39:57
going to look at the globular cluster messier 68. so here's a picture of it i mean to a first approximation when you've seen one globular cluster you've seen them all but actually that turns out to be what's interesting about this cluster is that it's pretty much the same as all the other globular clusters this is actually a hubble space telescope picture of it so it's not the whole of the cluster but it's sort of the inner parts so as you can see it really is a very spherical collection of lots and lots of stars tens of thousands hundreds of thousands of stars let me just remind you of a little bit of the details of what you can learn by studying these things this is a study of
this cluster messier 68 and it's ccd photometry so it's kind of modern imaging although it's from a fair while ago now from the 1990s so the blue the visual which is green light and i which is kind of very red light and the classic thing that astronomers do once they've made that measurement is you make one of these things called a color magnitude diagram where how bright the galaxy is in one magnitude is plotted on this axis here versus a color here so it's the difference between the magnitudes and two bands so basically bluish things are over here reddish things are over here faint things are at the bottom bright things were at the top so on this diagram for example the
things down here are main sequence stars those are stars which are turning hydrogen into helium the things up here are what are known as horizontal branch stars they're turning helium into carbon in their guts and actually you can learn a lot just by studying these diagrams like how far up this sequence runs basically tells you how old the cluster is those kinds of things so we're going to learn a lot about it and so it turns out again this is a completely typical globular cluster the shape of those sequences on the diagram are just what we would expect and coincidentally this object messier 68 was kind of used as an exemplar to explain why this commonality between clusters was an interesting phenomenon
way back in this paper 1920 paper by a guy called harlow shapley it was one of the founding figures of astronomy and one of the things that he's most famous for which is where this fits in was really understanding the size and shape of the milky way and where we were within it he actually picked this globular cluster messy a68 as kind of his poster child to illustrate the techniques he was using and the main thing that shapley was doing was trying to figure out the distances to globular clusters as a way of kind of mapping out the structure of our galaxy and actually he used three different methods on these globular clusters to figure out what their distances were so the first one which was kind of the technique that had
been around the longest was just to look at how big they appear to be and if you assume that they're actually all intrinsically the same size and if you see a big globular custard it's going to be close to you and if you see a very small one it's got to be a long way away the second thing he did was to start looking at the properties of the individual stars now he wasn't producing these color magnitude diagrams he was just looking at magnitude so he was only had photographs essentially in one color so he was just seeing how bright the stars were and he did two things with those brightnesses firstly he just looked at the brightest stars and again by picking out how bright they appear to
be if they're all intrinsically the same actual brightness you can figure out how far away the cluster is because if those brightest stars appear very faint that tells you it's a long way away if those brightest stars appear relatively bright that means it's relatively close the final technique was actually if we go back to our sort of better quality modern diagram and actually i want to just zoom in on this region okay and so conveniently there is a zoomed in version there we go and it turns out if we look at this one at the top here most stars just shine with the same brightness the whole time but there are some stars whose brightness varies with time and usually in a fairly characteristic way they tend to
oscillate in brightness and it turns out those stars all live in this region of the diagram it's the part of the diagram where stars are burning helium into carbon this thing called the horizontal branch and the nice thing about it is it's horizontal which if you look at this diagram again if things are horizontal that means they all have more or less the same value of brightness so all these variable stars are intrinsically the same brightness and again by picking out those variable stars you can play the same game of seeing how bright they actually appear to be you know what their intrinsic brightness is that tells you how far away the cluster has to be so we had these three different methods all of which were
essentially measuring the same thing and actually for this particular case which is could well be why he picked it as his typical example they all gave more or less the same answer and in fact the answer he ended up with was about 50 000 light years away a modern analysis of the same thing puts the cluster at about 34 35 000 light years away so you know he was in the right ballpark now the reason why this was kind of a an important measurement this one on its own you know getting the distance to one globular cluster is vaguely interesting but doesn't tell you a huge amount but he did dozens and dozens of globular clusters and what he found is that they distributed all around the sky
in a roughly sort of spherical halo and the center of that halo of globular clusters is nowhere near where the sun so the sun's kind of offset from the centre of this distribution which was sort of not what was expected because when people had measured sort of the structure of the milky way the other way which was just basically to count stars in different directions you came up with an answer which says that the sun is remarkably almost implausibly close to the middle of the galaxy and we now know why that is that's because there is all this obscuration out there this murk out there which means that actually if you count stars you can only see them to a certain distance away before they kind of kind of vanish into this
the obscuration into the merc the two things that shapley was able to show by doing this thing with the globular clusters was to show firstly we're nowhere near the middle of the galaxy we were out in one of the boring suburbs but actually secondly that the milky way was way bigger than had been thought to be in the past so he kind of revolutionized our view of our galaxy just by playing these games looking at the globular clusters around it of which messier 68 is kind of the classic example and it's quite an unsteady you've got your cup of coffee sitting in front of you and you look down what you'll see are these concentric rings you'll see this part and what that is a manifestation of something called a
bessel function
