Starship Asterisk*

Comet McNaught Passes NGC 1245

Explanation: Of the many comets named for discoverer Robert McNaught, the one cataloged as C/2009 R1 is gracing dawn skies for northern hemisphere observers this month. Seen here on June 13th from southern New Mexico, this Comet McNaught's long ion tail sweeps across the telescopic field of view (a negative image is inset). Remarkably, the ion tail easily stretches past background star cluster NGC 1245 (upper left) in the constellation Perseus, about 1.5 degrees from the comet's lovely greenish head or coma. The coma also sports a short, stubby, dust tail. Of course, the comet and background stars move at different rates through planet Earth's skies. But a digital processing of many short exposures allowed frames of comet and stars to be separated, registered, and recombined in the final image. To see the comet separate from the background stars, just slide your cursor over the image. The recombined frames show off both the rich star field and faint details of the comet. Easy to spot in binoculars for now, McNaught will sink into the twilight along the eastern horizon in the coming days as it heads toward perihelion (closest approach to the Sun) on July 2.

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So what causes the short stubby tail to go in a different direction than the long normal tail?

Are the ion/dust tails swapped in the text?

I thought that the ion tail was basically straight and pointed in the direction away from the sun, from radiation pressure.
And I thought that the dust tail roughly traced where the comet had been, so it is along the orbit direction.

beyond wrote:So what causes the short stubby tail to go in a different direction than the long normal tail?

duetosymmetry wrote:Are the ion/dust tails swapped in the text?

I thought that the ion tail was basically straight and pointed in the direction away from the sun, from radiation pressure.
And I thought that the dust tail roughly traced where the comet had been, so it is along the orbit direction.

duetosymmetry basically answered your question, beyond.

My understanding is that the ion tail is the positively charged gas ions interacting with the solar wind and it points away from the sun and is basically the same color as the coma. The dust tail is basically the debris trail left behind in the comets wake and points back along the comets path. The dust tail of previous comets is the source of various meteor showers as the Earth passes through the debris fields the comets left behind.

I do not claim to be a comet expert, but the ion tail (which shows some bends and distortion) is subject to variations in the solar wind / magnetic field. The dust tail follows the comet's path and should be straight(ish) or curved depending on where the comet is along it's elliptical path. A different view of the comet can be seen at http://www.enchantedskies.net/CometTrails.jpg. That image clearly shows the direction of the comet against the moving star field. The dust trail is the shorter stubbier (greener) trail in this case, and is pretty much in line with the movement of the comet relative to the stars. The longer ion tail is at quite a different angle compared to the star trails. The ion tail should be slightly bluish and is (but only very slightly so). I do not know why the dust trail appears greenish, but it could have to do with our viewing angle.

rr

Badsocref wrote:

The ion tail should be slightly bluish and is (but only very slightly so). I do not know why the dust trail appears greenish, but it could have to do with our viewing angle.

Indeed, the color balance is off here. This is the first time ever that I have seen a dust tail look green. And the ion tail should be bluer. Could it be that bright blue is regarded as a somewhat "unfashionable" color these days, and the photographer thought that a muted blue color would look better? But if so, I have no idea where the green dust tail comes from.



Comet Hale Bopp with a white, very slightly yellowish dust tail and a blue ion tail. No picture of Hale Bopp ever showed a green comet coma, probably because the white dust near the coma was so extraordinarily bright.



Comet Hyakutake had no visible dust tail, just a long blue ion tail. Here it appears to pass through a cone of zodiacal light.
Comet Holmes was almost colorless and seemingly tail-less from our point of view. You can see a faint green halo around the comet's white coma, and a wisp of blue behind the comet which might be a disconnected ion tail.

Ann

Additional images of this comet can be seen on this thread: http://asterisk.apod.com/vie ... 10&start=0

And Sky and Telescope has a good article on the comet here (linked to from the APOD text, too): http://www.skyandtelescope.com/observin ... 77259.html

Interesting, Owlice. Several other color pictures of this comet do indeed show that it has a remarkably green coma, a greenish dust tail and a not strikingly blue ion tail. It would seem that it is this particular comet's colors that are "off", not the photographers'.

My apologies to Rich Richins!

Ann

Ann, this is from the Sky & Telescope article:

The colors are real. Gas molecules of cyanogen (CN) and diatomic carbon (C2) in a comet's coma fluoresce green in sunlight. Ions of carbon monoxide (CO+) and carbon dioxide (CO2+) in the ion tail fluoresce blue.

owlice wrote:Ann, this is from the Sky & Telescope article:

The colors are real. Gas molecules of cyanogen (CN) and diatomic carbon (C2) in a comet's coma fluoresce green in sunlight. Ions of carbon monoxide (CO+) and carbon dioxide (CO2+) in the ion tail fluoresce blue.

I believe this shows that the chemical composition of the comet has a lot to do with it's color.

Until today all i had ever seen were single tail pictures on comets that always pointed away from the Sun. I suppose that was because the technology was not as good as it is now or perhaps this comet has a different enough composition that two tails show up??

beyond wrote:Until today all i had ever seen were single tail pictures on comets that always pointed away from the Sun. I suppose that was because the technology was not as good as it is now or perhaps this comet has a different enough composition that two tails show up??

I think it can only be because you somehow missed seeing the many images showing comets with two tails. It is nothing to do with imaging technology- Hale-Bopp strikingly showed two tails to the naked eye, even from the middle of bright cities.

Ann wrote:Indeed, the color balance is off here. This is the first time ever that I have seen a dust tail look green. And the ion tail should be bluer.

It does not appear to be off by very much. Do you calibrate your monitor? On my monitors (two, both calibrated) the dust trail appears to be seen in front of the coma, which makes it appear green. You sometimes see this coloration in the dust trail very close to the coma. How blue the ion tail looks is purely a matter of saturation, which is not related to color accuracy. To the eye, the ion tail is very faint blue because of the limits of photopic vision.

That said, I believe this image was made with a color camera (a DSLR), and these cameras are utterly incapable of producing highly accurate color on most astronomical targets, including comets. So getting close like this with such a limiting camera shows the processing skills of the imager. To get the best color it is necessary to use filtered systems, but that is usually not practical with comets because of the short exposures required, and because of the movement between subexposures (unless you don't mind losing the stars).

I think duetosymmetry is right, the ion tail should be the short one and the dust tail should be the long, wispy one.

Chris -- wasn't Hale-Bopp the comet that some "cult" killed themselves for so they could leave the Earth and board the comet to - somewhere??
If so, i saw that one through binoculars and i did not see two tails.

APOD: 2007 Mar 31 - Hale-Bopp: The Great Comet of 1997
APOD: 2000 Dec 27 - The Dust and Ion Tails of Comet Hale Bopp
APOD: 1997 Mar 14 - Comet Hale-Bopp's Developing Tail

beyond wrote:Chris -- wasn't Hale-Bopp the comet that some "cult" killed themselves for so they could leave the Earth and board the comet to - somewhere??
If so, i saw that one through binoculars and i did not see two tails.

That was the Heaven's Gate cult. Real nutjobs, but they did clean up the gene pool a bit.

You must have been observing under really bad conditions. Even from Los Angeles I easily saw both the white dust trail and the blue ion trail, no need for binoculars.

http://apod.nasa.gov/apod/ap070331.html
Hale Bopp was one of the few comets that I have seen. I was still working at the time on the night shift. We were able to climb on the roof and got a good view. Awesome!

Ok, the one i saw was not Hale-Bopp, because binoculars were needed. I've only ever seen that one comet. I do not get to see meteor showers either. Its always cloudy when the good stuff arrives here in the north-east

Heaven's Gate -- Yeah, i had forgotten their name. Thats normal - i leave the nuts for the squirrels.
The comet i saw with binoculars was about a 60 degree angle faceing North and it was heading West or South-West. I'm in the N/E corner of connecticut.
I do not remember if there was more than one comet around that time period, but i do remember that the one i saw was at its closest to Earth, according to the T.V. guys and i still needed binoculars. Also i think it was right around sunset or slightly after.

Chris wrote:

Ann wrote:
Indeed, the color balance is off here. This is the first time ever that I have seen a dust tail look green. And the ion tail should be bluer.

It does not appear to be off by very much. Do you calibrate your monitor? On my monitors (two, both calibrated) the dust trail appears to be seen in front of the coma, which makes it appear green. You sometimes see this coloration in the dust trail very close to the coma. How blue the ion tail looks is purely a matter of saturation, which is not related to color accuracy. To the eye, the ion tail is very faint blue because of the limits of photopic vision.

To me, there's no doubt whatsoever that the color of this comet is different from all other comets I have seen. It has nothing to do with calibrating my monitor, Chris. I look at existing pictures of different comets and compare their color balance. I don't need to calibrate my monitor for that.

Take a look at the link that Owlice provided to other pictures of this comet. John Chumack has a color picture of Comet McNaught passing NGC 891, and here both tails appear to be the same blue-green color. The blue color of the ion tail is very pale. Steve Yerby has another picture of Comet McNaught passing NGC 891, and here the color of the tails are slightly different, but the ion tail is still a very pale blue color and the dust tail is noticably greenish. Dale Ireland's picture shows the ion tail to be very faintly blue and the dust tail to be green. Rogelio Bernal Andreo's picture shows the ion tail to be a pale greyish blue, and the dust tail to be green. A bright star in the picture looks noticably bluer than the comet's ion tail. In Anthony Ayiomamitis' picture the coma and dust tail are very green, but the ion tail isn't blue at all. Here I think that the color balance of the picture is slightly off. The only picture where the ion tail looks really definitely blue is Andrea Tamanti's, but here the dust tail is still green. And in every single one of the images, the comet coma is very strikingly green.

There can be no doubt about it. This is a green comet. Owlice quoted Sky & Telescope and pointed out that a comet's coma looks green because of gas molecules of cyanogen (CN) and diatomic carbon (C2), whereas the ion tail is blue from ions of carbon monoxide (CO+) and carbon dioxide (CO2+). I'm going to guess that this comet may be particularly rich in gas molecules of cyanogen and diatomic carbon, but that the tail may be rather poor in ions of carbon monoxide and carbon dioxide.

Ann

Well, I've just done a general comet search, and it appears that there have been several green comets before. One comet whose colors seem to have been almost exactly the same as Comet McNaught's is Comet Lulin. Comet Lulin has even been described as "green comet Lulin".

http://philosophyofscienceportal.blogsp ... lulin.html

My general impression of "green comets" is that they are not all that active. They don't have huge dust tails and not very blue ion tails. They are dominated by the green light of CN and C2 molecules in their comas.

Ann

Ann wrote:There can be no doubt about it. This is a green comet.

I don't see anything unusual about this comet's color. Most comets show the same coloration: green coma, blue ion trail, white dust trail. With brighter comets these features can be seen in color to the eye, either aided or telescopically. Variations in the coloration are common in images simply because of the impossibility of matching imaging methods to the response of the eye.

I actually seen this comet the other night, if i'm not mistaken. I say night, but it was during sunset. I tell you one thing, it doesnt stick around the horizon very long.


Paul.

Chris Peterson wrote:

Ann wrote:Indeed, the color balance is off here. This is the first time ever that I have seen a dust tail look green. And the ion tail should be bluer.

It does not appear to be off by very much. Do you calibrate your monitor? On my monitors (two, both calibrated) the dust trail appears to be seen in front of the coma, which makes it appear green. You sometimes see this coloration in the dust trail very close to the coma. How blue the ion tail looks is purely a matter of saturation, which is not related to color accuracy. To the eye, the ion tail is very faint blue because of the limits of photopic vision.

That said, I believe this image was made with a color camera (a DSLR), and these cameras are utterly incapable of producing highly accurate color on most astronomical targets, including comets. So getting close like this with such a limiting camera shows the processing skills of the imager. To get the best color it is necessary to use filtered systems, but that is usually not practical with comets because of the short exposures required, and because of the movement between subexposures (unless you don't mind losing the stars).

Thanks for the complement regarding the processing skills, but I wouldn't say that the camera is utterly incapable of producing accurate colors. The DSLR used to take the shot (and the monitor used to process the image) have both been calibrated. I did nothing to change any of the colors except to enhance the colors of the stars. Essentially the same system was used to image Comet Lulin last year, and the subtle hues of blues and greens came out quite nicely (Feb 25 APOD). The blue ion tail was simply less blue this time around. I don't disagree that a dedicated CCD and color wheels are a superior imaging set-up, but newer DSLRs do a pretty decent job with color and sensitivity.

rr