Yet another Comet Holmes Photo
Published on Oct 29, 2007 at 5:37 pm.
11 Comments.
Filed under astrophotography, comets.
One of my readers, Don Spain, send me this image that he took of the comet. I am posting it with his permission. The object is 17P/Holmes. The photo was taken through a 152mm achromat with an IR cutoff filter.
In the photo, you see what look like two bright nuclei. In fact, these created quite a stir for a while. But, if you look at other photos of the comet taken a few hours before or after this one (and several others about this time), then you will see that one nucleus moves with the comet, but the other one stays in position with the background stars. It is a star shining through the coma.
However, the excitement about two nuclei is that we still are not exactly sure what happened to the comet to make it flare up so bright. Comets are known to sometimes break apart. When they do, they often release vast quantities of dust, causing them to brighten quite a bit. Had Comet Holmes done that, then it would explain a lot. But, if it did fragment, the fragments are still too close together to see as separate. The extra dot in the photo above is likely a star.
Most likely, part of the comet collapsed. Some comets are believed to be quite porous, so a cavity may form inside the nucleus. When the cavity collapses, it could quite possibly release an enormous amount of material as this has done. Exposing fresh material to sunlight allows for more sublimation of the cometary material, and thus yet more dust debris being shed. Alternately, something could have collided with the comet, excavating a crater, much like Deep Impact did with Comet Tempel 1 (9P/Tempel). While the impact hypothesis is fun to think about, I think that the prevailing idea is some sort of collapse. Whether or not that caused a piece of the comet’s nucleus to separate, I can’t say.
But, another point here is that the background star is shining through the coma. That tells you just how thin the coma really is. I have heard it said before that a comet is the closest thing that you can have to nothing and have it still be something (Actually, I think that the interstellar medium, or intergalactic medium, would qualify more!). The nucleus of a comet is only a few miles across. However, the coma can be tens of thousands of miles across (or more). The coma is composed of material released by the nucleus. Naturally, only a very tiny portion of the nucleus is released, yet that material is spread over a huge volume. So, you can see that, despite appearances, there really isn’t much to the coma of a comet. The tail is even thinner! So, the large and bright fuzz ball in the sky that is Comet Holmes is really mostly empty space. It seems bright because it is scattering a lot of sunlight. So, it is pretty easy for stars to shine through the comet.
And, it is bright. This comet still amazes me, as I have seen it from urban skies with both heavy light pollution and moonlight! For an old and venerable comet such as this, one that has been moving around the Sun for over a century without any major activity, that is pretty much unheard of. Of course, I covered comets in my astronomy class earlier this semester. I pointed out to students that a few years ago we had a model for comets that was generally accepted. Now, however, we’ve seen several comets up close, and we realize that the commonly accepted “dirty snowball” model is a bit simplistic. Each one of the comets that we’ve seen up close has been different from the others. Of course, that is also largely true of the asteroids that we’ve seen up close, too. Apparently the small bodies of the Solar System have a lot more variety to offer than had been suspected for so long.
Of greater interest to me is the structure in the apparent disk. There seems to be a darker region somewhat inside the edge of the disk. This is what you would expect if the outer disk were a sphere or a tube aligned along our line of sight. The edges of a sphere would look this way because we are seeing more material there. The same is true of a tube pointed towards us or away from us. I would suspect a tube pointing away from us, blown back by the solar wind and light pressure from the Sun. Of course, I am a stellar astronomer not a comet specialist, so I could be totally wrong on that.
So, I am eagerly awaiting news from the people who actually study comets. In the mean time, I am having tons of fun watching this one. It continues to be bright, but the light is now a bit more spread out, making the surface brightness a bit less. If you’ve got fairly sharp eyes, then you should clearly see a fuzzy cotton ball like patch where the comet should be in the sky. It is great in binoculars or low power telescopes, so if you are not an astronomer, you can still go see the comet without lots of equipment. After all, what got people so excited about this comet was that it suddenly, in less than a day, went from totally insignificant to being a naked eye object. So, go take a look!
-Astroprof
Image courtesy of Don Spain.
Martin Maguire on October 30, 2007 at 1:29 am: 1
I just came in after examining the coma closely & then found your website. I also see a darker region inside the edge of the disk. In fact the outer edge appears as a ring with a rather distinct outer boundary around approximately 300 degrees of the circumference. The more diffuse 60 degrees of the outer boundary is on the same side as the “open end” of the hyperbola of the inner coma. The inner boundary of the outer ring is more diffuse than its outer boundary.
All of this points to one conclusion: I agree with the theory that we are looking up a cone that is pointing towards us; the tail pointing away from us, it’s matter being blown into a wake by the solar wind. This is the same phenomena we see from the smaller stars in M42 with their wakes being blown back from the stellar wind of the stars of the Trapezium, the outer edge of their wakes being distinct, the inner edge being diffuse. We also see the same phenomena from wind tunnel studies & computer models; even from the wakes of power boats! It all fits the theory. Moreover, the diffuse 60-degrees of the ring being on the same side as the open end of the inner coma’s hyperbola tends to indicate that we not seeing this comet fully “end-on.” The axis of the tail seems to be at a slight angle to us.
Tom’s Astronomy Blog » Blog Archive » More Comet 17P/Holmes on October 31, 2007 at 5:27 pm: 2
[…] Astroprof wrote a very good post about the come, be sure to go on over and have a look. […]
maynard on November 1, 2007 at 12:17 pm: 3
This is the most interesting comet I’ve seen in years. Every night brings a different element to the comet.
GEMACHESNEY on November 3, 2007 at 8:55 pm: 4
COULD IT BE THATN ITS HEADING FOR US ,THE CORONA,WHAT IS IT MADE OF!
Astroprof on November 4, 2007 at 12:31 pm: 5
The comet is actually moving away from us. The coma is composed of dust (mostly silicates) and gas (mostly hydrogen).
rikta on November 7, 2007 at 5:11 am: 6
It’s only today that I heard about the Comet Holmes. I want to know more of it especially in what direction it rises so that I could see observe it?
Astroprof on November 7, 2007 at 12:21 pm: 7
Comet Holmes rises in the NE sky in the evenings. I have a photo of it here and a finder chart for it here . You can also check out http://www.spaceweather.com or http://www.skyandtelescope.com for additional charts of where it is moving.
M on November 9, 2007 at 1:09 pm: 8
Is there any way for less educated folk to find this comet? Like, is it anywhere near Orion?
Astroprof on November 9, 2007 at 4:19 pm: 9
It is not near Orion. It is near Perseus. The best way is to look at a current sky chart (it moves around a bit) like the one at Sky and Telescope’s web site. But, the comet has been getting harder to see by someone not familiar with the sky since Nov. 1 or so. So, even better is to find a local planetarium or astronomy club that is having a public program to observe the comet.
Mark on December 9, 2007 at 7:28 pm: 10
Is Holmes “The New Star Of Bethlehem”
http://www.prophecies.org It appears to be so!
Jeff Wilkins on December 4, 2008 at 4:02 pm: 11
Very cool picture, interesting that with all of the stargazing going on that we could find a new strange case.