APOD: Three Galaxies and a Comet (2013 Oct 20)

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APOD: Three Galaxies and a Comet (2013 Oct 20)

Post by APOD Robot » Sun Oct 20, 2013 4:06 am

Image Three Galaxies and a Comet

Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. The scene also captures the broad tail and bright coma of Comet McNaught, the Great Comet of 2007. Currently, many sky enthusiasts are following the development of Comet ISON, a comet which might become the Great Comet of 2013.

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Re: APOD: Three Galaxies and a Comet (2013 Oct 20)

Post by Beyond » Sun Oct 20, 2013 4:11 am

Wow, i had no trouble at all spotting all 4 objects. It must be a very good night. :yes:
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Re: APOD: Three Galaxies and a Comet (2013 Oct 20)

Post by Ann » Sun Oct 20, 2013 5:24 am

Very nice picture! Let's see if Comet ISON ever turns out to be that bright. For a comet that was predicted to become perhaps as bright as the full Moon, ISON has been a no-show so far.

For me as a color commentator, the picture is interesting. The tails of comets are generally white, if they are dust tails, and blue, if they are ion tails. Cometary comas are almost always green. The tail of Comet McNaught seems very faintly bluish, but washed out. The coma appears to be quite yellow! Perhaps the comet was a "dusk object", so that it was seen low in the sky and therefore quite reddened.

The colors of the three galaxies in today's APOD are also interesting. The bluest-looking object is the Small Magellanic Cloud. I think that is correct. The U-B and B-V indexes of the SMC are -0.200 and 0.450, respectively. Both values are blue. Also the far infrared magnitude of the SMC is comparatively faint, 0.7 magnitudes fainter than the B magnitude, which suggests low levels of dust in this galaxy. The SMC is made up of generally metal-poor and therefore somewhat bluish stars, and it lacks any sort of concentration of old yellow stars. It does have star formation and young blue stars.

My software contains no information on the Large Magellanic Cloud as a galaxy. :evil: Nevertheless, unlike the Small Cloud, the Large Cloud has a clear inner structure and an obvious organization of its stellar populations. The LMC has a bar, which is older and yellower than the rest of the galaxy. AURA/NOAO/NSF has taken a picture of the LMC where the bar of the galaxy looks unrealistically yellow. But other pictures too show that the bar of the LMC is yellower, or at least less blue, than the rest of the galaxy.

The LMC is seen to contain one intensely blue dot. What is this dot? Well, R136 of course, the huge cluster powering the Tarantula Nebula! But then where is the red light of the Tarantula? We don't see it because no Ha filter was used to produce this image. Bear in mind that the blue stars of R136 produce very much more light than the red nebula, so the filters or film used for this picture detected the stars but not the nebula. Compare with this image, which also shows the Tarantula region as very bright and blue.

Finally there is our own galaxy. While the Milky Way has a respectable rate of star formation, it also has a large population of old yellow stars and a thick central dust lane. All in all, it is clear that the overall color of the Milky Way is much yellower than the color of the Magellanic Clouds.

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Re: APOD: Three Galaxies and a Comet (2013 Oct 20)

Post by BDanielMayfield » Sun Oct 20, 2013 5:01 pm

Nice commentary Ann.

This image also shows how patchy the mostly central dust lane inside the Milky Ways' disk is.
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Re: APOD: Three Galaxies and a Comet (2013 Oct 20)

Post by Chris Peterson » Sun Oct 20, 2013 5:19 pm

Ann wrote:Cometary comas are almost always green.
That's not really the case. Cometary comas are composed of a combination of dust and diffusing gases. Two of these gases, cyanogen (CN)2 and diatomic carbon C2 emit green light when ionized (typically by the Sun's UV output). But that ionized glow is quite faint, and we only see it when the nucleus isn't producing much dust. Otherwise, the reflected light from the dust dominates, and we just see the color of the Sun- white from space, slightly yellow when seen through the atmosphere. The dust component of the coma increases gradually as the comet gets closer to the Sun. For most comets, we see the characteristic green coma when they are still quite far from the Sun, and that coma becomes white nearer perihelion. Virtually all naked eye comets show a whitish coma- even Hyakutake, an extremely blue comet, had a coma that displayed little color to the naked eye.

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Re: APOD: Three Galaxies and a Comet (2013 Oct 20)

Post by BillBixby » Sun Oct 20, 2013 5:59 pm

My first impression was wow, what a beautiful picture. After ogling it for awhile I wanted to know what the posters had to say and was surprised to find only 3 posts, counting Otto. Then, i remembered today is Sunday and most likely a rerun from the past. It was, though I don't think I was aware of APOD in 2007, so it was new to me :) . So, i checked out the historic discussion (and posted a link, below). Only eight posts, counting Otto, but a wealth of information. Upon my return to the 'here', five posts were up.

Thank you, all for sharing your knowledge with your fellow star-gazers.



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Re: APOD: Three Galaxies and a Comet (2013 Oct 20)

Post by Boomer12k » Sun Oct 20, 2013 7:30 pm

"Four Galaxies".....the Galaxy you DON'T see......The Galaxy of the MIND.....

Sweet shot.....very nice....

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