the way to the NGC 6559 region. It is obvious that the Lagoon Nebula and the NGC 6559 region are connected, and they were almost certainly born out the same large dust cloud, although this original dust cloud was thicker in some places and thinner in other places.
Fascinatingly, the Trifid Nebula appears to be a remnant of another elongated large cloud, which has been a site of multiple star formation. But in the case of the Trifid and its predecessors, all the gas has been used up and all the nebulosity is gone, except the gas and the nebulae of the Trfid itself. Note how an elongated "tail" of stars appears to stretch between the Trifid Nebula and the cluster M21 to the upper right of the Trifid. Also note the three bright blue stars which are sitting, widely separated, on the right side of the Trifid. Certainly M21, the "tail" stretching between them, and the bright stars to the right of the Trifid were born out of the same gas cloud whose last remnant is now lit up by a single O-type star in the center of the Trifid.
Note on the lower left the general yellow color of the background stars. These stars belong to the bulge of the Milky Way, which is dominated by old yellow stars.
What a beautiful and fascinating image!
Ann
Color Commentator
A Sagittarius Triplet
