The Cat's Eye Nebula Reveals Stunning Beauty and Cosmic Mysteries

We are at Caldwell 6, which means we're kind of working our way down from the far northern sky cuz when if you remember the Caldwell catalog is arranged in kind of declination order. So things closest to the North Pole are there at the start and we're kind of slowly working our way south. So we're still a long way north. It is a beautiful object. Um I actually have this is a pretty recent picture. Aren't they beautiful by definition? That's why they were picked. Yeah, but you know, beauty is on a scale, right? This is this Somebody once asked me what the most beautiful astro object in the whole sky was and this was the object I picked.

So this is in some sense it's not my favorite object cuz it's not a galaxy. So it's obviously it's not my favorite object, but it's you know, it is beautiful. It's stunningly spectacularly pretty. All right, you've built it up now. I might have oversold it a bit there. All right. So here's a This is a actually a very recent picture of it. Was released earlier this year. Uh taken by the Euclid satellite, which is a satellite which is kind of doing sort of surveys of the sky mostly looking at actually it's mostly looking galaxies and things like this. This is a big bright nearby galaxy. Mostly Euclid

is looking at little faint distant galaxies, but uh along the way because it's surveying large swaths of the sky, it also gets for free things like Caldwell 6. This is not at its best, right? Because actually the really beautiful bit is what's going on in the middle. So but I wanted to start with this cuz it's got it's big, right? It's actually got this sort of big surrounding area. But there's a very famous picture of the inner part of it. Let me show you the very famous picture of the inner part of it. It gets better than that? It gets way better than that. It's It's called the Cat's Eye Nebula. Cuz supposedly this bit in the middle looks a bit like a cat's eye um when it was first seen sort of only by eye, you

could just about see there was a kind of a black slit in the middle and a bright bit around it. So it looks a little bit like a cat's eye. But as you can see there's a whole load of stuff going on there. It's amazing that it looks good wide and it looks good zoomed in. It's a yeah, double threat. And it's kind of amazing in that it's actually different, right? In that you know, what's going on out here looks almost totally disconnected from what's going on in there, right? That actually has a completely different symmetry to it, a different shape to it. So, clearly this ain't thing has a kind of a long and complicated story that goes with it. Well, to be fair, it's a planetary nebula, which means

it's kind of the end of the life of a an intermediate mass star. So, a star a bit more massive than the sun. And that end of life is incredibly complicated, and we don't actually understand it all because things are happening on sort of relatively short time scales for stars and quite complicated clearly, you know, when we most of them when we're modeling stars, we tend to assume that they're spheres and everything's spherically symmetric and we can model all that, but clearly this is not spherically symmetric, right? So, actually there's clearly a lot more going on here. And let me pick on one bit just to kind of talk about one of the bits of it, which is kind of it's so I showed you the

big view and I just showed you the small view, but there's actually an intermediate view, which you can actually see in this image, too. There's this whole series of concentric rings around this thing. They're actually shells and you're kind of because you're seeing the shell kind of edge on, you're you're you kind of see it as rings on the sky. So, this whole series of very evenly spaced shells around it. And what this is telling you, so basically the stuff you're seeing furthest out has been traveling the longest, right? So, the stuff you're seeing furthest out is the thing that happened longest ago. So, what this is saying is that there's a whole series of events have all happened and they must have happened kind of equi- equi-spaced

in time to create that series of shells. And in fact, we can we know quite a lot about this object. For example, the Hubble Space Telescope has been looking at it for long enough that they've actually seen the shells move now. And because we can see them move, we can actually figure out how fast they're traveling. And from that we from how far apart they are, we can figure out how often they went off, right? How often one of these shells was puffed out. And it works out as about once every 1,500 years. One of these shells was kind of puffed off about once every 1,000 or 2,000 years. So, like tree rings tell us years, those tree rings are telling us 1,500 years.

Exactly. Each of them is one yeah, one growing season, if you like. And it turns out 1,500 years is kind of an awkward number because people have spent a long time trying to figure out, okay, so what is it that's creating this kind of clock that's making this happen? Usually things happen either they take billions of years in astronomy or they happen in a fraction of a second. 1,500 years is kind of a weird human type time scale. So, there are various things that people have thought about. For example, we know that stars undergo radial pulsations. And essentially all that's happening there is the star, you know, it starts out quite compact. It because it's compact, it's quite opaque. That means

the heat builds up. As the heat builds up, it then expands which cools it down and makes it less opaque which means it contracts back down again. So, there are various kinds of stars that undergo these kind of pulsations. And so, people have thought about that, but the time scales for those pulsations are typically around a year. You know, months to years kind of time scales. So, although it's sort of physically would create that kind of effect, you could imagine very strong pulsations might throw material off into space, it's the wrong time scale. It's too short. So, then people thought about, okay, so maybe it's something weird going on in the nuclear processes. This is a very late stage in of its evolution. There's

all sorts of complicated nuclear burning going on. There's burning going on in very intentionally in very hot shells of material within the star. And because that's very sensitive to temperature, sometimes you have, you know, very high rates of luminosity which you again cause a thing to expand a bit which cools it down a bit which slows down the process. So, you could imagine actually not just with the atmosphere of the star, maybe it's something to do with those nuclear processes. But it turns out the time scales for that is like tens of thousands to hundreds of thousands of years. So, it's the time scale we're at is completely wrong for any of the those kind of processes. The short answer is we still don't know like really what

causes them. But there is a theory. And the theory is, so around the star, you have gas that's kind of being coming off from the star, kind of coming in a very strong stellar wind, so it's being blown off. Its density builds up and builds up. Eventually it gets dense enough that you start forming grains in it, little bits of dust, little bits of soot. Okay? And those pieces of soot are very sensitive to radiation coming from the star. They get pushed by the radiation coming from the star. So, that soot then gets pushed out. And it actually drags the gas with it. There's kind of viscous effects that makes it drag the gas along with it. And so, because it then starts to expand, that will then reduce the density of the gas. And eventually, it'll reduce to the

point where you're no longer making that soot and dust and all that stuff. So, uh suddenly then the process goes away. So, that creates that kind of cycle that it will kind of the density will drop as it gets blown away. The soot stops forming. The gas then starts building up again. And then the whole cycle repeats itself. And so, people have done the calculations for this and said, "Okay, so what's the time scale on which you'd expect that kind of periodicity to occur?" And the answer is about 1,000 to 2,000 years. So, although it's not a particular, you know, it's not a very mainstream process. It's not particularly central to any physics that's going on. When people started

thinking about it, at least the time scales fit very well with what we see. So, it could just be that kind of cycle of soot being produced around the star. Would that have been when the star was alive and healthy and normal? Or is it relating to a star that's going through its death throes? It is dying. And because this is, you know, the stuff So, the stuff we saw in the first picture way out here, that was when it was still a star. Right? And that was just it throwing off its outer layers. As it got towards the end of its

life, it was starting to throw stuff off. But remember, as you go inwards, you're kind of looking closer and closer to the present day. So, that by the time you're in here, this really is pretty late on in the lifetime of this star. And it's probably looks like because these rings look like they're still being produced, it looks like it's still going on now. Is the star still alive? There is So, there is a star in the middle. It's not quite completely dead yet. There is still nuclear burning. It's a thing called a Wolf-Rayet star, which is kind of It's It's a star which has blown off all its outer envelope,

all its outer layers. So, you're looking basically at the naked core of a star. So, in that sense, it's still there. It's still hanging in there just about as a star. It's not kind of died off as a white dwarf yet. But it really is at that final stage of its life. I have I did is one last picture, which I kind of really rather like. It's a It's rather older picture taken from the ground. Um but I liked it because it kind of shows it's shows the whole structure of everything from this weird kind of structure that there was the outer envelope of the star getting blown off in its late stage of its life, all the way to the very complicated

structure. And actually the structure in the middle is really complicated. It's clear that there's kind of jets associated with it. There's stuff getting thrown out in jet-like uh outflows of material, but the jet seems to be precessing in that there's sometimes the jets are pointing that way and then a bit further in they're not pointing in the same direction. So again, it looks like the jet for some reason is changing its direction as time goes on. Perhaps there's a binary star in the middle, no one really knows. So it's it's incredibly complicated object, lots going on in it, lots we don't really understand, but that's part of what makes it so beautiful, really. held together by its own gravity.

Secondly, it formed quite a long way out in the Milky Way. That means that it didn't keep running into other things as it was kind of orbiting around the Milky Way. You know, if you're formed relatively close to the center of the Milky Way, you're going to keep the spiral arms and there are other clusters and everything's a bit crowded.