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APOD: A Sagittarius Triplet (2012 Jun 01)
Posted: Fri Jun 01, 2012 4:05 am
by APOD Robot
A Sagittarius Triplet
Explanation: These three bright nebulae are often featured in telescopic tours of the constellation
Sagittarius and the crowded starfields of the
central Milky Way. In fact, 18th century cosmic tourist
Charles Messier cataloged two of them; M8, the large nebula left of center, and colorful M20 on the right. The third,
NGC 6559, is above M8, separated from the larger nebula by a dark dust lane. All three are stellar nurseries about five thousand light-years or so distant. The expansive M8, over a hundred light-years across, is also known as the
Lagoon Nebula. M20's popular moniker is
the Trifid. Glowing hydrogen gas creates the dominant red color of the emission nebulae, with contrasting blue hues, most striking in the Trifid, due to dust
reflected starlight. This broad skyscape also includes one of Messier's open star clusters,
M21, just above and right of the Trifid.
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Re: APOD: A Sagittarius Triplet (2012 Jun 01)
Posted: Fri Jun 01, 2012 6:43 am
by Ann
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
Re: APOD: A Sagittarius Triplet (2012 Jun 01)
Posted: Fri Jun 01, 2012 12:29 pm
by saturno2
In this image there are of all.
Emission nebulaes ( red ), reflection nebulae ( blue ) and open globular cluster
( above and right Trifid)
Re: APOD: A Sagittarius Triplet (2012 Jun 01)
Posted: Fri Jun 01, 2012 2:54 pm
by ta152h0
Does the Mily Way galaxy have a catalog number ?
Re: APOD: A Sagittarius Triplet (2012 Jun 01)
Posted: Fri Jun 01, 2012 3:02 pm
by FloridaMike
Speaking of catalog numbers....
Astronomers, when you look at an image like this, what percentage of the stars visible here do you think have been cataloged? Roughly speaking of course.
Re: APOD: A Sagittarius Triplet (2012 Jun 01)
Posted: Fri Jun 01, 2012 3:11 pm
by Ann
ta152h0 wrote:Does the Mily Way galaxy have a catalog number ?
It doesn't. The closest you get is
M24, a large "star cloud" made up of mostly young stars. This cloud, the Small Sagittarius star cloud, "sits" in the thick Milky Way dust lane. The youth of its stars makes it different from other so called star clouds, such as the
Scutum star cloud and the
Large Sagittarius star cloud.
The Small Sagittarius star cloud also differs from the other two in that is has a Messier number.
Ann
Re: APOD: A Sagittarius Triplet (2012 Jun 01)
Posted: Fri Jun 01, 2012 3:59 pm
by Wiffle Snort
Ann wrote:ta152h0 wrote:Does the Mily Way galaxy have a catalog number ?
It doesn't.
I think we should give it one. I vote for "M00", then we could call it the "Double Naught Galaxy".
Re: APOD: A Sagittarius Triplet (2012 Jun 01)
Posted: Fri Jun 01, 2012 4:17 pm
by Ann
FloridaMike wrote:Speaking of catalog numbers....
Astronomers, when you look at an image like this, what percentage of the stars visible here do you think have been cataloged? Roughly speaking of course.
I'm not going to guess what percentage of the stars in this image have been cataloged, but I think it's worth mentioning that the Tycho-2 Catalogue probably catalogs the largest number of stars.
http://en.wikipedia.org/wiki/Tycho-2_Catalogue wrote:
The astrometric reference catalogue contain positions, proper motions, and two-color photometric data for the
2,539,913 of the brightest stars in the Milky Way, of which about 5000 are visible to the naked eye. Components of double stars with separations down to 0.8 arcseconds are included.
The catalog is 99% complete to V~11.0 and
90% complete to v~11.5.
I checked my software, and many of the tenth and in some cases even ninth magnitude stars that were inside the Lagoon Nebula didn't have Tycho numbers. But outside of the nebulosity, all eleventh magnitude stars that I looked up had Tycho numbers.
Ann