11:18
all right professor what do we got today messier 72 it's a globular cluster here's a picture of it's not a bad one it's nice some of the globular glasses are very centrally concentrated this isn't one of those it's kind of more of a loose scattering of stars it's a very long way away it's about 50 000 light years away so it's kind of on the other side of the galaxy what i was thinking about is well what can you do with a globular cluster you can take a picture of it right and then what can you do next well you could actually take another picture of it you'd expect the two pictures to be more or less identical but actually it turns out that's not quite the case because there are always a few stars in any
cluster that are actually variable and so you'll see a few stars have got brighter or fainter between the two images and if you take a whole series of images obviously you can then track to see if it's is it just fading away is it getting brighter or as it's almost always the case is it kind of periodically changing over time getting brighter and fainter again so that's exactly what these guys did so you see the time series photometry of the globular cluster ngc 6981 which is the name of messier 72 variable star census and physical parameter estimates one of the trickier things with globular clusters when you're trying to pick out the properties of individual stars is actually they're quite crowded
so you end up you know with this sort of confusion effect that you can't see if you're seeing one star or several stars superimposed or overlapping with each other so the neat technique they use in this particular analysis is a thing called difference imaging which essentially just means you take one image you take another one and you subtract one from the other and of course if nothing had changed then there'd be nothing left in the difference image but what you get is that if a star's got brighter then it'll suddenly pop up and if it's got fainter then it'll be a little it'll appear as kind of a negative star in the image and so from that you can actually pick out the variable stars so this plot up here
is one of these color magnitude diagrams so basically it's how bright are the individual stars and this is now the average brightness of them because of course these ones are varying a bit and how blue is it that way we blew things over here red things over there faint things at the bottom bright things at the top so there are this thing called the main sequence down here which is the ones that are turning hydrogen into helium there's this thing called the horizontal branch here which is where stars are turning helium into carbon basically what you're seeing are stars of slightly different masses that are at different stages of the stellar evolution and then the ones that are in
color here are the ones that they've identified as variable so what's really going on in these stars is there's something making it unstable making it pulsate and the something in this case is a weird instability basically the stars made mostly of hydrogen and helium sometimes you know in the later stages the helium turns into heavier elements but primordially when it started out it was just the hydrogen and helium and a little bit of heavy elements that were there in the universe so helium is two protons usually two neutrons and two electrons so you've got the nucleus and then the two electrons kind of orbiting around it when you heat helium up you rip one of those electrons off typically
and then it becomes singularly ionized helium and then if you heat it up some more you rip the second electron off and it becomes doubly ionized helium and so as you go down into the star it gets hotter and hotter you reach the point where the first electrons are getting ripped off the helium so it's singularly ionized helium now if you imagine taking that bit of the star and just randomly it just gets compressed a little bit when you compress it's going to heat up some more that act of heating up some more will make that singly ionized helium turn into doubly ionized helium so it's got hotter it loses its second electron so now we've just basically got two protons and two neutrons and no
electrons left around it what we've done now is we've compressed it a little bit we've created this doubly ionized helium suddenly we've created this layer which is much more opaque and that's essentially like putting a blanket on the thing it sort of holds the heat in because the light can't get out anymore so that then causes the interior of the star within that immediately within that sort of shell to get hotter because the radiation is getting trapped it heats up that act of heating up then makes the thing expand again the expansion makes it cool down and when it cools down that doubly ionized helium turns back into singly ionized helium again it recaptures its electrons and we're back where we started so it re-expands to where it was and then
keeps going and now it's a bit too fast and then it collapses back down again and the whole thing just repeats itself so it's this weird cycle that's essentially being driven just by the fact that the helium keeps changing between being singly ionized helium and doubly ionized helium that kind of drives this weird cycle within the star now only a very few stars are doing this though it's not like they're all pulsing like this they're not and so and that's the interesting thing so what you want for this effect to work is you need to have this region which is just at this magic temperature sort of goldilocks kind of temperature not too hot not too cold for to be just on that hairy edge of things becoming doubly
ionized i mean if the star's hot and hot enough then that region's gonna be right near the surface of the star so there isn't enough of the weight of the star to push down on it if it's too cool to style that then it you know it's very cool on the outside as you go downwards you'll eventually reach the point where it's hot enough for this effect to happen but that's now so deep in the guts of the star that all that weight of the star pushing down on it stops the process so you need it to be at just kind of the right depth in the star so remember this axis here is a measure of color but it's also a measure of temperature in the redder things are cooler and bluer things are hotter and it's only these
things in the middle that have this magic property and in fact if you go up through the color magnitude diagram there's this thing called the instability strip that kind of runs all the way up here any star that lies in this region is likely to show this instability so for example we talked about cepheid variables in the past they're driven by exactly the same process but they're very massive styles that are way up here somewhere the ones that were picked up here on this thing called the horizontal branch so these ones that are turning helium into carbon they're called ri lyric stars and then the two little ones that are down here are called essex phonesis stars these are also variable stars driven by the same process but the weird
thing about them is that most of the main sequence stars up here have disappeared because they were more massive stars that already died so the only ones that are left should be down here but these are actually up here so they're main sequence stars they're lying on the main sequence but they're actually more massive than should be in this cluster because anything this mass should already have disappeared again this is something we talked about before these are things called blue straggler stars so these are actually illustrating two phenomena at once they're illustrating this instability strip but they're also blue stragglers because they lie kind of beyond the end of the main sequence so they're that weird type of essex finishes variable star
professor would living on a planet going around one of these stars be interesting or catastrophic so in this particular case you'd be hard-pressed i think because so there's been a lot of discussion about whether you could have life in globular clusters and it's not terribly conducive to it partly because the stars are very close together and so there's a good chance that another star will come whizzing past and mess up your solar system but also they're very low in these heavy element abundances and so whether there's enough heavy elements there to actually form rocky planets and people i suspect probably not so these little ones down here the essex finishes ones the period of pulsation is about an
hour okay so it's actually varying on an hour's time scale so they're quite rapid variations so you would have this weird thing that your star would kind of visibly get brighter and fainter during the day because you know i guess we're used to that because it gets cloudy from time to time so it will really be that kind of effect that it would just sometimes it'd be a bit brighter sometimes a bit fainter but it wouldn't yes i wouldn't like there'd be a civilization destroying event every 24 hours no but i suspect probably you'd end up with some quite um strange myths coming up right you'd have you'd have some mythical reason as to why your style was doing these things the myth would come first the physics would come later
what's inside them but it just encapsulates for me just how unique this catalog is nice and gentle okay it's all right it's not too heavy okay brady would you like to take her feet yep okay
