Starship Asterisk*

Chris Peterson wrote:

Ron-Astro Pharmacist wrote:This comet's nucleus seems unusually bright as I look through images of other comets. Any idea why some comets have bright comas and some have bright nuclei?

You can't see the nucleus of this comet- or of any active comet. All you are seeing is coma and tail. The nature of the outgassing and dust ejection determines the radial density profile of the coma.

I was suspecting it would be hard to see something that small to identify it as a nucleus. Especially since most have such low albedo. It is almost surprising to me Rosetta's and Philae's cameras were able to contrast the darkness of the surface of Comet 67P/Churyumov-Gerasimenko against the background darkness. Thanks for the insight on why comets appear so different. http://en.wikipedia.org/wiki/Comet_nucleus

Even a dark comet surface is bright compared to the total blackness of empty space, though. Also, for those two images you posted, you must keep in mind exposure times and magnification. The first one is likely a widefield film exposure of a comet from many years ago. The second looks more like either a fake comet or perhaps it is a telescopic view of the coma. Edit: Actually they could easily both be fake/illustrations. Either way, imaging method and processing matters a lot.

geckzilla wrote:Even a dark comet surface is bright compared to the total blackness of empty space, though. Also, for those two images you posted, you must keep in mind exposure times and magnification. The first one is likely a widefield film exposure of a comet from many years ago. The second looks more like either a fake comet or perhaps it is a telescopic view of the coma. Edit: Actually they could easily both be fake/illustrations. Either way, imaging method and processing matters a lot.

Thanks Geck!! The images just intended to illustrate my question. My New Years resolution is to be less confusing.

It does seem to me that the tail of (this) Comet Lovejoy is relatively faint. In the January image submissions thread there are several images of Comet Lovejoy, taken by different people, and in most of theses pictures the tail of the comet is rather faint and often rather non-blue. Consider this image by André van der Hoeven, for example. There is also this image by Albert Barr and this image by Juan Carlos Casado, as well as this image by John Chumak.

Ann

http://www.universetoday.com/115493/two ... -pictoris/
As this first was described back in 1987 this is probably old hat for many but I found it almost too-hard-to-believe. If I truly understood current technology I'd be flabbergasted. We are about to see what occurs on the surface of a comet as it interacts with the sun and now, I discover, we can detect exo-comets as they existed back the early solar system in another star system. explains exactly how I should feel. What would Charles Messier say about the prospects of this technology only 300 years after his exploits?
"No Comet?"
Another "Meyer" describes his time much more eloquently.
http://www.icq.eps.harvard.edu/meyer_icq29_3t6.pdf
Then again, maybe he would have said, "No Comment" to comets discovered around another sun? Age has some privileges.

Ann wrote:It does seem to me that the tail of (this) Comet Lovejoy is relatively faint.

Ion trails are always faint. It's rare for the ion trail to even be visible to the eye. Dust trails are what get bright, and we don't see much dust with this comet. So the apparent brightness of this tail comes down to imaging technique more than anything else.

(I easily saw the comet with image intensified binoculars a few nights ago. But even with that, no tail was visible.)

Chris Peterson wrote:

Ann wrote:It does seem to me that the tail of (this) Comet Lovejoy is relatively faint.

Ion trails are always faint. It's rare for the ion trail to even be visible to the eye. Dust trails are what get bright, and we don't see much dust with this comet. So the apparent brightness of this tail comes down to imaging technique more than anything else.

(I easily saw the comet with image intensified binoculars a few nights ago. But even with that, no tail was visible.)

Thanks for pointing out the faintness of the ion tail, Chris.

It still seems to me that this ion tail is strangely non-blue. Most images show it as gray. In André van der Hoeven's image, it was yellowish.

Ann

Anything so dim would be prone to showing very little color and also very easy to shift color slightly during processing. Check out this picture... you can see weird color variation in the tail because the exposures were so long between shots that the tail was flowing and changing during them, not because it's a real color variation.
http://www.astrostudio.at/2_Bright%20Co ... 23_12C.jpg