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What do we got? Uh, Coldwell 3. Three. We're on the third one already. Cracking our way through them. Yeah. Two second reminder. What's the Coldwell catalog? Coldwell catalog is a catalog put together by Patrick Moore of objects that you might like to look at in the night sky that mostly aren't in the Messier catalog. Cold War 3. Actually, visually, it's not the most inspiring. This picture of it taken from my back garden. So, is it uninspiring because it's uninspiring or because you took a dodgy picture?
It's uninspiring just because it's quite faint. It's not a small galaxy. It's about maybe half the diameter of the moon, something like that. So, you know, it's quite large angular size, but it's faint. It's got what's called a low surface brightness. So, there aren't that many stars scattered over that large area, relatively speaking for a galaxy. It's in the general direction of one of the most impressive galaxies which is probably why it sort of pales into insignificance in that it's not that far in the sky from Messier 81 which is nice impressive spiral galaxy and it's probably part of the same group of galaxies. They're sort of gravitationally bound together. So here's a view of the sort of general
area of the sky. So our galaxy is up here and the main part Messier 81 and Messier 82 and a few other more dramatic galaxies are down there. So you can see it's kind of, you know, maybe it's connected, maybe it's not, but it's sort of, you know, roughly in the same bit of the sky. One nice thing about a low surface brightness galaxy is because the stars are relatively spread out, if you have a really good telescope, even though this is another galaxy, you can actually see the individual stars. So one of the things they did with the Hubble Space Telescope was look at a little bit of this galaxy.
Oh, now it's looking impressive. Well, here's the picture of the galaxy again. That's the whole galaxy that I was showing you before. Because it's big, the Hubble Space Telescope can't actually see it all. So, they picked out a little bit of it. And when you look at the little bit of it, they can actually start picking out the individual stars, measuring the properties of the stars, how bright they are, what color they are, those kinds of things. Are you telling me the Hubble's field of view is so small that even a galaxy that far away, it can't do the whole thing at once?
Yep. I mean, because as I say, it's a big galaxy, right? Half the size of the moon, so it's not a small thing that they're looking at. And yeah, the Hubble Space Telescope had a pretty small field of view. So you can actually only look at a little bit of at a time, but it doesn't really matter all that much in that if you're actually just interested in the stars in a galaxy, you can just pick a kind of representative region and study the stars. And there's plenty of stars in just this one little bit. So you don't need to study the whole thing. So if you measure properties of things, then that's what astronomers always like to do is plot one against the other. And the classic thing with stars is this
thing called a color magnitude diagram that you measure what color the stars are, which is essentially what temperature they are and what magnitude they are, which is essentially how bright they are, and plot one against the other. And so that's what these guys did. Took all this data from the Hubble Space Telescope for this galaxy and various other galaxies, measured the color magnitude diagram for this galaxy, NGC 4236, called World 3, but a bunch of other galaxies as well. This is NGC 4236 at the top here. and they looked at various regions in it, the whole thing and the central region and outer parts and so on. Each of these dots remember is a star measuring its color. So how
red to blue and how bright it is. But the neat part is that there is actually structure visible in here. And in particular, what I want to talk about is this bunch of stars up here kind of towards the top right of that spludge. These are red giant branch stars. These are individual red giant stars that form this sort of fairly clear structure here. And at some point you can see that they get redder and they get brighter and then they stop this. There are no brighter than this point here. And this point here is a thing called the tip of the red giant branch. And what I wanted to talk about is what's going on with those stars. Why they're up here and why they stop. Why there were no stars
brighter than that which turns out be useful for various things. But there's something rather cool to say about these red giant branch stars. First to do that we need to talk a little bit about the physics of stars. For most of its life, a star is turning hydrogen into helium. That's how it's getting its power. And so, in the core of the star, it's turning hydrogen into helium through fusion processes. That's liberating energy, which is what makes the star glow in the first place. Eventually, you run out of hydrogen in the middle. You've just got a helium core. And of course, if there's no hydrogen, you can't turn it into helium anymore. So, the nuclear fusion shuts down in the
center of the star. Okay? But there's still hydrogen further out. So there's a lot of hydrogen around there which is still burning which is still turning into helium. So it's fusion still going on. There's still an energy source. The star still glowing. So the star is still doing the business of being a star. It's still doing its thing. And in fact it structure is changing. This is when it starts to one of the things that happens when you got this very intense shell of hydrogen burning there is that starts to make the whole star puff up. So that's why it becomes a giant star. So the picture is you've got this sort of inert core of very hot helium in the middle, a shell of hydrogen around it, and then this kind of puffing up star
around it. That hydrogen then turns into helium in that shell which gets added to the core. So basically that shell works its way outwards using up hydrogen. Eventually that helium gets hot enough and dense enough that it will start burning itself. It actually turns into carbon. So the helium gets turned into carbon through the next kind of stage of the fusion process. But by the time it happens, that core of helium is so dense that the properties of it are no longer those of normal matter. Okay? It's become what's known as a degenerate body. It's like a white dwarf star, right? It's held up not by normal physics processes, normal kind of gas
pressure processes, but by quantum mechanics that you've crammed all the electrons so close together that they can't get any closer. So that's what's holding it up. So there's kind of most gases there's kind of this sort of self-regulating thing that if it gets a bit too hot it expands a bit and then it cools down again. Okay, for a degenerate gas as it gets hotter the pressure doesn't increase. So the gas gets hotter but nothing else changes. That means that you now have this potential for a runaway process. You have fusion starting which means that the thing gets hotter which makes the fusion happen faster which makes it gets hotter which means it fusion happens faster and
faster. So you have this runaway process without that safety valve of okay it'll just expand and cool down again which means that as soon as helium starts to burning to carbon to fuse into carbon you have this massive runaway process. So a huge amount of energy is generated. For a few seconds, that helium core is producing more energy than all the stars in the Milky Way put together. So briefly, this star is as bright as the rest of the Milky Way put together. But unfortunately, you've got all these layers of star over the top of it. So no one ever gets to see this thing. It's called the helium flash. It's this intense flash of light that's produced when helium ignition occurs, but it's
all muffled by the rest of the star. And most of that energy doesn't actually turn into light. It turns into lifting this degeneracy of turning this gas back into a normal gas again. So for those few seconds that energy goes into stopping this thing being this weird degenerate matter and become normal matter again which takes a huge amount of energy kind of sucks that energy up rather than letting it all eventually come out as light. And what happens after that uh helium flash occurs is that eventually the thing will settle down into a new state. it will actually move away from that state to a new more stable state on a thing called the horizontal branch which is basically
where you're now just more steadily burning helium into carbon. So I wanted to talk about this red giant branch and why it's got a kind of a well- definfined top to it. Okay, so that's the reason is that what's happening is that as you're adding more and more helium to that core, the stars move their way up this red giant branch until you reach this point called the helium flash where this hugely energetic rearrangement happens. That's this tip of the red giant branch. Nothing goes beyond that point because after that point the stars properties change entirely and on a matter of a few thousand years it moves away entirely. So the tips the stars you're seeing up at the top of this blob here are the ones that are probably just about to
undergo this helium flesh. They haven't quite got there yet but in a few tens of thousand hundreds of thousands of years they'll go through this process and then in a few thousand years they'll move away from that region entirely. Whenever you talk to me about stars and galaxies, things always happen on time scales that disappoint me. How slow they are. How, you know, you say this thing will explode over the course of a thousand years or something. This is one of the few things in a star that seems to happen really quickly. Seconds. Literally seconds. The whole process is all over in a matter of seconds because you got this massive runaway explosion going on in the center of the star because of quantum mechanics basically.
So what's what is it that dictates what the how bright that tip actually is? Well, actually, it's just how much, you know, how it's the point at which that helium flash occurs. It's the point at which that explosion occurs, which is always the same because that's just driven by the physics of quantum mechanics and what's going on in a, you know, in a helium to generate helium gas. So, always the cores that explode at the center of these stars, the cores that suddenly go through this flash process are the same size. They're what's driving the luminosity of the whole star because they what's powering the star. That means that the stars that are at that tip of the red giant branch at that point where that helium
flash occurs are always the same brightness. That makes them this wonderful thing that astronomers really like, a thing called a standard candle, right? Because they're always the same brightness, which means we can use them to figure out how far away something is because we know they're always the same brightness. So if in one galaxy they appear four times as bright as in another, we know that galaxy is twice as close. Um, so you can actually use them to measure the distances to things. That means that what these guys are actually doing by measuring the tip of the red giant branch in these galaxies in the M81 group was measuring their distances. So as well as being able to say where they are in the sky,
they can tell you what their distances were. So one of the things they did in this paper was kind of produced the 3D view of not only where on the sky are these galaxies, but how far away are they? And so here are all the various galaxies in the M81 group. There's the main kind of grouping of them. And here's Coldwell 3 up here NGC 4236. So it sort of doesn't completely resolve the issue in that it tells you that this galaxy is more or less at the same distance. It's clearly in the same kind of grouping of galaxies, but it's not one of the kind of the core members of the group, which you could kind of tell just from looking on the sky that it was clearly off to one side, but it's what
we now know from measuring these distances, it's not way in front or way behind. It is actually physically associated with that group. So Mike, if we do a color magnitude diagram of a of the stars in a galaxy, we find the tip of the red giant branch, identify those stars, measure their brightness, we can tell how far away the galaxy is. Yep. Has that become the standard way to measure the distance to galaxies or is it a bit of a niche way of doing it? It's a pretty widely used way. Of course, one of its advantages is these are some of the brightest stars in a galaxy, right? They're they're right at that tip of that red giant branch, which means you can see them a long way away.
So in that sense, these are good standard candles because not only are they standard candles, but they're bright standard candles, which means you can actually measure them even when they're looking in galaxies quite a long way away. So it's pretty widely used now. But you know, there are questions you always wonder a little bit, well, what if the properties of the stars are very different? What if there something very weird going on in this galaxy? So what astronomers like to do is have a whole slew of different methods that they use. So they use these stars, they use sephiid variable stars, they use planetary nebula, there's a whole bunch of different things. So, what you want to do is have a whole bunch of things
and hopefully when you measure distances using all these different methods, you get the same answer. Thanks for watching. If you happen to be a Patreon supporter, you can also view some bonus content from the making of this video. It's a fire alarm or a fire drill in various other galaxies. Look, if that doesn't make it worth becoming a Patreon supporter, I don't know what is. Details down below in the description. All the usual places.
