20:18
messier 47 what is it where is it what's going on it's an open cluster there we go looks like the pleiades it has a certain family resemblance to it so it's an open cluster like the pleiades so it's one of these sort of fairly ratty collections of stars in the plain of the milky way what else can i say about it's quite large in that actually it's about half a degree across which is about the same size as the full moon the other salient point is that there are both blue stars and red stars in it so the red stars are the ones that have kind of evolved away from their initial life on the main sequence when they're just turning hydrogen into helium to the late stages of their lives but the blue ones are the
ones that are still on the main sequence so they're the ones that are still just turning hydrogen into helium this interesting stage in that because it's got blue stars in it you know that actually stars that are blue on the main sequence are quite massive stars and quite massive stars have quite short lifetimes so the fact that you see blue stars there tells you that it can't be that old in fact if you go through looking in detail at the different distributions of the colours of the stars you come to the conclusion it's about 150 million years old it's not the youngest cluster it's not the oldest but it turns out it's an interesting sort of age and the reason it's an interesting sort of age relates to this paper here
it's a massive magnetic helium atmosphere white dwarf binary in a young star cluster and in fact the young star cluster in question is messier 47. so what the people who wrote this paper were trying to do is they were looking at the masses of white dwarves and one of the sort of unsolved questions in astronomy is how the mass of the star when it formed in the first place how that translates into its end state which is this white dwarf star it ends up as we've talked about wide doors before but basically they're the end stage of a star where the whole thing's collapsed down to about the size of the earth and it just basically sits there and cools down over time it's kind of the end
stage of a star's life is a star the heaviest it will ever be at the moment of its birth usually it's the short answer so the main processes that stars can go through after they've been born is to lose mass they'll shed their outer layers towards the ends of their lives even the sun's losing a little bit of mass at the moment through the solar wind the odd exceptions are ones where you have a close binary star and sometimes one star will steal mass from the other star so occasionally stars put on weight but usually they just kind of decrease in mass actually the end state for a star like the sun is that the at the end of its life it'll blow off all its outer layers in one of these things called a
planetary nebula and all that will be left will be this kind of core at the center but what these people are interested in is how that the mass that the star starts with translates into the mass that the white dwarf ends up with and in particular also what the most massive white dwarf that you'll ever find is because there's a theoretical limit thing called the chandrasekhar limit that says that actually you can't have a white dwarf more massive than about 1.4 times the mass of the sun so one interesting thing to do is to look at more and more massive stars to see if they end up with more and more massive white dwarves to see how close we get to that limit and even if that limit's exceeded which means that
that there'd be something wrong with the physics there so they're looking at trying to find massive white dwarfs which means they're looking for kind of massive progenitors the things that turn into the white doors now there's a problem with finding white dwarfs from massive progenitors in that what happens is star goes through its life blows up as a planetary nebula leaves a white dwarf the white dwarf then cools down and fades away and so you can't see it anymore so if you want to find one from a massive star and hopefully then a massive white dwarf that means that actually you've got to have had a massive star that's gone through its life blown up as a planetary nebula left the
white dwarf behind but the white dwarf hasn't faded so far that you can't see it anymore that's why messier 47 is such a good place to look because it's a cluster that's the right kind of age in that stars which are going to turn into white dwarfs have gone through their lives and blown up but the white dwarves won't have had time to fade away completely just a quick question about the planetary nebula which i think is one of the great misnomers in all astronomy can we look for those to help us like do those things hang around those sort of quite spectacular formations to help us find white dwarfs probably not long enough you'd have to find it in a young cluster of stars and
find a planetary nebula and then yeah you would find a really young white dwarf at that point but the planetary nebula doesn't last very long so actually that will have faded away before the white dwarf fades away so it's easier to just search for the white dwarfs rather than searching for planetary nebulae this was considered to be a happy hunting ground for recently formed white dwarfs and they found one they took a whole load of spectrum and looked at the colors and brightnesses and different things tried to identify where there was a white dwarf the other information that they have is they've got information on the proper motions of the stars in other words how the stars are moving around on the sky and so they were able to
identify this object which has all the properties of a white dwarf in terms of it's got the right colors the right brightness the right spectrum but it's actually moving with the cluster as well so they know it's a member of the cluster which gives them that evidence they need in order to date it so that's what they did they're adding to this diagram here this is how much mass the star had to start with this is how much mass the star ended up with so this is the mass of the white dwarf effectively and the object that they've just found is that point there so it's not the most massive white dwarf that's ever been found but it's right up there right there's a couple that are more massive
and you can see that if you start with a low mass star you end up with a low mass white dwarf if you start with a high mass star you end up with a high mass whiteboard how do you know the initial mass of the star that created the white dwarf okay so this is quite a clever bit so i need to show one of the other figures so this plot here is one of these things called a color magnitude diagram which again we talked about many times but basically is how blue or red the object is and how bright it is so it's faint to bright and blue to red white doors are sufficiently simple objects so we actually have a pretty good idea how they behave in terms of if you know the mass of a white dwarf for
example then that directly translates into what its radius has to be we know how big it's going to be so if we also knew its temperature which is how much it effectively tells you how much light it gives out per unit surface area and you know how big it is that tells you how bright it is so we know an awful lot about white dwarfs and so we can actually with reasonable confidence predict basically where they'll lie in this diagram and even how if you think about it if you just leave it's going to just cool down and fade away so these things are the fading curves as the white dwarfs cool they basically follow these lines like this okay so how far they are down the line tells you how long it is that they've been fading for and then the
different lines are actually for different masses of white dwarfs now white doors have this rather strange property that the more massive the white dwarf is the smaller it is and so that actually means that for a given temperature so a given amount of light coming out per unit surface area a more massive white dwarf because it's smaller will be fainter by seeing where this object is both in terms of its colour and its magnitude how bright it is we can see which of these lines it lies on so that's how they know its mass is 1.06 solar masses because it lies on the 1.06 solar mass line how far down this line it is that tells us how long it's been fading for and again by looking just at the models
here they can figure out okay this one's been fading for 75 million years and the cluster's 150 million years old that means that the star must have ended its life 75 million years ago because you know seven years so you've got a star that lived for 75 million years then it's been fading as a white dwarf for 75 million years that gets you to the 150 million years that the clusters ages we know what mass of stars live for 75 million years and the answer is a star that's about six times the mass of the sun so from just from this one figure we can figure out both what mass the star had to start with and what mass it's got now there's two other things that they've learned about it firstly actually it has a companion it's not alone it has an m dwarf so a very
faint star that's in a binary system with it the reason they know that is because there's a bit of excess light in the infrared which could only be the case if there's a close companion and that's quite exciting because one of the types of supernovae these type 1a supernovae are where you've got a white dwarf which is accreting material from a companion and we don't know whether these two things are close enough together for that equation to actually occur but it could be later on when this m dwarf goes through its evolutionary phases and starts losing mass itself that the white dwarf will acquit it and this might turn into a type one a supernova so it's possible this is a very early
stage of a progenitor of one of these type 18 supernovae the other thing that's weird is that when you actually look at the spectrum of this thing so split the light up right up into the colours of the rainbow so this is how much light there is as a function of wavelength from kind of blue light through to reddish light and there are these dips due to absorption this is what's called a helium white dwarf in almost all the lines you see a due to helium it's basically because you've exposed the kind of core of a star almost all that you're seeing there is the helium core of a star that's exposed you can see that this is one of those absorption lines but actually you can see it's actually split into three
and this is a phenomenon called the zeeman effect so atoms have these kind of transitions between energy levels that lead to these absorption and emission at very specific wavelengths if you take that atom and put it in a strong magnetic field then actually those energy levels get split depending on which way up the atom is effectively relative to the magnetic field and so instead of seeing a single line due to a transition between two of these energy levels everything gets split into three through this effect called the zeeman effect and the amount that the lines are split apart is related to how strong the magnetic field is so we can actually measure the magnetic field in this on the surface of this white door
and it works out it's about 200 tesla which is huge that's the only way to describe it so just behind me up the hill there we have one of the most powerful mri machines in the country that's a 10 tesla magnet and that's the kind of magnet where you don't want to be walking past it with a bunch of keys in your pocket because they'll just get kind of ripped out of your pocket right so that's a 10 tesla magnet this is 200 tesla which they're also excited about because actually magnet strongly magnetic white doors are quite rare um and what they found is that actually this is you know what it looks like whatever processes it is that go on in a when you start with a massive progenitor that turns into a wide all seems to be
producing these magnetic white dwarves so it could be that some of the other magnetic white dwarfs that we know about were formed from massive stars then this is the kind of leftover white dwarf it was a happy hunting ground for those astronomers wasn't it they got an awful lot out of this one object yeah all right ship in the 17th century or something but you were telling me before this is actually quite a modern piece that's been loaned to you by the met office that's right so this one will be from the 1940s but actually the design has remained the same since the 19th century
