Okay, professor. What we got today? Where are we going in space? So, I thought we'd have a look at this guy, which is Coldwell 4. That must be a Hubble image. It's so good. Nope. That's from my back garden. You took that last week. Very nice. Hot off the press. Cordwell 4. So, I don't know how many times we're going to do this, but do you want to give me a quick one-s sentence summary of the Cordwell catalog? Yeah. So reminder that this is a catalog that was put together by Patrick Moore. It's called Coldwell because M was already taken for the Messier catalog. So he wants it to be the Catalog. So he compiled this list of things which are

mostly not Messier objects, but that are nice to look at in the sky with a relatively small telescope. They're ordered somewhat unusually from kind of how far north they are down to how far south they are. So the low numbers are the ones that are very far north, which is why this is so good for my back garden because I can actually see the northern part of the sky really clearly. my house doesn't get in the way. The trees don't get in the way. Uh so it's an ideal one for me to look at. Okay. What is it? It looks kind of cloudy and fuzzy. It's one of these things called a reflection nebula. So nebula come in various different types. There are absorption nebula which are just where

light is being absorbed. In fact, there's a bit of an absorption. You can probably see there's kind of a dark patch up here where there are fewer stars. There's actually a lot of absorption around this thing. So there is an absorption nebula there as well. There are emission nebula which are where the gas has been excited to glow all by itself. And there were these things called reflection nebula which are just basically reprocessing light. So you can see there's a bright star in the middle here. And what you're mostly seeing here is light that's being kind of emitted by that star and then scattered back towards us. Professor, when I think of a nebula, the first thing I go to is something like the Crab

Nebula. It's kind of the debris from an explosion. Is that what this is? Has something exploded and this is the debris? or is this like stuff that's going to form a star or Yeah, this is very much more the beginning of the life rather than the end of the life. So, there is a cluster of stars that has formed here, but there's a load of gas and dust left over. And so, you're seeing the leftover bits and pieces from that process of that forming a cluster, which maybe one day will form some more stars. So, if there's a bunch of stars in there, what's kind of lighting things up? It feels like there's a source in there somewhere.

There is one particularly bright star here. That's mainly what you're seeing is that bright star is doing most of the illuminating there. What I wanted to talk about really about this thing, I mean other than saying how pretty it is, um is to talk about why it's blue. And so I thought I'd talk about why reflection nebula in generally are blue. It's sort of related to something else which we've never talked about which we should have done which is why the sky is blue. Why is the sky blue? So the sky is blue through a process called rad scattering. If you think about it, where you're seeing the sky, it's not where the sun is. So clearly, it's not light that's just come straight to us from the sun.

It's light that's come from the sun, ended up in a bit of the atmosphere, and then been scattered to us. And so we see it light up because it's something sunlight that's kind of been scattered in our general direction. And the process of that scattering was first figured out by Lordley. What's really happening is kind of at the molecular level. You've got some light coming along. There's a molecule there. Molecule is bunch of positive charges with electrons around it. Lights an electromagnetic wave. So it means it's got a varying electric field. As it passes that molecule, it jiggles the electrons around. It tugs them backwards

and forwards as the light goes past. So the sort of cloud of electrons in that molecule gets tugged left and right as the light goes past. Essentially that means that charge is being accelerated cuz it's moving first one way then the other way. And when charges are accelerated that produces electromagnetic radiation. It's one of the ways you can produce electromagnetic radiation. So you produce more light. And so basically the light comes in, it jiggles the electrons around which produces light and that light can go off in any direction. Some of it comes back towards us. So that's the kind of the physics of the scattering process. The reason why it's blue is because if

you've got high frequency light coming in, so bluer light, that jiggles the electrons backwards and forwards very quickly. That means they're being accelerated very quickly. And when you accelerate charge very quickly, you produce more intense light. If redder light comes along, that will also tug it backwards and forwards, but it does it more slowly, more gently, and that means it produces less light. That means that this scattering process preferentially produces blue light rather than red light because blue light is more readily scattered than red light. Is that Lord Riley up there? He was a president of the Royal Society.

It is indeed. Yes. We're in Ed Copeland's office. I gave that picture to Ed when he won the Raleigh Medal. Yeah. And that Yeah. So there we go. That was not planned. Uh anyway, so that's why the sky is blue. Yeah. And it's it's a very strong effect in that you know blue light is very much more strongly scattered than red light which is why the sky when there's no clouds around does appear a very deep shade of blue because really you're mostly just seeing blue light but sunsets being pink and red that's a whole other thing. No that's actually the same thing in that what's happening when the sun's setting is that this there's a lot of atmosphere between you and the sun and

this scattering process is working there. So which means that the light that was coming in your direction, some of it gets scattered away. And because this scattering process works more effectively on blue light than red light, more of the blue light's being scattered away, which means more of the red light gets through to you unaffected. So here's the story. What I've just told you about works in the limit where the thing that's doing the scattering is very small compared to the wavelength of the light. Okay? And that's what was able to figure out. You know, in the late 19th century, he did the mathematics to figure out what happens in that limit. In the case of an object like Cold War 4 here, the thing that's doing the scattering is this

stuff that astronomers refer to as dust, which is little grains of material, carbon and other sy material and all sorts of bits and pieces. And they're comparable in size to the wavelength of light. And that really complicates the physics of what's going on. And you can kind of understand it. And if you think about it, remember what I told you before is that well, what's going on is that the electrons are being tugged backwards and forwards as the light passes. If you've got a larger grain comparable to the wavelength of the light, that means that as the wave's passing, you know, some of the bits of it are being tugged one way and some are being tugged another way just because

they're in different bits of that wave. It's like the sum of all the tugs and the different right and but they're doing it in a kind of a coherent fashion in that you know some bits are being tugged one way and exactly one wavelength away other bits are being tugged the other way and so you know it's just a kind of a more complicated story is what's going on that was first figured out by a guy called Gustaf Mi German physicist at the beginning of the 20th century. So the math was more complicated. So it took a little bit longer to figure it out. This more kind of general thing where the object's bigger is a thing called MI scattering. The most important aspect of this is that you get the strongest

scattering when the wavelength is kind of comparable to the size of the object. So that means that you know if you've got lots of very big grains then you'll end up scattering red light effectively because there the wavelength is longer to match up to the size of it. If you've got lots of very little grains then blue light scattered more effectively. So the reason why this thing appears blue is because astrophysically almost always there are more small grains than big grains which means you end up seeing the object being blue. So it is still rally scattering because it is the electrons excited electrons giving us the light. In some sense scattering is a special case of mi scattering right in that it's

the limit where you've got very small grains. Mi scattering deals with much more general things where you can have the grains big smaller and in between. But in this case you know the grains really are big enough that you can't deal with it in that limit. So you have to do the whole me scattering analysis. Okay. Done. There you go. And why the sky is blue. But you've got more to say. I have more. Yes. I want to talk about So let's go back to looking at this image again. You can see that it's basically blue. But if you look really carefully, you can see there are bits of it that kind of redder in color. And so

one of the still to an extent unsolved problems in astrophysics is what's called extended red emission or er it tends to get called. Well, I've got a theory based on what you've already told me. there are patches that have bigger grains which indeed is one of the theories that people looked at and it doesn't seem to work but actually you know the there's no particular I mean you know there's no huge difference between these different bits so the question is why would something which starts out blue suddenly end you know where it's a single feature parts of it are blue parts of it are red so it seems likely

there's some other emission mechanism and there are things that will emit red light for example if you got little uh grains of amorphous carbon so kind of little uh chunks of dirt basically um they'll emit in the infrared and all the way through to the red part of the spectrum if you excite them just right. So there are ways that you can do it, but no one quite knew what the answer was. And it turns out that one of the important clues to figuring this out came from studying this object called Wolf. So if you look at this region just to the kind of lower left of the center in my picture here, they pointed one of the instrument on the Hubble Space Telescope towards that little bit. I

think it's a different way up in its orientation. And here's the emission from that gas and dust around it which is I have to say absolutely beautiful. It's kind of that beautiful kind of wispy slightly ethereal nature to it. But you can also see that there's kind of red edges to here. And so that's what they were studying. And what they were interested in is something that we've already talked a little bit about which is what kind of light is it that's exciting that emission in the first place? What light is it that's coming in that's then causing that red glow? And the analysis these guys did and I've got

their paper here. Here it is. It's called the excitation of extended red emission new constraints on its carrier from Hubble Space Telescope observations of NGC723. And NGC723 is another name for Coldwell 4, which I should also have said is also called the Iris Nebula cuz it looked a little bit like an iris, the iris flower, right? Yeah. This was the analysis that these people did looking at this emission to try and figure out where this extended red emission was coming from, what its cause was. And they use this rather clever method which is that they looked at they noticed that it was just this edge here. And they said, "Well, I know why you only see an edge there. It's because whatever the light is that's

causing it is only penetrating a certain distance into that cloud. Different uh wavelengths of light will penetrate to different depths. Generally speaking, you know, like for example, far ultraviolet won't get very far in, whereas the infrared will get a long way in. So from the depth it's penetrated to, we can actually figure out what kind of light it is that's causing that emission in the first place. And you can see it's quite a long paper, right? They went through pages and pages of analysis to figure it out. But basically, they were able to figure out the wavelength of light that does it. It's about 100 nanometers, which is kind of far ultraviolet radiation cuz it's actually

restricted to this very narrow layer, which tells you that it's the kind of radiation that doesn't penetrate very far, which is what this far ultraviolet radiation does. Where's the light come from though? It's come from a star. It's coming from Yeah. It's the same star that's illuminating everything else is emitting ultraviolet, far ultraviolet radiation. It's quite a hot star. And so that far ultraviolet radiation is kind of illuminating this layer doing something which is then causing it to emit red light. But then you hit a snag which is that you can figure out how much energy is coming in that far ultraviolet radiation and it's less than the amount of energy that's coming out

in that red light which immediately destroys the idea that it's just you know that radiation being reprocessed because somehow you're getting energy for free then right? there's actually more light coming out, more energy coming out than when it went in the first place, which kind of complicates the story. So the only way they can figure it out is okay, so what's happening is that far ultraviolet radiation is coming in and is creating something in that thin layer which then interacts with all the rest of the light that's coming in and produces the red stuff that we're seeing. And there is a candidate which they suggest which is there are these things called PH's, polycyclic aromatic hydrocarbons.

They're basically molecules that are made up of multiple rings of carbon. You have to talk to the chemist if you want to know more about them, I'm afraid, but basically they're these kind of multiple benzene rings linked together to produce these largest molecules. We know they exist in space because we can see their emission lines and things. So, they've been detected in space. If you take one of those and bombard it with this far ultraviolet radiation, you can ionize it. You can rip off some of its electrons. And it turns out that the key thing here is that there's enough energy in those far ultraviolet photonss to rip off two electrons. So you take one of these PHAH's and then you rip off two electrons and then it

just sits there as an ionized PH with, you know, missing two electrons. All the rest of the light coming in then interacts with these ionized PHEs and it turns out that produces red light more or less exactly as seen here. So there is at least now a plausible story if a rather long one as to what it is that's producing this extended red emission. It's due to ionized polycyclic aromatic hydrocarbons. So when we look at that huge cloud around Coldwell 4 as a whole, you kind of think of it as all being the same gas, but it's not. There are some patches that are rich in some material and there are other ones that are rich in another material and different patches have different personalities

to an extent. Although there are probably pH all the way through here, it's just remember that far ultraviolet radiation can only ionize the ones that are very close to the edge. So there we go. That's the story. Nice dimensional and comets like Hailbop often come in from very oblique angles. It was coming in from the south, crossing the plane of the ecliptic, going over the top as it rounded the corner near the sun, and then plunging back down