 Hi everyone, my name is Christian Reddy, and by now you've probably seen really cool animations and videos of common Ison swinging around the Sun. They're absolutely spectacular to watch, but they can be a little bit confusing if you're not used to looking at images taken from space-based observatories. So I thought this video might help us in understanding exactly what's going on in some of these movies, and, you know, take a look at just what the heck happened to Ison. So what we're looking at right now is an image of the Sun taken from the Solar and Heliospheric Observatory. It's a spacecraft known as SOHO that looks at the Sun's outer atmosphere. This region is known as the corona, and in order to view the corona, you have to first block out the Sun. Otherwise the Sun's light would simply wash all this out and you wouldn't be able to see all of this fine detail. So SOHO is equipped with an occulting disc, that's what you see here, and this is the arm of the occulting disc holding it into place. And there are actually two instruments on board SOHO, both looking at the Sun. This is the Wide Field Channel, known as Lasco Charlie 3, and I can bring up the Narrow Field Channel, known as Lasco Charlie 2, right in here. So this gives us a little more of a complete picture as to what's going on, both in the outer and the inner coronal region of the Sun. So when we run the movie, we'll just go ahead and start a movie. This begins at the beginning of the day of the 27th of November. This is the day before Thanksgiving and Ison makes its way around the Sun and then comes back out and it takes us all the way up until about 7 in the evening of the 30th of November. So we're basically walking all the way from Wednesday morning through Saturday evening of Comet Ison's Path Around the Sun. And it's very cool to look at, and it's worth just pausing here and just taking a quick look at what is going on with our comet as it makes its approach to the Sun. Well, the comet has a very well-defined central region. This is called the comet's nucleus. And it has two very well-defined tails. One is a gas tail. The other one is an ion tail. And the comet is so bright that it's actually saturating SOHO's detector and it's creating this artificial, what's called blooming effect. So the comet itself isn't fragmenting at this point or doesn't appear to be doing anything out of the ordinary except that it's just brightening up so much that it's just saturating the detector. So these horns are not real. They are just artifacts of the detector. So as Ison makes its closest approach, we'll go ahead and advance the movie. And you'll notice that by this point, Ison begins to change. It's no longer quite as bright. So by the time it goes from here, right from about this point to this point, Ison's starting to get a little bit dimmer. And that was a bit of a worrying sign because in a perfect world, the comet would stay bright and maybe even get a little bit more bright as it made its closest approach to the Sun. The closest point to the Sun is a location known as Perihelion by the way. In fact, I'll just go ahead and just inlay an image of the Sun. So there is the disc of the Sun and by the time Ison reaches right around here, it's at Perihelion. And as the comet gets closer to Perihelion, it's beginning to fade. So that was a bit of a worry there as Ison made its way through. So we'll just go ahead and advance our movie here a little bit further. And now we're going to enter the C2 detector and I'll go ahead and turn off the C3 detector and we'll take a closer look at what's going on. Notice that Ison is now at its brightest somewhere here in the middle of the tail and not here, whereas before it was brightest up at the front and a little bit dimmer toward the rear. That tells us that Ison is probably breaking up at this point. It's probably just gotten boiled off or maybe there's some ice in there that just got cooked away or steamed away by the Sun. And now the largest parts of the comet are now falling toward the back and some lighter elements are coming up in front. That would be a model that would help explain this elongated shape. So as Ison marches closer, now you can see it's getting quite a bit dimmer, quite a bit more spread out. And it doesn't seem that Ison is expected to be surviving at this point. Now as it turns out, Soho wasn't the only spacecraft looking at the Sun. There was another spacecraft that's also in a position relatively close to Earth known as the Solar Dynamics Observatory. So this is the SDO image of the Sun right here. As you can see, Ison is about to make perihelion. And the SDO team were also watching Ison make its approach. And they were curious to look at the Sun particularly in this ultraviolet filter. As you can see what's going on right here is that by this point, SDO has been shifted. It's shifted its pointing in an effort to maybe catch Ison passing through. And we're looking at an ultraviolet filter because this region is sensitive to ionized oxygen, which is something you'd expect to see from a comet. Because comets, you know, they're made of water and ice and there's a lot of hydrogen and oxygen and you'd expect that as the Sun strikes those H2O molecules, it breaks them up into their component atoms of hydrogen and oxygen and then proceeds to ionize the oxygen. So you'd hope to see an ion tail, if you will, of ionized oxygen. And yet that's not what's being shown in the SDO imagery. So that was a bit of a disappointment. This has been detected by SDO in the past in previous close encounter comets. But, you know, they don't always get them, right? So as Ison traveled through, SDO was repointed again. Now we are just about at perihelion, I believe. Or actually I think we're passing past perihelion. And then finally, yeah, here we go. For the departure sequence, SDO was repositioned yet again in an effort to catch Ison and it could not. When we turn this off and we start to zoom out a little bit more, we'll get back to our SOHO spacecraft and now we'll go ahead and continue to watch. And now we can begin to see something emerging in the SOHO imagery. Now the first thing we noticed is that it's a lot dimmer, right? So, yes, something survived, but it's not the Ison that we had just a few hours ago. It's a much fainter Ison and, you know, we can't really tell exactly what's going on. We can't see any individual pieces, but it's entirely possible that the comet, or what's left of it, may have fragmented or outright disintegrated. Now it appears to be getting a little bit brighter here and starting to look a little bit more comet-like-ish, if you will. So let's go ahead and turn on the C3 detector. And now the comet appears very bright. Well, I should explain that. You see, when you're looking here through the C2 detector, you're imaging a region that's fairly close to the sun. So, you know, everything in this region is inherently relatively bright because there's a lot of illumination coming from the sun, but everything in the outer regions are a little bit less luminous. And therefore the detector for C3 is very sensitive, and therefore everything's going to appear brighter. So the apparent brightness change of Ison is really an artifact of the detector that it happens to be in. So if we zoom back out, we can go ahead and continue to track the evolution of, well, of whatever this is, the former Ison. And it's still starting to look a little bit more comet-like. I mean, now we've got a very nice pronounced fan tail, but it's no longer nearly as linear or line-like as the tail that it once had. So that tells us that whatever's coming out from around the sun and departing is very different in nature than what arrived. So, as Ison continues to make its way out from the sun, you'll notice it's getting fainter and fainter and fainter until finally we arrive at about, let's see, back up a little bit here. And so finally we arrive, yeah, there we are, there we are. We're now looking at this at 200 hours. So this would be about 8 o'clock universal time on Saturday the 30th. So, yeah, Ison has changed, and it's not the first time something like this has happened. There have been several of these close approach comets known as sun grazer comets that have just completely fizzled out. And there have been others that have survived intact. And then there's Ison, which seems to be sort of kind of surviving, maybe. And at this point it's really anyone's guess. Maybe in a few days it will become visible once again in telescopes and maybe even bright enough to be binoculars. So will it become naked eye visible again? Who knows. But all I could say is that, you know, just think about the fact that this is a piece of the outer solar system that was formed four and a half, five billion years ago maybe. And all within a matter of hours it becomes a very different and possibly forever changed object. And quite possibly maybe never to return back into the solar system again. Who knows. But that's just a really exciting thing to think about something that was formed for, you know, billions of years ago that existed in more or less the same state and then in a very short time underwent a very massive substantial change. It's very cool to see the universe in action. So I hope you enjoyed this explanation and thank you for watching the video. My name is Christian Ready and you can follow me on Twitter at ChristianReady and I blog at ChristianReady.com. Thank you so much for watching. Bye-bye.