How Were The Stars Formed? - Professor Joseph Silk

okay good afternoon everybody welcome to the new series of question lectures on astronomy that I'll be giving this is the first one and I'm going to tell you about how stars form now when you talk about birth which I'll be telling you something about what I have to think about death as well so part of my story will be how they die okay so it's all to do really with how we compare deep into murky clouds in our galaxy and in other galaxies and it's been a bit of a revolution in in the past decade or so so here for example is a field of stars that many of us know and love one of the easiest constellations to see in the sky or Ryan and but in Orion actually there are regions in here which turn out to be the birthplace of stars and they're teeming with newly born stars it turns out the stars in this particular we call it a nebula which you can't even see in you know in ordinary light I include some very massive stars and in terrorist's a little further away I'm also in a cloud that you can't see directly on this picture there are stars like the Sun being born now how do we know all of this well basically you have to go into space first of all to take photographs at wavelengths that you can't really do very well out on the earth to see the stars that are much more massive than the Sun they're very hot you need ultraviolet and thanks to the Hubble Space Telescope we have beautiful images and I'm going to show you some and so the Space Telescope the Hubble telescope has been up since 1990 it's only a two point four meter telescope very small but nevertheless it's been able to do very well for us and because it's not very high 335 miles it's been a by the space shuttle to do repairs and that's why in fact it's the astronauts done a good job repairing it on these two missions and refitting it and so it's survived a long time another telescope launch rather more recently peered into the infrared there isn't one more recent than this even which I'll tell you about soon this is called the spitzer space infrared telescope and it was launched in 2003 and it was limited in what he could do because to look in the infrared you have to cool down your camera and so when I stand the coolants run out after a while so Spitzer was good and it's now been replaced by being replaced by other telescopes anyway so when you look at this same region of space with an infrared telescope you see something incredibly different it's full of glowing dust particles actually and some of them are hotter than others and they give you slightly brighter light in this color-coded picture and that's where stars are buried inside the dust inside denser parts of the nebula and I'm actually being born anyway I'm going to tell you more about how that all works but the basic part important part is that we live in a very dusty universe it's you know you might think it's pollution it is for the astronomers actually because the dust particles get in the way of seeing what's going on in the innards of clouds but here is a star cluster as seen in the optical ultraviolet with the Hubble telescope you see the very bright dots very massive stars and and the less faint dots are more ordinary stars more like the Sun and and we know as I'm going to explain to you that these massive stars they die very rapidly actually so they're rather young when you see them so that's why we know we're seeing the birthplace of stars but you look in the infrared and this is the nebula you see or within that nebula there are many more stars that you don't see in this picture and the stars buried deep in the nebula the ones in the process of birth so this is in a sense is like a cosmic womb for us an amazing story really ok so now let me folk some birth and then we'll get on to death in a bit but let's do birth okay so one of the most spectacular pictures taken by the Hubble telescope is this one it's been called I think the pillars of creation among other things but all we're really looking at are regions where there are very massive stars producing lots of hot radiation hot ionizing photons that destroy and eject away much of the dust and debris around them begin to see them but buried inside that these regions there are stars yet to be born and we know that because we peer inside these regions we can see stars breaking through over here and we look in the infrared we can see stars newly forming inside this dark nebulae so this is an example of something very similar to one of these or one of these pillars and so all of the this once was you know a much larger nebula and all of this stuff has been eaten away by by radiation from young newly formed stars like this one and these are the densest remnants left over this is the Eagle Nebula and the one on the right is the cone nebula okay so now let's look at the Eagle Nebula in the far infrared so this is yet another telescope this is the more a more recent one called the Herschel Space Telescope it's a bigger one three and a half meters in diameter and the beauty of it is as they looks at very very long wavelengths we call this the far infrared and now the longer the wavelength in some sense the cooler the dust must be that emits at that wavelength and so you're looking at the coldest densest parts of the nebula and and within that and it's color-coded so don't be fooled by the colors of thinking that it's that there's any warmth or heat in here this stuff is all relatively cold but there are some regions that are warmer than others and those the regions which are be aware where stars are actually forming in these bright spots over here inside this very dense Dark Nebula so here's an example of this tell this is this at the Herschel telescope it's actually in a very interesting orbit one-half million kilometers away from the earth on the opposing side from the Sun which is what we call in a garage point is very stable points and the telescope cameras the gravity of of the very of its orbit more is balanced by that of the Sun so we could stay there essentially while only we're seeing a very small correct correcting boost and it's not bothered by sunlight because the sun's of the opposite side of the Earth anyway it's a great place though astronomy this Lagrange point and that's where many more satellites are being poor eventually in the future so here is the far infrared view of the Eagle Nebula ok now there are many many more regions that we've mapped out and these are just beautiful pictures you the colors are slightly artificial in the sense they're they're coded to give you relative change in temperature the darker the browner the cooler the whiter in some cases the green are the slightly warmer stuff but in all these cases you see nebulosity clouds basically like thunder clouds and things are happening inside them thought you know lightning will arise etc things break out in some sense that's you have to think of these clouds in spaces they're subject not to thunderstorms but but to actually break out of star formation like explain why that works and that produces energy that heats up and partially destroys the clouds and you're seeing the process of cloud destruction revealing the stars being born underneath so this nebula is one of the more beautiful ones called the horsehead nebula cuz if you squint at it carefully you can see a shape that might look like a horse's head and there's one over here called the monkey nebula again I'm not quite sure where these names all came from but in any case if you look hard enough you can but ok so over here we have the stars basically heating up and breaking out in some cases and many of these are brighter spots in the dark cloud of stars in the process of being born here there's a whole cluster of stars buried beneath the dust we're actually looking at this this picture was taken with the Hubble Space Telescope this isn't a very near roofer an optical picture but nevertheless when you look at this in the infrared you you would see far more stars than you hear and another beautiful example and the Omega nebula again I'm not sure where the name comes from maybe you can vaguely see an Omega shape there but here where stars are being born here stars have been born and they've eaten away at the dark that now could heated up the gas and you can see light from the stars being reflected and diffused by the dust grains that's why you don't see a single bright star because the dust scatters it's like looking through a fog basically and one of the more beautiful examples is this one and I'll come back to this this is the Carina Nebula or again you see these these regions in which stars are forming and regions where stars have recently formed and it's all very active stuff that you just get no no idea if you looked at this in the optical you just see blackness over here basically with for a few foreground stars if you ever look to the Milky Way at night you notice there are regions which we see apparently very few stars other stars around them those dark regions are just huge dust clouds when you look in the far infrared especially they light up in these and show this amazing activity going on that's buried from from our normal telescopes and which you can only see in space ok because the Earth's atmosphere is very good at blocking most of this interesting long wavelength radiation and we have a neighbor near the nearest galaxy to us the Large Magellanic Cloud and again those of you who have seen this in the southern hemisphere from Australia maybe well you know it's a beautiful thing but it's just a fuzzy spot on the sky that's all you can see with the naked eye when you look with telescopes you can resolve it into stars but when you look with the Hubble Space Telescope you can suddenly see this is exquisite pattern of this is a whole cluster millions of stars with some of the most massive stars that we know in the central region being formed and and those stars as I'll explain are intrinsically rather young just millions of years old remember our earth is four-and-a-half billion years old our Sun is about four and half billion years old but these stars are just millions of years old they're there youngsters and they presumably they come and go we're just seeing the recent generation of them and over here is a dark cloud where again when you peer in at longer wavelengths you can see many more stars and this is waiting to be to emerge they're waiting the new stars away from this region over here okay so when you look at the very center of the Milky Way that's near the constellation of Sagittarius that's low on the horizon and North Lima so you can see it well from the southern hemisphere with a again with an infrared telescope in this case you I mean again it's just darkness I mean if you look at it with an optical telescope but in the infrared you suddenly see it's teeming with activity and the center of the galaxy is roughly in them in the middle of this picture the actual geometrical center where there's a huge black hole lurking actually I'm not going to talk about that today but that you know it's a dynamic place to be lots of things going on and we have no idea about this with our ordinary telescopes until we begin the adventures of infrared astronomy to peer deep into these things so that's how we've begun to understand where the stars are forming okay so I try to explain where the star is our galaxy contains a lot of stars something like a hundred billion stars mostly not too different from the Sun but some of them are much more massive than our Sun is and the massive stars actually have a short lifetime because they consume their fuel I'll explain what it is in a moment very quickly in millions of years and they explode and I'll show you the explosions in a moment and the explosions of the debris from the star form the dust that you see in in in the pictures I showed you and the interstellar clouds and from those clouds new generations of stars will form so our Sun is produced from the ashes of previous generations of stars and the earth in particular and we but all that we're made of we our atoms once went through this very quite fiery furnace of stars we're the relics of all day in some sense we're form for the ages of stars okay so how then are stars actually formed let's try to shed some light on that so historically James Gene's was a well-known astronomer and science popularizer in the early years of the 20th century and he had really a brilliant insight into why it is that a cloud of gas would would make stars so he said that if this cloud of gas becomes large enough it accretes enough matter by bumping into other gas clouds which stick to it much like you know you see clouds build up in the sky when especially when a storm is coming based they're very sticky things they'd graduate together they coalesce together he argued that well clouds in our galaxy would aggregate together and eventually they're become massive enough so their own gravity takes over now this never happens to clouds in the Earth's atmosphere they're far too light for that but in the interstellar medium one can really accrete millions of solar masses worth of gas eventually and I'll explain a little bit how that said in a moment but why don't you do this once gravity is important then gravity causes the gas to collapse the cloud whole cloud collapses under its gravity just can't resist anymore now what happens is a diffused gas cloud can cool down rather easily okay and so genes realize that this cloud under gravity there'd be no resistance to gravity would cool down and as it cooled down it would automatically have to break up into smaller bits and pieces as the cloud got denser and these smaller fragments he said would be the stars and eventually the fragments of even the smallest clouds that made the stars would be the planet so that was his insight and he showed that it was just really a competition between the gravity which caused wanted that to make the cloud collapse and the ability of the cloud to stop gravity by just having pressure atoms bouncing bouncing around but if they lost energy by calling into space that that resistance would weaken okay and so then the cloud would shrink and break up okay so that didn't quite if you think about it I mean that says fine clouds break up into smaller clouds but why stars and it was interesting around the same time another pioneering English astronomer called Arthur Eddington had a tremendous insight into why they had to be stars and he said just imagine a physicist who always lived on a cloudy planet okay she could never see the Stars he said maybe he said he in those days right but whatever and anyway one could she could deduce there had to be stars it was inevitable given a knowledge of clouds and so clouds stars have to be there and the so the logic of that is interesting and I'm going to explain that to you but let's now just look at a cloud so this is a typical cloud as seen in the optical part of the spectrum with ordinary telescopes okay even the Hubble Space Telescope would see a cloud like this and we see many of these in the Milky Way and it's only when you look at the infrared that you realize that this is teeming with young stars but anyway this is a dark cloud it's dog because there's dust a lot of dust along with the gas and the dust absorbs star line okay and so you just don't it just looks black and this stars you see arrow stars in the foreground between us and the cloud and so you see none of these many background stars and look at this okay so this is a typical room for star formation okay and this cloud when it grows and it get acquired enough mass but running into other clouds will then be the birthplace of stalks that's the idea so we see some clouds which don't make stars there's starless clouds a few there the precursor stage and we see many that apparently are forming stars okay so here again is a beautiful image from an infrared telescope of star-forming region in our Milky Way and again this is these are the stars that have burnt away their surroundings emerge from the shrouds of dust and these regions are still shrouded by dust and and these are the stars in the foreground and you can infer there's stars inside them because regions of the duster heat being heated up and this is where star star stars are actively forming and in these regions they will form in the future we're certain just by extrapolation and here there's been some really impressive activity in forming stars so this is the way things go you know the way the reason we're so innocent in these pictures is that you know it's a bit like you're looking at a population population of stars so when you study people you go to a large enough City and there you have a sample of you know young people babies old people sick people well people it said for etc you see all sorts of things with a large enough population so that's why we astronomers care about building up this catalog of clouds with many many environments and they're going to catch the young stars the new stars the to be stars and the dead stars and the dying stars they'll all be there just as they are in any in any population so that's why it's so important to analyze all these images and get them in the first place which is not easy because the Earth's atmosphere and you know good for us because otherwise we'll be burnt up by the UV etc and either horrible radiation it blocks out much of this interesting stuff and we have to go into space to see this stuff the infrared actually is blocked out by water vapor in the Earth's atmosphere that you know what is a good thing to have but again if you go to the moon that's a great place though astronomy there's no atmosphere you're not blocked out so that in the future I'm sure we'll be having telescopes on the moon but that's another story okay so now let's zero in to a very young star where we've been able to get images by peering through the infrared this was taken with the Herschel Space Telescope so this is an example of a star very young star in the center and it's surrounded by what looks like a disc or a ring of material and in this corner of the ring there's an object which probably is going to be something like Jupiter in the future that is a massive planet and from this this which can barely resolve with this telescope we infer this probably is the precursor from which the planets were made and because we don't know exactly how planets are made and we need smaller bodies to build up into them we've entered a word for that and we call them planetesimals you can think of them as a huge crowd of asteroids or rocks in space similar to what we see around Saturn actually Saturn's rings are a nice laboratory for studying how structures how moons might form the Rings have set and our future moons perhaps and you know and you know of course it has its many satellites of its own whether it's clear the path through those rings anyway so here we think is a space where and an example of a young star with a future that will include planets okay so this is one of the latest most impressive images I want to show you of which tells a lot about how planets are formed so this is one of the nearest young stars to us in the constellation of terrorists and this has been imaged with with a telescope I'll show you pick no second it's a it's a telescope which works a microwave wavelengths it's a whole series of telescopes actually and the microwaves compare even more deeply into the dust than any infrared telescope can and let's come up with this amazing image and so what you can see there's a star in the middle this is its name HCl terrorist astronomers use alphabetical coding and characters for the constellation of terrorists and it's surrounded by a series of disks and they're a break and there are gaps in between the disks so this is very reminiscent of what we found with the Cassini spacecraft when it imaged the rings around Saturn its saw rings gaps and satellites and so we think this is the early stage before even the planets are formed we're looking at potentially there may be planets inside these gaps sweeping up the material or maybe they have some other origin but in any case all this stuff is the future you know dusty rings which will coalesce in the future into into planets so that's some and this star we know from how bright it is how cold it is it's only a hundred thousand years old it's the youngest star that we've ever found okay so it's a beautiful thing to to stare at this is the Alma telescopes if ever you go to the Atacama Desert in Chile at 16,000 feet you will find this array of 66 radio telescopes ok paid for by Europe by the USA and by Japan each of them is 10 or 12 metres across and basically they are used microwaves and because microwaves are penetrating they can peer deep into clouds into the densest clouds and so that's the way we're studying how planets are being born and the most interesting case is this one so this is actually we think an earth-like planet information so here is here is another object in the constellation of Hydra a very young star also this stars a million years old again we see these gaps and rings and then what Alma was able to do as you that those telescopes I showed you that they're movable you can move them further apart and so when you extend the separation to the max and distance in on the Atacama Desert where they have you know the rails all this stuff set up for this that the further apart they are the more resolution you get that gives you it is like parallax right if along the baseline so when they zoomed in at the very central region right and this turns out to be roughly the earth-sun distance now okay so we're looking at the place before the disk I showed you was way beyond you know the orbit of Jupiter or something it was you know just one massive planet but in this case we're looking in at the region where an earth-like it could form and amazingly we see the star here and then on this roughly earth-sun distance scale which is roughly this we see a ring and lumps in the ring which suggests that the planetesimals are at work there acquitting together wait another few hundred thousand years ago they'll all merge and just leave one you know it's like you know the rich get richer the poor get poorer the biggest one is going to dominate eventually and that'll be the earth-like twin that we've been looking for for so long you know information so it's you know you know planets are forming everywhere and this just remember this is just a star only a couple of hundred light years away from us that's why we can resolve it in so much detail our galaxy is teeming with hundreds of billions of stars and presumably hundreds of billions of planets and who knows how many what fraction of those will be at the right sort of earth-sun distance to be so called earth-like planets and you know with all sorts of interesting consequences which we are just beginning to explore ok so here's an artist's conception so this is something that we cannot quite resolve yet with our biggest telescopes so the artist then from the Keck Observatory said well ok so here we have a young star and we have these disks of debris around it and they're going to be in this early stage of aggregation running into each other and they eventually will coalesce and the biggest ones in each orbit will dominate and be planets and then it's interesting so at the outer part it's very very cold a long way from so near that the young star it's fairly hot and only the the most heat resistance heat was in material survive ok that is if you have ice that will just evaporate right volatilize so if you go far enough away you have icy debris if you're close you have you know rock like debris so this is sort of why the earth is a rock like planet so is Mars so is Venus but when you get to Saturn Jupiter Neptune arian so cold that you have you know most the planet is basically volatiles gas and ice okay for another Pluto for example - so there is a rocky core inside but it's a fairly small one whereas opposite roof or the other plate anywhere that so that's the artist you okay so why is it that we're so sure that stars can be aged dated from how bright they burn so the bright how about how bright they shine so the brighter they are the more they use up their energy supply and the less time they have to live now the big discovery after Ellington conjectured that stars had to be there and I didn't really explain why that had to be but the reason was that stars of this continuing battle between pressure and gravity and there's a sweet spot as it were where the two can balance each other and Eddington realized that and that's how he conjectured that that was a natural place to be but let's now imagine that we have a series of possible masses of stars it turns out that the supply of fuel comes from nuclear reactions it's thermonuclear fusion basically which power stars and this was realized in the 1930s 1940s has studied the most each other then adding they didn't actually know that but nevertheless he conjectured that stars but anyway so we know now what the fuel supply is we've been tested that by matching the debris from the reactions with observations we've actually measured neutrinos as ghostly particles produced only by nuclear reactions from the Sun proving the Sun really is burning by thermonuclear fusion okay the same reactions basically the power of hydrogen bomb now someday we'll will master that on the earth and have a new generation of energy reactors powered by fusion the beauty of that will be in the future in a centuries time alone no doubt is that we'll be using hydrogen as our fuel just as stars is hydrogen and there's a huge supplier that in the ocean so we will not ever run out of fuel and what is more by sticking with hard as your fuel that's really actively a very very clean supply because you're not making anything heavy like uranium or whatever the isotopes that are toxic okay anyway so that's by the way but stars do burn peacefully for the most part of thermonuclear reactions so it turns out though that the energy the star loses while its fuel supply just depends how much massive s there are massive stars are more fuel it's a very prolific prolific user of fuel and the amount of fuel it uses because roughly is the cube of the mess so it when solar mass star will burn a certain amount like the Sun does but a star that's ten times the mass of our Sun will burn and shine a thousand times more than the Sun does it use up its fuel a thousand times or so we know just from the simple arguments that balance gravity and pressure that stars more massive ones to shine much more and that's how we infer their masses actually and but we now can estimate how long they will live because they've run out of fuel and that's why we're sure that say a star that's thirty times the mass of Sun will only survive a million years the Sun will keep on going for ten billion years okay it's we've got a long future we're not going to you know you may worry what will happen to the earth in you know when the Sun does die I'll explain that but we do have a long way to go okay so in billions of years who knows what may happen on the earth we have a you know a long way ahead of us this sort of thing does make you think for a bit the Sun itself is halfway along only five billion years old for 4.6 actually but there are other stars around us that are a little bit less massive than the Sun that were formed long before the Sun that maybe billions of years older than our Sun several billion years older even and if they have planets you know that leaves you all sorts of conjectures about what they may you know what state they may be in if they are planetary inhabitants did care too much about climate change er if they did whatever anyway so that's another story but anyway so a million years is a tiny instant at the time of the Milky Way and we're seeing stars come and go all the time and that's the beauty of these um infrared telescopes and we should see now stars not just being born but stars dying so I've talked about star birth but now let me tell you a bit about star death okay so fortunately our son is going to die what I will call a gentle death now it may not seem so gentle in some of you in a moment but anyway but there are stars that will explode and have a much more violent death and those are the massive stars with much more mass and so when they go bad bad things can happen okay when these also they feel bad things from the Sun because it's got less mass will die more quiescent eventually okay that's a few billion years ago so this is a forecast really based on studying the populations around us of what will happen to the star in about four billion years time to our Sun in about four billion years time so here is there's a star in in the center which you can see over there I think and that's a very compact star it's called a white dwarf star it's it's what happens when the parent star collapsed ran out of fuel and collapsed and this he'd collapsed it's very compact remnant behind with no ability to get any more energy from it by burning but at the same time the outer parts all expanded into what we call a planetary nebula a new planets but we these are beautiful objects that are the debris from this dying star and to give you some feeling for the scale I mean around this central star there'll be planets and they will be overtaken by this debris and impacted now that in itself is not the worst of it the worst thing would have happened before then but let's just look at one or two more cases of white dwarf so here's a pure example where as this star the star that preceded this one aged it went through a series of puffing out you know shells of gas as gradually erratically you know bits of fuel came in and from the other star was a little bit spinning and beating you know that dying not in a regular way but you know panting as it were puffing and panting and going through periods of convulsion if you like and so eventually you know these are the remnants of all there and this very compact star we call it a white dwarf is what's left behind okay so that's the sort of thing and another beautiful example of so some of the deaths of stars are slightly more are more gentle than that and then this is an example of a star that again has left its white dwarf behind and here's all the outer material you know maybe a tenth of the mass that once was in the star maybe more is being blown out as a shell and this is the debris that will pollute other clouds so what is interesting is one of the most important elements is response for life on the earth is carbon and we believe strongly the stars like this results for producing carbon so this nebula is is often rich with carbon and that carbon get mixed into the gas there's a hydrogen from then new stars will form and which would be more carbon rich and eventually be able to have the right ingredients you know for life has developed in the earth for example but before then it would have been rather hard okay now let's turn to the massive counterparts those are stars that were a horde of roughly you know one or twice that the size that the mass of the Sun okay and I should say that you know the bad news for the earth would be of course at these these remnants the hot gas that that comes out from the star as it dies will overtake the earth and burn it up basically burn up the atmosphere anyway maybe not the rocky core of the earth and so the earth if you think we're doing bad things to climate change imagine what a dying the dying Sun will do but that is a long time away so presumably we'll have solved that solution or left the earth or whatever you know that's four billion years in the future so okay anyway so let's now turn to it more short-lived stars massive star $30 by the Sun and as it dies and what happened so first of all it you know it began with pure hydrogen it burnt the hardened helium by thermonuclear fusion that went round of helium it would shrink a bit and then it would start burning the helium and the helium burns into carbon but then as it runs out more makes nitrogen eventually makes iron that iron has the property of being the most stable of all the elements once you've made iron you can't get any more energy out of it by nuclear burning that's it okay it's the ultimate it's like a the stars turned into a gigantic slag heap if you were okay but a very very compact ball of iron okay and so that's what's left but in the meantime so much energy is released at the end but its final collapse that there's a huge explosion and that we call a supernova explosion and it's from the those ashes all these other stuff mostly the heavier elements the nitrogen and the heavier stuff you know end up being very important for again producing all the most material solid material the solar system for example and so these are different stages that you might see for this massive star so here is a massive star before it explodes and it goes through a phase when it it's a bit unstable and again you see this ionizing globe a shell of material is ejected okay and these are these objects beautiful things are photograph on the sky so we see many of them their particular name or phrase does and here's another star that we think is going to be a supernova very soon okay because it really is going through you know unstable phases we see varying from year to year the weird things going on and there's a massive star in the middle and it's already been ejecting lots of gas but we think one day it's going to go boom and that'll be it for that one anytime you know it's it's thousands of light years away so there's no immediate problem for us but and so here again is a beautiful case of a star after the explosion so in the center you see the iron object that's left behind what is that it's gotten so compact and we call that a neutron star it's even smaller than the white glogg it's as though the whole star has shrunk into basically you know the size of London actually okay or even a tenth of that okay it's it's the miles across that star and but that's the realm of the explosion and because this is shrunk to such as it's such small size unlike the white dwarfs a thousand times bigger where you get what I call gentle energy here it's violent you're shrunk so much so much energy comes out but that's that's the explosion and that causes this rapidly moving nebula which then expands and we can follow the progress of that nebula for a long time by looking at objects in the sky there are different stages of their evolution so puris exact examples so these are some of them again beautiful things in the sky so these are supernovae the remnants of supernovae and these were seen some of them on the earth let me take you through the history of this the Crab Nebula okay seen by Chinese I'm not sure I call them astrology astronomers may be the strategy in those days or related things whatever but they saw a star come and go over the course of less than a year and when we look in the same place and they were good enough to give us the way to tell us where to look okay in the ancient Chinese records lo and behold one sees a nebula expanding in a thousand clubs a second okay and if you follow that back in time bingo 1054 ad that's where it came from and in the center we've discovered a neutron star spinning rapidly we call those pulsars okay because they beam radio waves at us here's another one found by Tycho Brahe a first seen in 1572 this is how it looks today as seen with an x-ray satellite and telescope and and there's this expanding nebula nebulosity with his very hot shell we made a glowing in x-rays around it this is an infrared view of the crab saying that lots of dust is being created and in this explosion - that's where some of the dust comes in neat form in the universe and another famous astronomer Kepler Johannes Kepler also found one about thirty years after Tycho Brahe hey and this is the remnant of his supernova seen today magnificent glowing in x-rays because they're expanding so rapidly and and then if you go back to the earliest one that we've found again in Chinese records I think there's there's some arguments for this not as good as for the crab this is the remnant but they did tell us where to look and this is where we look now in 386 ad and that's as you see it today so that's that's wonderful not everyone has been seen though this is this is another one in Cassiopeia which you can date from the expansion as being three hundred years old there are three hundred years ago that you know 1717 or whatever but nobody reported it it's amazing this should have been you know brighter than Venus whatever and lastly just a you know a few months but there's no record of any human record of anything of this being reported so that's a one of the big puzzles well as we hear this is an x-ray view of of Cassiopeia A which is a very bright radio source - that's what he means there okay so those are hundreds of a thousand years ago that we see lots more over along the timescale because more stars exploded so we can see stars explosions remnants of supernovae that are ten thousand years old and here are a few of them and they're beautiful things so this is you know hundreds of light years across here this is called the Cygnus loop so it's glowing in the optical and if you look at part of this in the x-rays it looks really you know just expanding shockwaves basically heating up that they get the gas around them and there's a similar image in the ultraviolet of all this rich material in here so so these are interesting objects again here are some more of them after 10,000 years so this is the relic of a massive star that exploded ten thousand years ago there's not much left behind but if you look at it in the x-rays typically you can begin to pick something up or in the ultraviolet light so this is one in the V in the velar conservation you see and all these little rings are bits of debris from the massive star doing stuff before the explosion ejecting pulses of bit of its material and then the huge blast wave from the supernova running into it okay and that's why the shot you see shocks curving around these things okay and then but you know not not all stars die again in this more you know chaotic way here's one that clearly left a very smooth environment around it and this is the remnant that we see now an almost perfectly spherical shell here's another old one too okay okay so let me tell you briefly the idea behind edingtons conjecture I just want to explain this a little more than I did before so Eddington's told us that clouds fragment and break up as they shrink down so this shows you the the history of again what jeans James James defined as the critical mass to break up into stars so genes get gave that and Eddington ran with it okay and so so the two between them they realized that as the cloud got denser and denser as it shrank the critical mass for it to break up that James claimed was when you ran out of pressure got smaller and smaller as you get denser and denser but then then basically Eddington realized that at some point the cloud would begin to get her pay he couldn't let its radiation out and radiation pressure builds up and then the cloud has to stop fragmenting okay so that was the idea and basically from the sort of arguments simple arguments not so different Millington you can figure out what it is that it takes you know is there some special value this might have any and you can show I mean anything conjecture was the balance of radiation and mass that limited the mass of the star but you can you can show in a more much simpler way that the minimum mass here just depends on fundamental constants and turns out to be a fraction of the mass of the Sun with almost no assumptions whatsoever so this cloud Aladin should break up into small bits unfortunately that's not the right answer for making stars now we see these small bits they might be asteroids or planet or something and free-floating ones perhaps but they're not stars and the only way we make stars is because stuff falls into them okay and they grow so you make the cloud breaks up it's a bit like Humpty Dumpty and putting Humpty Dumpty back together again so the cloud builds up by what we call accretion okay it's that's the combination and the accretion then you ask why on earth is the cloud built up and something get enormously and you only massive stars why solar-type stars left behind and the answer again is another bit of physics and this just shows you how commonly the whole story is is to do with magnetic fields so let me show you how that works magnetic fields are everywhere in the universe and in particular they act like sources of tension okay and if you wind up a magnetic field it resists gravity so here is fields permeating this cloud done from a numerical simulation and the fields twist up and that stops the collapse eventually okay and it gives you so much energy as you compress a field that actually generates outbursts okay and indeed this is an example of a of a young star and it's actually shedding stuff okay so let me I'm going to show you more examples of that in a second so here is what we call a star a very young starts no not to be powered by nuclear reactions yet we think it's a million years old and apparently shedding all this stuff so this star has stopped its accretion this is why stars end up being most of them just like the Sun in mass a few resist this and get to much higher masses and this is the sort of shedding that that's happening and the shedding process is very exciting because it happens everywhere we see young stars and so this is critical for understanding the structures of the nebulae and why not everything formed stars where there's so much gas around so we hear examples of two star-forming nebula and all of these projections you see are rejected from from the young stars like these they eject stuff because of the winding up of the fields and that's what limits the massive stars so that's and again a great discovery from peering into these clouds with the big infrared telescopes okay and in this coming back to the Carina Nebula here's a close-up one of the most beautiful of all the space telescope images you can see all these jets and things where you're spatially from the stars being formed that are stopping too much gas occurring so this is a great great these images a great inspiration for a variety I'm sure and the other side of stories that one another way we see this energy from the young stars is their x-ray sources - so here is an x-ray telescope launched in 1999 still it's still taking data x-rays are hard things to focus right but nevertheless you can design x-ray telescopes and they can work and these red things are x-rays which can penetrate leasco can see them from young stars so lots of hot young stars blowing accreting star blowing stuff out energizing themselves and being and so that that'll proves that you know there's a whole wealth of things going on in the nebulae that make young stars ok so at the end of the day we're left with this a rich field of stars young stars all stars everything when the dust will be blown away and and they stars not alone and many of them like to forming clusters here's a pure example of a globular star cluster several hundred these around the Milky Way in the Milky Way galaxy with each one with millions of stars in mostly old stars here the all the younger stars are long since dead so this is only the stars that are as old as the Sun basically okay so final point is why on earth the clouds collapse okay why do they grow I said gravity does it all but what how it why do they get to this critical point so the idea is that the clouds move around the galaxy in circles so here's an image of our galaxy a real image and you can't see much though we've reconstructed a map of the galaxy from three-dimensional tomography or of this image of looking at the stars getting their distances and you learned that in the center of the galaxy there's a huge bar shaped theory of stars and the other stars are in orbits going around what we call the spiral arms okay now the stars go in circular orbits basically it's a rotating disk but as this bar which swings around it perturbs the orbits of the stars and basically it's like creating congestion on the highway okay you're causing some size to slow down others to speed up your your deviations from from the circular paths and in this cosmic traffic jam the clouds collide then they coalesce together and they've got massive enough to make stars and what is more because the outer parts of the galaxy spin have softly the same rotation as the inner part that means in in the inside the speeds the same that the clouds can orbit more rapidly the things wind up into a spiral structure so that's where we get these beautiful spiral arms that this map shows and we see in galaxies like this so these are basically this is thought to be the twin of our galaxy more or less and this is something similar seen from a different perspective the Andromeda galaxy and these are where the young stars are forming because of the clouds that have collided and this is the same sort of thing happening with a different perspective so there you have the story I've taken you from star birth to star death and back to formation and back to the whole grand universe full of each galaxy has 100 billion stars now universe has so far seen through the biggest telescopes properly has you know a hundred billion galaxies similar mass to these so it it makes us just wonder what all the potential of this will be for the next generations of astronomers there'll be many things to discover so thank you [Applause] so I think I've time for a few questions okay so I think you should wait for the microphone do we know what the mechanism will serve the very very first generation stars after the Big Bang because there were no seeds star formation so that's a good question so how do you make the first stars if you don't have these then this carbon or whatever to help you make molecules that can cool down atoms that can help the nebula cool down so the answer is it's difficult but you do have one what do have is abundant hydrogen that's for sure and the hydrogen will occasionally form molecules of hydrogen and so in the first clouds there are some hydrogen molecules no hydrogen molecule it's two atoms but just like a hardly atom which has electrons in different energy states and if you can excite it by colliding with another atom then the energy state gets lowered as the electron falls down you've raided energy the same thing happens molecules because they can also vibrate they can spend they have different energy levels so again if when they collide with atoms they were very excited and they're and they were radiated and their radiation is in the infrared and escapes really from the first cloud so we think that's the trick the first clouds have and and the first stars and the interesting consequence of that is that because these hydrogen molecules the first molecules are not very good very effective at cooling they do call the first clouds that make the first stars are watch warmer than today's clouds and so that means they they will they will make predominance of stars so we think the first generation stars was stars maybe a hundred times the mass of the Sun from these simple arguments so that's how we think started yeah so you mentioned the formation of elements in in stars but you didn't go on to talk about supernovas my understanding is that the heavier elements were formed in supernova explosions is that correct and are we gonna see more elements so that's absolutely right so in the supernova you have elements like silicon and oxygen which stay in shells of gas around when the center of the star collapses and they will get a big explosion so the supernova remnants are rich in these elements they're very important and so they're made in the course of evolution of a massive star the low mass star tends to finish its life time before it's had much time to make these elements a supernova has time to do it and then everything gets ejected and you could even in a more complicated way make the rare elements the heavier elements because some of the stuff during the explosion falls back onto this central very very hot you know neutron star that's forming and you get extra reactions that can even take you further up the Ferranti table so we think everything in principle comes from a combination of stars like the party nebulae and supernovae between the two one can explain most of what we see it's not a proof exactly but the model seems to hang together maybe we're time for one more question we're running out here do we know okay so the questions how long after the Big Bang was the first light yeah okay so the we refer to something called the dark ages which is when there were no stars at all and that was something like began or maybe I should say ended about a billion years ago so essentially you know thirteen or fourteen billion years after the Big Bang the first stars are made and that was the age of light if you like many more stars are made after that and that is as far as we can tell at the end of it we do beginning to see the dark ages whether a very few stars so we think the story altogether so I think we're out of time thank you very much and I look forward to meeting you in a month [Applause]

Channel: Gresham College

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Length: 55min 2sec (3302 seconds)

Published: Tue Oct 03 2017