Saw this gorgeous photo of Comet ISON on Phil Plait’s blog (though he links to the wrong picture there) and saw that beautiful but bizarre tail structure. I’m trying to figure out what’s going on with it and why, what with the concentric tail structure and the apparent blooming of the tail some distance away from the head.
As best as I can tell, it looks like the tail consists of a solid cylindrical core surrounded by two cylindrical shells (the flanking tails look like they’re limb-brightened) in a larger diffuse tail. As for the spreading, it looks like the tail stays compact for the length of the coma, spreads out for about five coma-lengths, and then spreads at a slower rate after that. No point in speculating about what might be causing these features if I’m wrong about what the features are, though!
So this might be premature and we need more data before we know what’s going on with this amazing comet, but if they’re familiar features I’d love to know what’s going on.
Remember, the tail consists of dust and gas that are being ejected from a rotating core. That leads to an uneven distribution of debris in the tail. You are seeing a snapshot in time - if you were to look at a motion picture of the comet, the tail would evolve somewhat like water sprinkler.
ETA: also, the dust and gas tails are affected by light pressure differently. The gas tail always points normal to the sun, and the dust tail follows the path of the comet.
Hubble found a new unknown object in August that has science-types confused.
I suppose it’s possible that Ison has a similar structure, outgassing from 2 or 3 jets that are being pushed back/molded by the solar wind.
Although I’m on a high point in my area, I think the treeline to my east is just too close/high, since I’ve not been able to pick it out in the mornings just yet, and I’m just too lazy or tired to schlep a telescope a mile down the road to a hilltop with a clear view that way.
For those of you who don’t know where to look, or how to find things, please download Stellarium. It’s free, it’s easy, and a lot of fun.
You can track any comet, satellite, planet, the ISS, and more. Show it to the kids and let them have some fun. Clear winter skies are coming, and even binoculars on a tripod can show them wonderful things.
That makes sense. So if I’m understanding correctly, there are several gas jets at different points, each making a separate tail, and as the comet rotates the tails that come from points further from the axis of rotation sweep out a bigger spiral than those closer. In three dimensions, they look like spirals on the surface of a cone, rather than a sheet covering the surface of the cone. Though in that case, I’d expect to see regular variation in the brightness of the tails.
Or else it’s rotating perpendicular to the the tail, and the thick tail in the middle is from venting more-or-less directly from the equator, and the fainter lines surrounding it actually are individual tails. That would explain why there seems to be a faint third line on the top and only two lines on the bottom, but then there’s the apparent limb-brightening, because they definitely get brighter gradually as you go away from the center and suddenly get darker again.
So I guess it would help to know how fast it’s rotating, and around what axis.
Regarding the dust versus gas tails, I’m not sure how relevant that is to that particular picture. Earlier photos show it with the classical two-tailed comet appearance, which I don’t see in the photo in the OP. It may have been taken with a filter that hides one of the tails so I’ll try to get more information about the photo itself.
Comets don’t eject significant debris until they get close enough to the Sun to be warmed to the point that volatile elements start to boil. So, they could have been orbiting in space for ages, and not lost significant mass.
Comets that are periodic (like Halleys’) do lose mass on each orbit, and will eventually disappear.
That’s an awesome image of the comet. I don’t remember seeing one with so much detail in the tails.
Also, Comet ISON has brightened dramatically in the last few days. They suspect it’s starting to become unstable and is perhaps in the beginning throes of breaking up due to its close proximity to the Sun. That could lead to multiple jets of material.
You raise a good point. If comets had been in their current orbits since the beginning of the Solar System, they would have all evaporated away by now. Even long period comets with orbital periods of thousands of years.
So that tells us that they haven’t been in their current orbits that long. Instead, they are thought to have resided in one of two reservoirs for most of the age of the Solar System. The one for long period comets, such as Comet ISON, is the Oort Cloud. Passing stars perturb objects in the Oort Cloud and send them into the inner system. The Oort Cloud is so far away that it takes them thousands, if not millions, of years to fall that far.
Comet ISON, by the way, is on its first pass through the inner system. Usually first time comets are a disappointment. They usually have a thin outer layer of ice which evaporates when the comet is fairly far from the sun. This makes them brighter than other comets at that distance. There’s a general rule about how much a comet will brighten and applying it to first timers based on these distant observations gives predictions of very bright comets.
But since the layer of ice is thin, it all evaporates away quickly and the comet turns out to be a dud, at least compared to what its predictions were. I thought Comet Kohoutec was a first timer, but the Wiki page says differently. It was a major dud, though.
Apologies if this is getting boring but another thought has occurred to me.
Why do comets have tails at all?
There is no atmosphere in space to drag off the debris and even if some stuff did detach it’s self it would be traveling at the same speed so would not form a tail.
So Phil posted an update with new photos but a quick skim doesn’t say much about what’s giving Comet ISON that cool tail.
The animation on this page is very helpful, and it lines up with what beowulff said from the start. If the axis of rotation is perpendicular to both the line of the tail and our line of sight, then jets shooting towards us would appear to diverge, and as they rotate away from up they’d appear to converge, which is exactly what that animation shows. I guess the individual jets become more prominent as they heat up and become more powerful. Amazing how fast that thing moves against the background stars in only 70 minutes!
The other picture of interest here doesn’t show much of the layering structure, but it still has that blooming I mentioned in the OP. If you draw a straight line from the edge of the visible structure to the center of the coma, you’ll see that the central tail has a concave shape. The fainter, diffuse rays seem perfectly straight though. Is there some process keeping it confined until it reaches a certain distance, or is this like the “cones” and also an artifact of the rotation, where the apparent width of the core of the tail varies with the rotation of the comet, and both these pictures happen to have been taken when the fresh part of the tail was narrower than the older parts?
