The Lives of Stars - Professor Carolin Crawford

well good afternoon everyone oh great we feel like a teacher I do respond like that all right today's talk is about the most easily observable objects in the night sky in other words the Stars and also easily observable in the day given that the nearest example of a star of course is our Sun and all the stars you see in the night skies are Suns just like our own and they're all the same entity in the sense that they are enormous spheres of burning gas producing heat producing light and as you see their intrinsic properties differ a lot they they're different size and mass and temperature but they're all the same basic thing an object is a star while it's producing that energy to counteract gravity because the whole of the star's life is a battle against gravity the gravity of the mass of the star wants to squeeze it it was to collapse it in and the star resists that by producing energy in an outwards pressure at its core in this case from nuclear fusion it's burning elements to heavier elements but as long as it can produce that fuel in the core it can withstand the collapse of gravity and for that time it's known as a star but stars have lives they do change obviously on timescales much greater than human lifetime so to us they seem eternal everlasting but nonetheless even now stars are being born throughout the galaxy stars are dying throughout the galaxy through other galaxies as well so stars do have lives and galaxies are sort of dynamic places things are changing all the time in terms of these populations of stars but if we want to start off with some basic observations you'd be surprised how much you can learn about stars just from fundamental observations you you can get with your na to die for example the start one that everybody notices that not all stars are the same brightness now obviously that's related to their distance away from us and I'll come back to that but you still have variation brightness of stars not just that they're not uniformly distributed us the sky they make patterns there are clumps they are fairly randomly distributed and also stars are different colors from each other we know the Sun especially observe it from not within our atmosphere but out in space it's as a yellow a white color but there are blue stars out there gold red stars brown stars you also have very blue white stars you can't see this so well with your na to die if you're familiar the constellation of Orion you about your best bet for seeing an orangey red star is Betelgeuse at its top shoulder and you could compare that with Rigel Derna Disney and the two stars are very very different colors Rigel the blue-white Betelgeuse is that orangey red in general you have to rely on images digital images perhaps like this because the colors are quite subtle and you don't get enough light from a star to for it to certify the visual receptors in your eye but a lot of the pictures it comes apparent that these colors are very very different from each other so just these basic observations as I'll show you allow us to infer a lot of properties fundamental properties about stars and what we discover is that our Sun it's a fairly average and quite dull storming that's actually a good thing but it's very kind of average and there are many more extremes of behavior out there so for example stars varying brightness they can be one ten thousandth the brightness of our Sun up to a million times more luminous in terms of temperature our Sun is about and I'm talking surface temperature the bit of the star you actually seen we get the light from in our Sun it's about 5,600 degrees on the surface but they range from cool dim stars at about 3,000 degrees to hot blue stars that are about 30,000 degrees sizes also very enorm enormous ly our Suns about 1.4 million kilometers in diameter you can go down to admittedly a very extreme star called a neutron star there's only 10 kilometers in diameter but you can also go up to stars that over the order two billion kilometers across that's about 1400 times wider than the Sun enormous objects masses very there is limit to you know the smallest mass you can get is about 1/10 out of the Sun after you know for various reasons it doesn't become a star unless it gets over that threshold and similarly stars only survive up to about a mass of about a hundred times the Sun so there's quite a narrow bass range in terms of ages some stars are just being born they're less than a million years old others have existed for almost as long as the universe has existed so you're looking at ages of eleven to twelve billion years old and chemical composition some stars are primordial mainly hydrogen healing some have heavier elements mixed in with that so again there's a slight difference in composition and finally also environment our Sun is relatively unusual in being quite so isolated as it is many other stars are distributed differently and that's to do with their formation as well some form binary systems where one star goes around the other and many of the stars we see in the night sky our binary systems some are even quadruple or sextuplet systems there are many combinations of tiny groups of stars moving on up the scale you can also have what we call open clusters so these are loose clusters of stars there's a nice example here where you have perhaps a few tens to maybe a hundred stars that are all as you discover about the same age about the same composition and they just live out their lives within this group and then you could get the intense swarms of what's known as a globular cluster where you have hundreds of thousands of stars all living their lives alongside and in these kind of environments some of the processes of stellar development I'm going to be talking about can be a little bit accelerated so you see there's this this huge range of properties so let's just start off with what you can observe and what you can infer from what you observe and where that takes us because all of these features reveal fundamental properties about those stars which then dictate that they go on and live very different lives so to start with the brightness of stars now I know that these stars that form up feed the plow are all about 100 light years away they happen to be bright because they're relatively nearby realized and you lighters may not sound nearby but in terms of our galaxy it is and a lot of the other stars you see in the background a fainter because they're further away so you have this strong you can see a variation of brightness and if you know the distance to the star you can work out intrinsically how luminous the star is and of course it drops off as the square of the distance if you have the light from a star it gets all that light emitted from the surface of the star gets diluted over increasingly large spheres of space so for example the luminosity the star is given out through an area 4pi say d squared at at this particular distance but if you move out twice as far distance to twice the distance the whole area of the sphere that light spread out with has gone up by a factor of four if you move out to three times further the area's gone up by a factor of nine so it drops off pretty rapidly but if you can work out for various means and that's going to be my talk in April where we talk about distances to stars I'm going to put it on one side for the minute but if you know the distance to the star the flux is what you measure of the brightness of the star you can infer its intrinsic luminosity so different stars let's say you can do that you can work out the different brightnesses the real brightnesses of stars when this was originally done we didn't have telescopes we didn't have cameras we didn't have photographs and astronomers use is very archaic scale so if any of you are amateur astronomers or have read and think that amateur astronomer you see we talk about brightnesses in terms of magnitudes and it's worth just telling you a bit about magnitudes because this was developed in the second century BC by this Greek chap Hipparchus and he put the the most luminous stars the brightest star in the night sky he said they were first magnitude and then the very faintest ones he could see was six magnitude and then he faced all the other stars in between according to their brightness so you just have a ranking the brightest and the faintest stars you can see with the unaided eye so that's a good system but it's limited to what he at the time could see with his eye we still use this system but of course because that very archaic start is developed to something that's quite a little bit cumbersome can be a little bit confusing to begin with so for example if you lose binoculars and you have a clear night you could probably go down to a magnitude 10 when this scale was started he decided a certain star was this first magnitude the brightest star but actually there are other stars that are brighter to include those in the scale you have to go through 0 and up to negative magnitudes so for example Sirius the brightest star in the sky is minus 1 in magnitude if you want to look at Jupiter in that very bright and light sky at the minute that's minus 2 and of course you want to go to the moon in the Sun they kind of way off the scale simile fainter objects that we now see with very powerful telescopes we talked about going to a magnitude of 27 20 25 up to 30 so you have a very strange scale in that it goes the wrong way around greater magnitude means fainter negative magnitude means bright the other thing is an example of a logarithmic scale the visual receptors in your eyes don't work linearly so say difference between one magnitude and another magnitude is to drop in flux of about a factor of two and a half there's an object that's five magnitudes fainter it's a hundred magnitudes and there is a hundred times less flux so it's a logarithmic scale but we still go and despite the fact that this dates from Greek times this is the scale that all astronomers use to refer to the brightness of objects but it's still an observed brightness and so we need to change it all to something called apparent magnitude which is where you basically say how bright what magnitude that star would be if you put it at a distance of not entirely an arbitrary distance of thirty two point six light-years and at that point you've got all your stars and so we turn it into again an intrinsic a real luminosity so you can measure brightness we know how bright the stars are this is complicated a bit by the fact I just told you they're different color and the color with star is related to his temperature the color star radiates its most it's like most strongly in depends on its temperature because it's a basically thermal emission it's due to the heat of the body and if you distribute the colors that come off from bodies at different temperatures it follows what's known as a blackbody spectrum so here for example this is a spectrum so you've got the color if you like wavelength of the light given off by the body against the brightness and you can see you have this at a regular family of curves and you see where the peak goes so here it's very blue for a very hot object it moves into more visible for a cooler object and a very very cold object in terms of stars 3,000 degrees the pitt peaks in the red and so you don't just quote a magnitude you have to say what color that magnitude is measured in so it starts to get more complicated already so very hot blue stars you know we can say the temperature is about 12,000 degrees stars like the Sun again 6000 they peak in the visible spectrum cold dim red stars peak perhaps just in more towards the foreign for the near-infrared but this allows us to measure the temperature if you look at two bands in the visible you take pictures and you say okay I'm going to compare the light in a star maybe at this kind of blue band and maybe this kind of yellow band you can see there's a big difference from there to there the ratio of those two is very different from so the ratio of these there's a big change between them here there's a small change and here the change goes the other way so if you had just have the the brightness and start in two different color bands you have a measure of its temperature so the color gives you the temperature if you ever see anything about color index all it means is the very show if you like between the colors in two different bands this is say between four thousand so 400 nanometers to about five hundred nanometers and you can see hot blue things versus red things you can discriminate color therefore you can discriminate cut and the temperature so here you have fundamental properties of star its luminosity and it's ten just from the simplest observations from just taking images of the night sky originally again from just naked eye observations so where does that get you you've got two things that you can measure but after that you have to start being a bit cleverer but what you can get from those two points is you can get a measure of the size of the star now size of a star is very difficult to determine they're all a long way away nearly all of them even through the biggest telescopes appear as point sources of light there are a few exceptions here's a picture of beatlejuice beatlejuice you can resolve the disk of the star and I'll talk about what all this rest of this governs is in a minute but you can get the size of the star just because it is a phenomenally big star and it's relatively close at only 650 light-years away from it so fortuitously we can actually see the surface of Betelgeuse we can even resolve structures on the surface you know like star spots but because uses very much the exception even with the most powerful telescopes that nearly all the other stars appear as point sources of light so to get around this you use the luminosity in the temperature so the lumen I'll see if you plot that the well if you look at the equation that gives you that blackbody spectrum you find it's this now don't frighten anybody doesn't like equations very simple for pi r-squared is the surface area of a sphere so that's the surface area of the star and it radiates at this temperature T to give off this luminosity L so it's effectively you know what temperature this area is radiating at related by Sigma is just a constant here that gives you the intrinsic luminosity of the star so I've already told you from the brightness and the color we can measure the luminosity in the temperature we know Sigma that's just a constant therefore you for every luminosity and temperature you have a unique radius for that star and so immediately you know the sizes of stars from this to observations and the way it goes if you look at the proportionality if luminosity was fixed if you increase the temperature then to keep luminosity constant as the radius has to decrease so hotter stars are smaller simly you could say well okay let's keep the temperature fixed and look at the way luminosity and radiance very well the more luminous of star and it's got fixed temperature it has to be radiating out through a bigger radius and so more luminous stars are larger and from that you get three fundamental properties luminosity temperature and its size so where does this take us if we want to compare stars and obviously we want to go and do that and see how they live out different lives maybe you want to compare those intrinsic properties and we do this in what's known as the hertzsprung-russell diagram after a couple of a couple of astronomers who first thought through this process and there are two versions of this we just lost it slightly off the end here but this is basically temperature versus luminosity so you can see these plots either in terms of color versus magnitude which are the things you observe or when they've been translated to the real properties of the stars in other words the temperature versus the luminosity and if you just plot loads of stars observations of all stars into this diagram their luminosity temperature and remember for every one there's a unique radius you find they're not scattered throughout the plot but they concentrate and very different and very specific areas of this plot and this is telling us something about what stars do during their lives so for example 90% of stars fall on what's known as the main sequence which goes from hot blue stars down to cool red dim stars and then perhaps there are other smatterings out to this side and this side off to the right and down here now what these regions are telling us they're telling us where stars spend most of their lives okay you can't watch an individual star their lifetimes are much longer than human lifetimes you don't watch it change either in its luminosity is temperature any of these properties because it's happening on much longer timescales than we can see however if you look at a whole ensemble of stars where most of them congregate although they kind of where they have the properties shows you that where they'd spend most of their lives regions that are sparsely populated so like the gaps here or here are where they move through those regions or then if they ever have that combination luminosity in temperature it's only for a very short while and so sparsa regions out here they don't tend to spend so much of their lives but it's still a good fraction of their life so again how this is populated tells us where the relative length of their lifetime in these different areas and it also tells us remember about the sizes of the stars and this is what gives the name two different kinds of stars so remember we were doing with that luminosity temperature relation we can apply it to here so remember if we fix a luminosity a hotter star is smaller than a cooler star or we do it the other way around if you fix a temperature the more luminous star up here is larger than the last luminous and so that is immediately telling you you have a gradation that these are small stars and these are big stars and so this lends other names that you may have heard the stars you have supergiant's the very biggest you have giant stars and in the opposite end you have things called white dwarfs they're hot and they're small and these are all key periods within a star's life especially a star like our Sun and - when I'm talking about giant soup jar let's just show how big these are this is a lovely movie that goes around a new tube which is starting off from the moon and working up in size through the planets and so you've got the four rocky planets and then of course it's a big step to the gas giants of our solar system you know remember you could fit 1,300 earths inside inside Jupiter the biggest planet so that's the relative scale of those two but you would still need a thousand Jupiter's to fit inside the Sun and I'm telling you the Sun is a very average kind of star so when I'm talking about giant stars you're looking at things that are ten to a hundred times wider than the Sun but they aren't the biggest stars you get supergiant so now we have a class called hyper Jones when you're looking up to a thousand time to the diameter of the Sun and this is just progressing on up and we're moving up to the largest known star it could be that there's one even bigger but this is the largest one we know about and it's called V Y Canis Majoris and it's going to be the next star and it's a red hyper giant now we're just zoomed back and you'll see the size of Earth in comparison to this star you know it's going to be small yeah I bet even a challenge to see that right from the back so I mean it just gives you an idea that wouldn't be calling them super giant and giant they really are massive you think talking things 2 billion kilometers across so we've got color we've got size we've got temperature now we've got to get a bit crafty another key parameter about a star you know fundamental property is its mass and a mass is something that's difficult to measure you don't go out and just observe the mass of a star you can observe its gravitational attraction and that's when it's useful if you've got stars in a binary system you can see the gravitational effect of one on the other and you can start to measure masses of different kind of stars and so you have to get the mass by indirect means but if you put that as a property on top of this color magnitude diagram you find there's a very clear correlation of mass with this main sequence if I just put the masses on you find the most massive stars here are the hot blue ones the Sun is about here midway down the main sequence and the cool red stars go down to about a tenth of the solar mass now the reason you've got these kind of boundaries of 1/10 to 100 times the solar mass are because if it's very tiny you don't you don't have enough gravitational compression to squeeze the certain center of the star to temperatures where you can actually initiate nuclear fusion so below that you get objects called brown dwarfs will sit between Jupiter's and start and you know cool dim stars and you haven't got nuclear missions so it's not nuclear fusion so it's not quite a star it's not quite a planet above 1/10 solar mass you've get enough temperature in the core that you've got nuclear fusion and then it like a star but at the other end of the scale you can't just have an infinitely big infinitely massive star because stars produce light and light has pressure if you have a hundred solar mass star you can see is going to be very hot it's going to be very luminous just continuing this trend and the light is going there's going to be so much pressure from that light that it ends up pulling the star apart so it's that difficult balance between too much light at the center versus the gravity and we reckon above 100 120 solar masses stars are not stable they can't exist at those masses so generally go from a tenth to 100 solar masses and the line follows the line of the main sequence this makes sense if you have a very massive star it's got more mass it's got more gravity squeezing on it it has to produce more energy at its core and it has to produce this energy faster so it burns hotter it burns brighter because it's producing more energy and it has to produce you know even though it's a very massive star to begin with it has to produce so much energy that it goes through its fuel much quicker so those tend to be the short-lived stars you have a cool dim star it doesn't have so much mass it doesn't have to go through its fuel at such a prodigious rate and it burns for a lot longer so not only do you get this distribution of mass down the main sequence you get a distribution of life times so a star like our Sun can last about 10 15 billion years the very hot massive stars have lifetimes of tens of millions of years and even cooler stars maybe are the much older systems so again math is what determines where our star is in the main sequence and math is something that is virtually unchanged through most of the lifetime of a star may forget stellar winds forget you know the final throes where it maybe will lose mass very rapidly for most of the lifetime of a star it's got the same mass so it's start doesn't move up and down the sequence it's just a distribution of behavior and the key thing that you know determines where a star is on that is its mass okay so to the story of stars now I told you they're being born all over the place and they are born from the reservoir of material it is our interstellar medium and there's a lot of matter out there the space between the stars is not completely empty it's filled with gas is filled with atoms it's filled with ionized atoms it's got particle there's small solid particles of dust that I talked about my first year's lecture here and all of this is mixed together and within the interstellar medium you've got different phases so for example in the space between stars there's a lot of hot gas this is an ionized plasma is so hot but big it's very sparse it's very diffuse it's not very dense and so even though it occupies a lot of the volume that isn't so much mass in it and then embedded within that you've got cooler clouds denser clouds now these aren't so big but they tend because they don't so they tend to carry a lot of the mass but they're cold you can't see them unless you're looking in radio wave bands they're giving off radio emission you might see them if they happen to be in proximity to one of these bright blue stars they can ionize the gas atoms and make them appear as these pink nebulae that accompany the blue stars so in terms of the reservoir is the cold denser regions that happen to be the best places for star formation and in particular with embedded within all this complex a very sort of hot gas and cooler gas there's the ones that almost appear Paik its structures like this if I just zoom in and some of these these are regions where the stuff is so condensed I mean obviously nowhere near is condensed as the air you're breathing now this is condensed for the interstellar medium okay but it's still the densest regions the interstellar medium you've got so much dust there that they begin to look a peg and these are very cold clouds they're small clouds and these are the regions where star formation starts happening especially as they tend to get shredded by the light of surrounding stars and they break apart into these small cocoons that will go on and form the stars I would just say I always love this particular cloud always doing a particularly rude hand gesture so one of these little cocoons stars to collapse down maybe it's it's more dense than a regions it's got more gravity gravity pulls things together you've got gravitational contraction conservation of energy within the cloud if things lose gravitational potential energies they fall in their energy goes into heat and so you've got accumulation of stuff towards the center of the cloud or a dense region in the cloud and then it heats up and at a point it reaches 15 18 million degrees that's when nuclear fusion begins and you switch on your protostar I mean of course it's nowhere near that simple it goes through phases where it collapses and then it becomes much denser the dust becomes more opaque it doesn't let the heat of the star out and so the whole cloud heats up you have to wait till the dust is kind of zapped away before radiation can escape and it can carry on collapsing so it's going to go in fits and starts it's not just a quick straight once down to a star it's an elongated process that takes a while to happen and also you've got things like rotation you've got magnetic fields in the cloud which and add to inhibit the star formation but in simplest level you've got gravitational attraction leaving two heating at the core and you end up with a protostar that's still surrounded by a disk of matter and the whole you know time for a star to collapse down to protostar does depend a bit again on the mass of the star it's very difficult to tell of course because all of this is hidden it's not something we can readily observe and map out this is what we think happens is that if you've got a star like our Sun it probably takes ten million years to go from a clump down to a star but if you've got something that's like you know 15 20 times more massive maybe it just takes a couple of hundred thousand years to reach to that stage and there are interesting things that happen I've talked about protoplanetary disks before the material that forms a torus around the star eventually goes on to form a planetary system we see some of these in nearby star formation regions such as the Ryans nebula and this is just an artist's impression where you got the protis star in the middle surrounded by this bigger tape opaque torus of Tyrael but this also funnels some of the material that flows from the star and that protist iron is very early phases produces twin jets that come out of the rotational axis and they blast out probably for a couple of light years into space now again this is a very short-lived period you know maybe a few tens of thousands of the protis stars life and as part of this adjustment working on how much energy it needs to produce at the cord counteract gravity is an unstable situation and one of the net things is that you have these twin flows of gas and they're coming up out of probably sort of tens of thousands what's at hundreds of kilometers per second and when you look at them they're followed along the rotational axis and they indicate that even though you don't see the protests start there's something really exciting going on in that cloud so for example here you've got the twin Jets the protests are responsible is very deep in that cloud there's another one down here I love this particular C these long thin jets and again what's happening the protostar is all enveloped in this cloud we only know what's going on because of these Jets and this is something we do see change in human lifetimes the star protis star is very deep within a kind of shell of material here it's just lighting up this area and then you've got this jet going out and where it impacts on the surrounding interstellar medium you get this kind of knot of glowing gas this called her big hair objects they're quite rare because it is such a short transient part of the star's life but they're very spectacular and then of course the protis star grad she gets rid of this disc material either it settles down to planets and some of it is blasted off out into space from the winds and from the radiation from the star and the stars exposed so you have a point we where it'll start off some around here on your host Brian Russell diagram your color magnitude diagram and they'll follow these tracks which eventually land it right on the mount right part of the main sequence for its particular mass and then it spends most of its life time as a star recognizable star like our Sun on this main sequence it's worth just saying why star formation is still happening all around us I've told you it comes from gravitational collapse of a gas cloud and whether or not a gas cloud collapses depends on a couple of subtle things it's very easy to see that if you have a couple of slight over densities you know imagine a bit one of those sort of cocoons you've got ever such a slight fluctuation of density there compared to there this has got slightly more self-gravity of all the particles in the cloud then maybe other regions and so to gradually pull stuff in and you get those condensations begin to form conversations of mass within the cloud and then because they're slightly denser than the surrounding regions they have more gravity they'll pull in more material and you start getting a runaway process a gravitational collapse which is fine except there are things that stall that and one of those things is that is fine of all the particles in the cloud a completely stationary but they're not they have wolf they have temperature even if there are only a few degrees above absolute zero they're still jiggling and moving and vibrating and that gives them energy that can resist the gravitational collapse and so you know again slight exaggeration but if they're all moving around all the time the hotter they are they're more they're moving because of course temperature is just a measure of this at average kinetic energy of the particles in the cloud so the hotter clouds particles are moving more and they resist the gravitational collapse more so whether or not a star collapses to for a Sariah cloud collapses to form stars depends on this balance between it the gravitational force versus the thermal pressure of the motions in other words density versus temperature so where you're going to form stars preferentially is going to be where it's cold because they're not moving about so much and where it's dense you've got stronger gravity pulling things together and you have those kind of regions so that's why it's those cold dense regions that go on to form the stars so this also answers the question about why some stars are forming even now because if our galaxy is 11 to 12 billion years old there's gas clouds all around is why do they sit there for all this time and only now start to form stars one of these two things has to change density versus temperature it can sit there in an equilibrium State maybe it's slightly too sparse maybe it's slightly too hot to actually go on and full contract down to form stars but it's something then just tips it over and changes one of these properties that's when that'll start it's difficult to change the temperature of a whole cloud especially to make it cooler but you can change the density of a cloud very quickly by just squeezing it so maybe you have a supernova that goes off and that producer you know since when a star explodes it produces a blast wave that compresses the gas in the cloud maybe you got the gravitational pull of another cloud that gets too close to it and that compresses the cloud and so there are many different ways that you can suddenly get a cloud tipped over into forming stars and you see this within the disk of our galaxy where you have vigorous star formation and remember I've said blue stars are the most massive stars they go through their lifetimes quickly so they're the youngest stars is in the spiral arms of the disk and you get these spiral arms not because the blue young stars are the ones that are just rotating around like that instead you've got all the gas in the disk and you have waves of density that come and compress the gas in the disc and then trigger it to form collapses into stars and so you see these blue star clusters marking where the density wave has moved through the disk so be clear these spiral arms are not just due to different rates of rotation of different stars as you go out through the disk if you did that you'd soon find the start with spiral arms would wind up very rapidly the fact we see them as long-lived structures and almost every spiral galaxy means that they're being continually replenished the density wave sweeps through the disk and just triggers star formation in its wake and in terms of what this density web was reducing this compression within a galaxy it's just when material bunches up gets close together and gets squeezed and it's just due to the all bits of gas clouds perhaps around our galaxy so for example here are some orbits around the center the galaxies everything is rotating round we don't necessarily go nicely circular orbits you just have the slightest eccentricity slightly elongated orbits and as things were take round at different rates the axis of these orbits get slightly disturbed or unaligned you begin very rapidly to get a spiral shape where stuff is bunched up and compressed and it forms very naturally the pattern of the stars we see so the spiral waves in our galaxy regions of star formation where you've got this bunching in this star formation triggered and of course one gas cloud doesn't usually just collapse down from one star usually fragments and forms whole pockets of stars so you have star clusters like this double star cluster in Perseus where all these stars in each of these clouds each collapse from one giant cloud at about the same time when you look at a star cluster most of them they form from gas the same chemical composition and they all formed it around the same time how they differ is in their mass what mass star forms from each of those fragments and that then dictates how bright it burns what is temperature is and the length of its lifetime and when you see these stars again in the spiral arms star formation is not 100% efficient you still get they remain to the diffuse gas cloud around the stars that form these nebulae that you see lies in the rosette nebula here being lit up by the light from these stars so again if you see the remnants of the gas cloud around it it again is an indication that there young stars and these ones are about four million years old in the rosette nebula so different star clusters they all form at the same time they have different ages so here for example you have three different star clusters of different ages this one over here is the oldest it's lived sufficiently long that many the stars have gone supernova within it and I'll talk about what that means but they've lived their lives and they've reached the end of their lives you've got other ones that are younger and younger still and it could be that the supernova explosions producing blast waves that then trigger the formation of subsequent star clusters so you have one thing triggering another thing triggering another thing again just pushing things over into that collapse but you can see I'm very definite about aging clusters of stars and it's very easy to do because you take a picture of them in different colors and you plot out their luminosity versus their temperature because when you have that color magnitude diagram whereas star is it depends on it depends on its age whether it's still on that main part of main sequence or it moves off to become another kind of star and it's to do with age remember more massive stars less massive stars these have shorter ages then and shorter lifetimes than these so if you start off with a full range of masses all the way up all the bin sequence is distributed and then after about 10 million years these move off they stop being on the main sequence in fact as you see they're become supergiant or Giants they disappear if the clusters about 10 million 100 million years old these ones are no longer on the main sequence and they disappear and so you have a gradual peeling back of the main sequence so for example here's some theoretical color magnitude diagram again color versus magnitude this is a really young cluster about million years old but as soon as you get to 10 million years old the first ones are evolving developing away from the main sequence and becoming giants and as we step up the time you can see there is some peeling bag and if you have a really old system like some of the globular clusters you see a very different distribution where this is really just completely peeled back so we can use the color magnitude diagram to definitively age stars so here's a fairly young cluster this is about ten million years old there's just one red giant in its midst and you can see it's got the four main sequence of stars here's another one where there are lots of red giants in their midst this is probably about two billion years old and you can see it's very different in terms of the color magnitude distribution okay so what's happening on the main sequence this is when a star has stopped being a protostar and is doing that nuclear fusion and nuclear fusion is the way obtains the energy and just very briefly you've got atomic nuclei I created of protons and neutrons and they're surrounded by this cloud of electrons and different elements have different numbers of protons in the nucleus again long story short but basic idea is that and so what she do in the core of the Sun is that you have four protons and you smash them together through various processes and you end up with a helium nucleus now this has to happen you need a lot of energy to get protons to combine to form a helium nucleus you smash things together at high speed and at high temperature this will only happen right in the core of the Sun where you get in those temperatures well above ten million degrees so it doesn't happen throughout the Sun only right at the center and how you get the energy out well if you measure the mass of four protons compared to your helium nucleus that they make you find that there's a difference in masses only about 0.7% but if that accounts for quite a bit of energy through your mass equivalence relationship equals mc-squared and if every one of these collisions these combinations you release a bit of energy and that goes into supplying the Sun with that outwards energy that's going to let it shine and counteractive and counteract the gravitational collapse and this is going on at prodigious rates at the Sun the Sun burn you know goes through a fuel at you know billions of kilograms a second but of course you know it's a huge object so it's got plenty of fuel and as long as it can do that it stays in the main sequence is turning hydrogen in Helium and as I say that only happens right in the core and a star like our Sun it can spend 10 15 billion years happily turning all that hydrogen into helium right in the core just where its hottest and the outer envelope you don't have the nuclear fusion going on but at some point after 10 15 billion years you run out of sufficient hydrogen at the center there's hot enough and you can no longer withstand the gravity and at that point the outer layers of the star begin to press in they squeeze the star and the core gets compressed it's now mainly in a helium it gets compressed it gets heated up and that heats up another layer outside the core to temperatures such that you can then start burning hydrogen again and you go through these cycles where you've got the inert helium core surrounded by shell of hydrogen and you go through cycles of running that being exhausted the star shrinking the star called just outside the core heating up enough that you can regain hydrogen and it's balanced for a while so you've now moving off the main sequence behavior because the structure of the star begins to change and this is where it becomes a red giant because this helium shell is doesn't go all the way through the envelope but you're directing more energy out into the outer layers of the star they expand into few serwe and you get the red giant and this is where it becomes enormous you you it gets to the scale of you know a red jar like Betelgeuse if you plunk it in the center of our solar system it's a radius would be out around Jupiter again enormous structures and this will happen to our Sun when it revolves off the main sequence and if it's a big enough star a massive enough star it'll perhaps reach a point where it can gets compressed enough in the core it reaches temperatures of a hundred million degrees and you can start burning helium into things like carbon but at some point you know a star that's maybe up to about ten times the mass of our Sun you reach a point where it can no longer keep on being compressed such that other nuclear reactions using the products the previous ones will continue and then it even then stops being a red giant you can no longer carry on with the fusion at the core there are some interesting things that happen once a red giant it's expanded the outer layers have gone further away from the source of energy to their cooler that's why it's redder there was a further away from most of the gravity of the stars they're more loosely bound so red giants so again this is this is going back to Betelgeuse it's surrounded by clouds of material which are billowing away from the surface because they're only tenuously taut the outer envelope to the star and it tenuously tied to it and even on bigger scales of we shrink that down you can begin to see some of the matter that's being just blown away these stellar winds from Betelgeuse and this continues the red giant will eventually lose all of its outer envelope and it does this when you can no longer produce fusion at the core no longer produce energy at the core and at that point gravity wins and it squeezes the core down and that collapses down suddenly to form a white dwarf and the outer layers of the star expand away there is one thing that happens within the red giant you can create heavier elements than the carbon or the helium and you do this by a process of slowly capturing new neutrons so if you've got chemicals or you know heavier elements than helium within the envelope of the gas of the star and that's because they'll have been in the cloud that the star is near collapse from it can accrete some neutrons that are produced from the core of the star within the atomic processes and if you add a neutron you know here's your bog-standard element you add a neutron after a while that neutron decays it's called beta decay to form proton an electron so the minute you just increase the number of where you've got an atomic number of that's number of protons you've increased the mass but you haven't changed the nature of the element but once that decays a way to form a proton and electron and the electron is kept you've increased the atomic number of protons within their element you've changed the element to a new one and this is a very slow process but nonetheless within the envelopes of red giants you can create some of these heavier elements but then it's going to puff away all the outer layers and this is when you get these dramatic structures known as planetary nebula nothing to do with planets it just means that when they were first seen they were kind of disliked and I had sort of a greenish shoe reminded the Victorian astronomers of the newly discovered planet Uranus and the center of the white dwarf so the center of the star forms this white dwarf which is now exposed and that is held up by electron degeneracy pressure in other ways stuff is really compressed if you compress electrons to a certain state they don't like being squeezed anymore and they can resist it and they can resist the gravity of this white dwarf and it's again compressed down it's heated up and it's sending out lots of I mean it's a temperatures of a few tens of thousands of degrees it will gradually cool but for you know over a few billion years it's still going to be hot and radiates it excites the gas atoms in this envelope of material is going to expand away over the next few tens of thousands of years so the outer layers the star disappear into space often in very dramatic structures they can form shells this is the spirograph nebula yeah different structures or you have the Eskimo know below those his head this is Parker hood and so you get winds you get the cat's eye nebula where you've got jets and winds and it's not just one eruption of material from the surface it's given off different eruptive shells you know every like 1,500 years and this one you shrink that right down in a much bigger scale you can see that it's surrounded by much larger structure and thousands of years before so red giants go through this process and stages they puff off material for a while before they finally get rid of it all and collapse down to form the white dwarf and also it's worth mentioning that some of that early material that's given off can govern the distribution of the later expanding stuff so you get this bimodal and it tree nebula here's a good example you've got a disk of dust that's been given off by the red giant when it's in that Beetlejuice phase it's kind of losing matter and then the subsequent outer layers of the star expand away and they're focused in two direction so you get these fantastic structures from stars like our Sun when they reach the end of their lives now of course if you have a more massive star it can it's got it's got to reduce energy faster and if you got more mass it's hotter in the center and you can burn hydrogen to helium through a much more efficient process which is a carbon nitrogen oxygen as as catalysts so the net result is you're producing a lot more energy it's much more efficient you can hold up the mass of the star but it goes through its lifetime that much more quickly now you go at the end of the main sequence you will go the same thing it would turn to a red giant and you'll get an inert helium core surrounded by hydrogen shell burning but of course because it's got more mass it'll reach higher temperatures in the course a helium burning will be commenced to form and carbon and then if it's a big enough star so between 10 and 50 times the mass of the Sun you're going to go through these cycles compression and heating and every subsequent inert core then is used as fuel for the next set of nuclear reactions and you're building up subsequently heavier elements in its core and until if you have a very massive star it gets to such temperatures at the core that you can actually form silicon to iron now these all happen faster the processes are gradually less efficient in in terms of the energy release so they have to happen a lot faster and so the lifetimes in each of these phases it may be you know 10 million years in the main sequence hydrogen to helium but by the time you turn in silicon to iron it's only about a week doing that and once you get to iron and it's similar compounds you've got a problem because you can you can't get energy by splitting on it's the most tightly bound element nucleus and at that point the star can again it may be a supergiant it may be a giant it may have all these different shell of different elements burning but once you hit iron in the core you cannot get any more energy out and it undergoes the dramatic collapse because it can no longer sustain itself against gravity again the core collapses down there's more mass it goes past that electron degeneracy phase of the white dwarf it collapses down to form a neutron star where again neutrons don't like being squeezed so they resist the gravitational pull and the outer layers I mean that collapse of the iron core down to the neutron star so you're going from something which is perhaps a few million kilometers across to something is ten kilometers across in less than a second and you're kind of leaving all the outer layers of the stars not knowing quite what's happened they're sitting there unsupported and they crash down moments later they rebound off this incompressible surface of the neutron star and then rebound out into space to form a supernova so this is the endpoint of a really massive star like and you know much bigger than our Sun it's going to explode away form a supernova this material is going to travel out at some thousands kilometers per second this is an example the Crab Nebula relatively young one some of this material in the outer edges is still expanding away at a thousand kilometers per second or so and the core of the star is right down in here if i zoom in its that and it's a neutron star and so this is an you know again we I can't show you beautiful picture of a neutron star here's just like an artist's impression and this supernova explosion releases a lot of energy they're so bright they can outshine the light of the galaxies they sit in and you've got to realize that light is only one hundredth of all the energy released from the supernova most of it comes to us in the form of neutrinos so violent explosion and the outer layers of the star are plowed into space and a blast wave and mixed up with the material the interstellar medium and of course being a massive star is created a lot of heavier elements at its core and they get dispersed and recycled into the interstellar medium not just that but it's within the supernova explosion that you create those heavy elements because there's a flood of neutrons released and you have much rapid much more rapid process you can have an element it requires not just one but two or three neutrons before they undergo this beta decay and then these disperse and you suddenly increase the elements by a number of three and so in the envelope of a red giant star is a very slow process neutrons are required very kind of slowly and there's lots of radioactive decay but in the supernova explosion you generate all those really heavy elements the elements that are heavier than lead and you can plot what you expect the S is the slow that's the kind of elements this is Neutron number against atomic number and the the isotopes you get and what you get in the supernova explosion and it counts on the distribution of elements out there okay so I'm cutting this a little bit short but you get the idea that you the massive star explodes away and you form a neutron star and if you've got a really really massive star something that is like fifty to a hundred times the mass of the Sun well actually you're producing so much light in by the red giant phase that it blasts off the outer layers of the star so as it evolves off the main sequence it never is a red giant because you produce so much energy at the core the outer layers have been dispersed and these are objects called wolf rays stars there's the core of the star and in you just the outer layers are blown away into space and it never undergoes the red giant phase it never undergoes that shell burning and that affects a lot of any kind of elements that you form within the very massive stars but they will still go supernova at the end of their lives and disperse so that they have created at the core into the stellar medium so there is this cosmic recycling these stars generate all the heavy elements in the gas most of the interstellar medium is primordial hydrogen and helium form from the very early universe but all the heavier elements have all been processed the core of stars if you have a nebula and it collapses down to form stars well obviously you get some the ones that never quite become stars those are the brown dwarfs those are the things less than 1/10 the mass of the Sun you might get solar mass stars which evolved to form red giants go through a planet and also planets around the stars of course and they will go to form planetary nebulas you get the very and white dwarfs then you get the very massive stars they'll form the super giants and these are the ones that will go supernova and form a neutron star even a black hole if it's one of those really massive stars and all of these processes recycle those elements into the interstellar medium you've got winds from the Giants and the red giants they puff off those outer layers you've got the planetary nebula that expands away after the red giant phase now there's a fairly gentle mixing and they're not so enriched in the heavy elements the dramatic one of course the supernova you've got lots of heavy elements and it's thrown off with a great energy so they're very violently mixed into the interstellar medium so this is a very effective return of material you could remember big luminous stars are rare only about 5% of all the stars that have ever been formed have actually gone through the supernova phase so it's not you know it's not the case that there's a lot of the heavy elements they recycled into the interstellar medium it's still just a trace of those heavy elements visible and the other thing of course is that although you have this recycling some of these chemicals are locked away to form what one of my colleagues calls the cosmic ash heap because if you got your neutron star or your black hole or your planets or your brown - also your white doors they're all inert compact objects they're just going to cool and dim with time and those a lot of those heavy elements are still locked in those bodies so the recycling perhaps is not as efficient as one might think and so it's still even though you can map generations of stars that are forming from progressively in which stars so that I'm sorry progressively enriched clouds so the more recent stars formed from more enriched clouds because they've under there's been time for various generations of massive star formation this is a slow process but is one way you can actually see generations of stars being formed where their chemical composition is changing an exciting thing of course that allows you to date different stages of formation within a galaxy and start to look at galaxy formation processes more or less by doing stellar archaeology and looking the chemical composition of those stars so that's it's obviously a lot more to do with stellar lives but let's just kinda light your potted history and I hope it gives you a flavor of our galaxy's a very dynamic medium my next lecture will be the other extreme no longer looking at the universe around us but looking at the very earliest light of the universe so join me in March for echoes of the Big Bang thank you very much

Channel: Gresham College

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Keywords: stars, astronomy, nebula, nebulae, star, space, planetary nebula, star magnitude, basic astronomy, beginner astronomy, brown dwarfs, white dwarfs, red giants, the sun, sun, solar, solar system, neutron star, black hole, supernova, super nova, back holes, astrophysics, stellar, gresham lecture, astronomy lecture, interstellar medium

Id: JW0cc11YSKY

Length: 58min 25sec (3505 seconds)

Published: Wed Feb 26 2014