Transcript for:
Lecture 10

good evening ladies and gentlemen how is everyone doing this evening that is good to yes so last week we ended off with the formation of stars so we looked at our low mass stars form we looked at how high mass stars form and we know where they all fit in on the hr diagram and now we're going to see and start looking at what happens at the end of the life cycle of stars so what happens when they all eventually die out so to get a bit understanding of what this lecture is about and to understand this tonight's election tomorrow's lecture better i'm going to show you three short videos of the work we are going to cover stars are in a constant struggle between gravity trying to collapse them and their internal heat trying to inflate them for most of a star's life these two forces are at an uneasy truce for a star like the sun the balance tips in its twilight years for a brief glorious moment it expands but then blows away its outer layers leaving behind the gravitationally compressed core it goes out with a whimper well a whimper from a two octillion ton barely constrained nuclear-powered fireball but more massive stars aren't quite as resigned to their fate when they go out they go out with a bang a very very big bang [Music] in the core of a star pressure and temperature are high enough that atomic nuclei can get squeezed together and fuse this releases energy and creates heavier elements hydrogen fusion makes helium helium fusion makes carbon and each heavier element in general takes higher temperatures and pressures to fuse lower mass stars like the sun stop at carbon once that builds up in the core the star's fate is sealed but if the star has more than about eight times the sun's mass it can create temperatures in its core in excess of 500 million degrees celsius and then carbon will fuse there are actually a lot of steps in this process but in the end you get carbon fusing into neon magnesium and some sodium what happens next harkens back to what we found goes on in the sun's core as it ages fuse an element create a heavier one then that heavier one builds up until the core contracts and heats up enough to start fusing it so carbon fusion makes neon magnesium and sodium and those accumulate the core heats up and when it reaches about a billion degrees neon will fuse neon fusion creates more magnesium as well as some oxygen these build up in the core it shrinks heats up to about 1.5 billion degrees and then oxygen fuses creating silicon then that builds up until the temperature hits about 2 to 3 billion degrees whereupon silicon can fuse among a pile of other elements silicon fusion creates iron and that's trouble big big trouble once silicon fusion starts the star is a ticking time bomb but before we light that fuse let's take a step back what's happening to the outer layers of the star what do we see if we're outside looking back at it because the star was born massive it spent its hydrogen fusing days as a blue main sequence star stars like this are extremely luminous and can be seen for tremendous distances like the sun though a massive star changes when hydrogen fusion stops its core contracts and then helium fusion begins it swells up just as the sun will but instead of becoming a red giant it generates so much energy it becomes a red supergiant these are incredibly huge stars some over a billion kilometers across and they are luminous for example beetlejuice and orion is a red supergiant and one of the brightest stars in the sky despite being over 600 light years away from that distance you'd need a decent telescope to see the sun at all and that's nothing compared to vy canis majoris the largest known star which is a staggering 2 billion kilometers across we even have a special term for it a hyper giant as the core switches back and forth from one fusion reaction to the next the outer layers respond by contracting and expanding so a red supergiant can shrink and become a blue supergiant rigel another star in orion is a blue supergiant putting out over 100 000 times as much energy as the sun okay let's go back to the core it now looks like an onion with multiple layers iron is building up in the center surrounded by fusing silicon outside that is a layer of fusing oxygen then neon then carbon then helium and finally hydrogen you might think massive stars would last longer because they have more fuel than lower mass stars but the cores of these monsters are far hotter and fuse elements at far higher rates running out of fuel more quickly a star like the sun can happily fuse hydrogen into helium for over 10 billion years but a star twice as massive as the sun runs out of hydrogen in just 2 billion years a star with 8 times the sun's mass runs out in only 100 million years or so and each step in the fusion process happens faster than the one before it in an extreme case like for a star 20 times the mass of the sun it'll fuse helium for about a million years carbon for about a thousand and neon fusion will use up all its fuel in a single year oxygen lasts for only a few months silicon fuses at a ridiculously high rate the star will go through its entire supply in get this a day yes one day the vast majority of a star's life is spent fusing hydrogen the rest happens in the metaphorical blink of an eye silicon fuses into a bunch of different elements including iron that inert iron builds up in the core just like all those elements did before and just like before the iron core shrinks and heats up but there's a huge difference here in all the previous fusion stages energy is created that energy transforms into heat and that helps support the soul-crushing amount of stellar mass above the core but iron is different when it fuses it actually sucks up energy instead of creating it instead of providing energy for the star it removes it this accelerates the shrinking compressing the core heating it up even more even worse at these temperatures and pressures the iron nuclei suck up electrons that are whizzing around which are also helping support the core it's a double whammy both major means of support for the star are removed in an instant silicon fusing into iron is happening so fast this literally takes a fraction of a second once it gets started the core gets its legs kicked out from under it it doesn't shrink it collapses the gravity of the core is so mind-bogglingly strong that the outer parts crash down on the inner parts at a significant fraction of the speed of light this slams down on the central core collapsing from several hundred kilometers across down to a couple of dozen kilometers across in just a few thousandths of a second the star is doomed because all hell is about to break loose now at this point one of two things can happen if the star has less than about 20 times the sun's mass the core collapse stops when it's still 20 or so kilometers wide it forms what's called a neutron star which i'll cover in the next episode if the star is more massive than this then the collapse cannot be stopped by any force in the universe the core collapses all the way down down to a point the gravity becomes so intense that not even light can escape a black hole is born we'll cover black holes in a future episode as well but for now what happens when the core collapses and suddenly stops the core of the star whether it's a neutron star or a black hole is now extremely small with terrifyingly strong gravity it pulls on the stars matter above it hard this stuff comes crashing down at a fantastic speed and gets hugely compressed ferociously heating up at the same time two things happen in the core while this stuff is falling in a monster shock wave created by the collapse of the core moves outward and slams into the incoming material the explosive energy is so insane it slows down that material substantially the second event is that the complicated quantum physics brewing in the core generates vast numbers of subatomic particles called neutrinos the total energy carried by these little neutrinos is almost beyond reason in a fraction of a second they carry away 100 times as much energy as the sun will produce over its entire lifetime that's an incredible amount of energy now these little beasties are seriously elusive and hate to interact with normal matter one single neutrino could pass through trillions of kilometers of lead without even noticing but so many are created in the core collapse and material barreling down on the core so dense that a huge number of them are absorbed this vast wave of neutrinos slams into the oncoming material like a bullet train hitting a slice of warm butter the material stops its in-fall reverses coarse and blasts outward the star explodes it explodes this is called a supernova and it is one of the most violent and terrifying events the universe can offer an entire star tears itself to shreds and the expanding gas blasts outward at 10 percent the speed of light the energy released is so huge they can be seen literally halfway across the universe they outshine all the stars in the rest of the galaxy combined the expanding material called the supernova remnant forms fantastic shapes the most famous is the crab nebula from a star we saw blow up in the year 1054. the tendrils form as the material expands into the gas and dust that surrounded the progenitor star as remnants expand and age they become more tenuous some have bright rims as they push into material between the stars others form complex webs of filaments i'm often asked if there are any stars near enough to hurt us when they explode the quick answer is no even though supernovae are incredibly violent space is big a supernova would have to be at least as close as a hundred light years from us before we start feeling any real effects the nearest star that might explode in this way is spica in virgo and it's well over a hundred light years away i say might explode because it's at the lower mass limit for going supernova it might not explode at all beetlejuice will certainly explode someday but it's too far away to hurt us we're pretty safe from this particular threat i'll note that after all this there is another kind of supernova involving white dwarfs which we'll cover in our episode about binary stars happily we're probably safe from them too breathe easy as terrifying and dangerous as supernovae are there's a very important aspect of them you need to know supernovae are capable of great destruction but they're also critical for our own existence when the star explodes the gas gets so hot and is compressed so violently by the blast that it undergoes fusion what astronomers call explosive nucleosynthesis literally creating heavy elements explosively new elements are produced in quantities that dwarf the earth's mass calcium phosphorus nickel more iron all made in the hellish forge of the supernova heat and flung outward into the universe it takes millennia or longer but this material mixes with the other gas and dust clouds floating in space sometimes these clouds will be actively forming stars sometimes the collapse of the cloud to form stars may even be triggered by the supernova slamming into it either way the heavy elements created in the supernova will become part of the next generation of stars and planets supernovae are how the majority of heavy elements in the universe are created and scattered the calcium in your bones the iron in your blood the phosphorus in your dna all created in the heart of the titanic death of a star that star blew up more than 5 billion years ago but parts of it go on in you today you learn that massive stars fuse heavy hey folks phil plate here in the last episode of crash course astronomy i talked about the eventual fate of the sun and other low-mass stars like it after a series of expansions and contractions they blow off their outer layers become white dwarfs and fade away over billions of years the end except not so much first white dwarfs are pretty awesome objects and worth investigating and second when a star becomes a white dwarf it produces what is quite simply one of the most gorgeous objects in space [Music] to recap when the sun ages it undergoes a series of changes in its core it's fusing hydrogen into helium now today and will eventually fuse helium into carbon and it'll make a dash of oxygen and neon too but when it runs out of helium to fuse it's in trouble it doesn't have enough mass to squeeze the carbon nuclei together so they can't fuse fusion is the sun's energy source once the core is nearly pure carbon that power is switched off by this time nearly 8 billion years from now the sun's outer layers are gone expelled by all the shenanigans going on in the core what's left of our star is just its core exposed to the dark of space over the next few billion years it'll cool and fade to black that might seem like the end of the story but i skipped a step and it's a beauty when helium fusion stops the sun's core will have about half the mass of what the sun does today the rest will have blown away into space around it what remains is basically composed of electrons and carbon nuclei mixed with a small amount of a few other elements so what kind of an object are we left with here you may know that light charges repel electrons have a negative charge and repel each other the tighter you squeeze them the stronger that pressure there's also a second force called electron degeneracy pressure it's a result of some of the weird rules of quantum mechanics for you qm nerds it's the poly exclusion principle this describes how subatomic particles behave on teeny scales one of these rules is that electrons really hate to be squeezed together above and beyond simple electric repulsion this resistance is phenomenally strong and becomes the dominant force in supporting the core of a star once helium fusion stops by the time this electron degeneracy pressure balances the core's immense gravity the core is only about the size of earth one percent the original width of the sun and it's called a white dwarf listing its characteristics is enough to melt your brain ironically everything about it gets amplified as its size shrinks it becomes ridiculously dense a single cubic centimeter of it the size of a six-sided die has a mass of a million grams one metric ton an ice cream scoop of white dwarf material outweighs 60 cars because it's so dense the gravity at the surface of a white dwarf screams up easily topping a hundred thousand times the earth's gravity if you have a mass of 75 kilos you'd weigh 7 500 tons if you stood on the surface of a white dwarf not that you can stand there i mean you'd be flattened into a greasy smear but not for long newborn white dwarfs are hot they can glow at temperatures of upwards of a hundred thousand degrees celsius if you were on the surface you'd be a vaporized and ionized smear their intense heat makes them white and they're small hence their name they're so hot they also glow in the ultraviolet even in x-rays weirdly though because they're so small they're actually quite faint the closest one to us sirius b can only be seen with a telescope even though it's nine light years away one of the ten closest known stars over ten thousand white dwarfs have now been found in our galaxy still any gas near a newly formed white dwarf is likely to be affected by the intense radiation pouring out of it and hey wait a sec when a star like the sun is in its final death throes it expels its outer layers as a gaseous wind you don't suppose yep by the time that white dwarf forms the gas blown off hasn't gotten very far from it at most a light year or two that's plenty close enough to get zapped by the white dwarf radiation causing that gas to glow in response what does something like that look like why it looks like this this object is what we call a planetary nebula it's a funny name and like so many other names it's left over from when these objects were first discovered the astronomer william herschel the same man who discovered infrared light in the planet uranus gave them that name because they appeared like small green discs through the eyepiece the first planetary nebula was discovered in 1764 by the french astronomer charles messier who spent years scanning the skies looking for comets he kept seeing faint fuzzy objects that he mistook for comets so he decided to make a catalog of them a sort of avoid these objects list that list is now a staple of amateur astronomers because ironically it contains some of the best and brightest objects to observe among them is m27 the 27th object on messier's list one of the biggest planetary nebulae in the sky and one of my favorites i love seeing it through my telescope when it's up high in the summer planetary nebulae can be a bit tough to observe most are small and faint on film even with long exposures they can appear to be little more than discs for a long time they weren't thought to be terribly complicated when a star becomes a red giant blows off its outer layers it's rotating very slowly so the wind should blow away in a sphere surrounding the star many planetaries as we call them for short are round and look like soap bubbles pretty much what you expect when you look at an expanding shell of gas but with the advent of digital detectors their fainter structures became clearer and the true beauty of these phenomenal objects was revealed some are elongated some have spiral patterns some have jets shooting out on either side some have delicate tendrils streaming away from them in fact only a handful of the hundreds known are actually circular clearly there's more to planetaries than meets the eye if the wind from a star blows off in a sphere how can planetaries come in all these fantastic shapes it turns out the real situation is more complicated as usual when a star is a red giant it spins slowly and blows off a dense but slow solar wind if there's nothing else happening to the star then that wind will blow outward in all directions spherically however as those outer layers of the star peel away they expose the deeper hotter part of the star the star starts to blow a much faster though far less dense wind that wind catches up with and slams into the slower wind when that happens you get that idealized soap bubble nebula but some stars are binary two stars that orbit very close together we'll go into detail on them in a later episode but if the dying star has a companion they may circle each other rapidly that will shape the wind forcing more of it outward in the plane of the star's orbits due to centrifugal force the overall shape of the expanding gas is flattened like a beach ball someone sat on when the fast wind kicks in it slams into that stuff in the orbital plane and slows down but there's less stuff in the polar direction up and down out of the plane it's easier for the wind to expand in those directions and it forms huge lobes of material stretching for trillions of kilometers that's a very common shape for planetary nebulae but to explain many of the shapes we see the two stars would have to orbit improbably close most binaries aren't that tight so what can cause these shapes when i was in graduate school my master's degree advisor gnome soaker came up with a nutty idea maybe the stars had planets like in our solar system if the star expanded into a red giant and swallowed them it would take millions of years or more for the planets to vaporize and for all that time they'd be orbiting inside the star moving faster than the star itself like using a whisk to beat eggs the planets inside the star would spin it up causing it to rotate faster fast enough to explain the shapes of planetaries that was in the early 1990s a few years later the first exoplanets were found and we saw that massive planets orbiting very close to their stars were common i suspect this is why we see so many weird and fantastic shapes and planetaries their progenitor stars swallowed their planets so planetary nebulae really may owe their existence to planets and we come full circle the glow in a planetary nebula is due to the hot central white dwarf exciting the gas most of the gas is hydrogen which glows in the red however a lot of the gas is oxygen there's not nearly as much oxygen as hydrogen but kilo for kilo oxygen glows more brightly than hydrogen due to the atomic physics involved this oxygen glows green giving planetaries their characteristic hue funny when this green glow was first analyzed spectroscopically astronomers couldn't identify the responsible element making it they dubbed it nebulium but eventually figured out it was just extremely tenuous oxygen other colors can be found too nitrogen and sulfur glow red and oxygen can emit blue as well all adding to the beauty of these celestial bobbles but these aren't just pretty pictures the structure color and shape of a planetary nebula tells us about the life of the star that formed it we learn even more about stellar evolution by studying how stars die mind you the gas in a planetary nebula is still expanding cruising outward from its initial momentum of being thrown off the star eventually the gas expands so much it thins out and stops glowing that takes a few thousand years so when you see a planetary nebula you're seeing a very short snapshot of the life the death of a star and that's why we don't see many though there are billions of stars in the galaxy that die this way this phase is very brief enjoy looking at them while you can and what of the sun will it one day in the distant future be at the center of a planetary nebula it expels as it dies probably not when the sun becomes a white dwarf it most likely won't be energetic enough to make the surrounding gas glow most planetaries start off as stars more massive and hotter than the sun when our sun dies it'll go quietly and without a lot of visible fanfare alien astronomers if they're out there in eight billion years may not even notice but more massive stars do make quite the spectacle and if they're really massive more than about eight times the sun's mass they really and truly make a scene when they die they explode but that's for next week today you learned that when low when an 8 to 20 solar mass star ends its life it does so with a bang a supernova and when it's all over there's a couple of octillion tons of superheated plasma expanding away from the explosion site at a fraction of the speed of light a whole mess of energy released in the form of light and neutrinos and a bizarre little ball of quantum nastiness in the center composed almost entirely of neutrons the properties of this neutron star are almost as bizarre as things get in the universe and if it all seems rather alien to you well that's okay for a little while astronomers wondered if aliens really were behind what they were seeing no i'm not saying aliens [Music] when we last left the core of a high mass star it was in a bad way milliseconds ago it was fusing silicon into iron but now it's collapsing under its own immensely powerful gravity the collapse takes a fraction of a second but a lot happens in that fraction of a second in lower mass stars the core supports itself via electron degeneracy pressure the result of a rule in quantum mechanics that says electrons vehemently resist being squeezed together but even electron degeneracy fails to stop the collapse if the core has a mass more than about 1.4 times the mass of the sun that's just too much of a load to bear and the collapse continues under these huge pressures a funny thing happens protons electrons and other subatomic particles get smashed together and they merge to form neutrons and this happens to almost all of them when the core collapses down to about 20 kilometers in diameter it's essentially a ball of neutrons with some protons and electrons here and there that survived and a crust of normal but highly compressed matter on top when this happens yet another effect comes into play neutron degeneracy like electrons neutrons resist being squeezed too tightly together but this time the strength of the pressure is far far stronger than for electrons if the core is less than about 2.8 times the sun's mass the collapse runs into a wall it stops this generates a huge shock wave which along with a flood of energetic subatomic particles called neutrinos blasts outwards blowing up the star what's left of the core after the metaphorical smoke clears is a neutron star one of the most bizarre objects in the universe such a star would be extremely weird or really just extreme its mass would be more than that of the entire sun all packed into a sphere maybe 20 kilometers across now let's just stop there for a sec and let that sink in the sun has a mass 300 000 times the earth imagine packing that all into a ball the size of a small city too mind-boggling okay think of it this way you are mostly empty space every atom in your body has a nucleus made of tightly packed neutrons and protons and electrons whizzing around outside them if you could magnify an atom to be a hundred meters across the nucleus would be roughly the size of a marble imagine all that empty space between the nucleus and the electrons that's a normal atom but in a neutron star all of that space would be filled with neutrons all of it every nook and cranny inside the neutron star has matter in it all the way down to the scale of an atomic nucleus this is what gives a neutron star its mind-crushing properties i'm now going to barrage you with very large numbers so take a deep breath and you might want to sit down neutron stars are ridiculously dense a single cubic centimeter of neutronium as neutron star stuff is usually called has a mass of about 400 million tons want some perspective on that number well very roughly that's the total mass of every single car and truck in the united states imagine a couple of hundred million vehicles crushed down until they could all fit inside this six-sided dye that's neutronium it's so dense that as far as it's concerned normal matter is a slightly polluted vacuum if you set it on the ground it would fall right through the earth now anything that dense has a huge gravitational pull if you were on the surface of a neutron star well you'd be very dead obviously like immediately flattened down to a thickness of just a few atoms and that's because a typical neutron star has a surface gravity 100 billion times stronger than earth's i have a mass of about 77 kilos and here on earth i weigh about 170 pounds on a neutron star i'd weigh 17 trillion pounds that's 23 000 times the weight of the empire state building but wait there's more in our introduction to the solar system i mentioned that when you take a spinning object and shrink it the spin will increase the usual example is an ice skater drawing in his arms increasing his rotation until he's a blur the same is true for the star's core when it collapses into a neutron star it may have had a very slow spin before the supernova maybe even taking weeks to spin once but when it shrinks down to just 20 kilometers across and becomes a neutron star that rotation will increase by a huge factor a freshly minted neutron star might spin several times per second the magnetic field increases as well a star like the sun has an overall magnetic strength not too different from the earth's but when that core collapses the strength of the field skyrockets and a neutron star can easily have a magnetic field several trillion times stronger than the sun's that's strong enough to erase your credit card from a hundred thousand kilometers away see ridiculous all of these properties are brain melting but are they real could an object like this really exist oh my yes the first neutron star was detected in 1965 though not recognized for what it was at the time a couple years later another one was found and this time was correctly identified as a neutron star but then things got weird in 1967 jocelyn bell was a graduate student helping build a radio telescope there was a persistent noise in their data they couldn't seem to fix bell studied it night after night finally figuring out that the pattern wasn't a problem with their data it was from an actual astronomical object she had discovered the first known pulsar what's a pulsar you ask pulsars are neutron stars in a nutshell their rapid rotation coupled with their incredibly strong magnetic fields launched twin beams of energy away from the star like the beams from a lighthouse the beams sweep around as the star rotates and from earth we see this as a pulse a blip of increased brightness this pulse can be detected in visible light radio waves and even x-rays the spin of a neutron star is amazingly stable making these pulses act like a very accurately timed cosmic clock in 1967 no one could believe a natural object could do this and this object was half jokingly given the name lgm1 little green men won now we know of over a thousand pulsars in just our galaxy alone and we know they're the leftover cores of massive stars that exploded some spin with periods many seconds or even minutes long some are in binary systems another normal star orbits them if they're close enough together the neutron star can rip material off the other star and feed on it this increases the pulsar spin and we know of a few that have incredibly rapid rotation rates some spin hundreds of times per second these are called millisecond pulsars and if they spun much faster the centrifugal force would rip them apart despite their tremendous gravity even after a thousand years a pulsar can still be a force to reckon with there's a pulsar in the center of the crab nebula the remains of a star that exploded to create that supernova remnant a substantial fraction of the light emitted from the nebula is powered by the pulsar itself its fierce output energizes the nebula causing it to glow brightly even after a millennium i'm telling you thinking about neutron stars makes the hair on the back of my neck stand up and i haven't even mentioned magnetars yet neutron stars are more than just weird little balls of neutrons they have a crust probably a few centimeters thick made of highly compressed but more or less normal matter squeezed into a kind of highly stiffened crystal state the magnetic field of the star penetrates this crystalline crust and stretches out for quite a distance in some neutron stars the magnetic field is even stronger than usual and can be get this a quadrillion times stronger than the suns these uber-powerful neutron stars are given the name magnetars they're relatively rare maybe 10 percent of all neutron stars are born as magnetars and they have short lifetimes the magnetic field is so strong it acts like the brakes on a car slowing the neutron star spin that spin helps power the magnetic field so the field weakens as the star slows but while they're around magnetars are the most magnetic objects in the universe and with great power comes great responsibility if your responsibility is to be one of the scariest objects in the universe why in a neutron star the crust and magnetic field are locked together so a change in one affects the other the crust of the star is under incredible strain due to the intense gravity and rapid rotation if the structure slips it can snap creating a starquake like an earthquake but just a wee bit stronger in an earthquake huge amounts of energy released when the earth's crust shifts and snaps enough to destroy buildings and quite literally move mountains but in a neutron star this effect is multiplied hugely remember the crust is phenomenally dense and the gravity is enormous if the crust strains and snaps dropping to the crust shaking it this also shakes the magnetic field which reacts poorly when the sun's magnetic field throws a tantrum we get a solar flare which can be as powerful as billions of nuclear bombs a magnetar flare dwarfs that into insignificance it can be trillions of times stronger than a solar flare in a fraction of a second a magnetar can release as much energy as the sun gives off in a quarter of a million years in 2004 astronomers were stunned when a huge blast of x-rays slammed into orbiting satellites one of these satellites named swift actually had its detectors saturated with x-rays even though it wasn't even pointing at the source at the time the x-rays literally came right through the side of the satellite with such intensity that swift which was designed to detect powerful x-ray sources was momentarily blinded by them the source of this x-ray burst was quickly determined to be a magnetar called sgr 1806-20 and the effects were incredible it actually compressed the earth's magnetic field and partially ionized the earth's upper atmosphere oh did i mention that this magnetar is 50 000 light years away that's halfway across the galaxy that's incredible at a distance of 500 quadrillion kilometers its effects were felt more strongly than a powerful flare from the sun the good news is that there are very few magnetars like this in the galaxy probably fewer than a dozen also catastrophic explosions like the one in 2004 are rare if one had happened any other time in the past 40 years or so we would have detected it and frankly it's really cool that we had astronomical satellites orbiting the earth which could study it we've come a long way in understanding neutron stars since they were first discovered but there's much about them we don't understand every time we find out more we find out they're even weirder than we first thought and yet for all that they're not the weirdest things in the sky not by a long shot that place is held pretty securely by the other type of object created in a supernova a black hole stay tuned today you learn that when a star between 8 and 20 times the sun's mass explodes so ladies and gentlemen this is what we're going to be discussing in this lecture and the upcoming lectures so i'm just going to use a google drive slideshow just for the first intro lecture to show you a few animations then i will swap to the normal slides again so for this lecture we are going to look at the interstellar medium so making stars from the interstellar medium young stellar objects and proto-stellar discs stellar structure and nuclear fusion then how that fits together with main sequence stars so in a previous chapter you discovered that a wide range of differences within the family of stars so in this chapter you will combine observations and hypothesis to understand how nature makes stars and learn the answers to five important questions so i said we're ready to upload the wrong lecture slides let me just quickly check on my side now this is the correct slides we are in so um something might sound similar but this is the correct slideshow but thank you for checking up so the first question is how do astronomers study the gas and dust between the stars called the interstellar medium secondly how does stars from redustella meet how these stars form from the interstellar medium how the stars remain stable how do stars make energy lastly why do the luminosities and lifestyles of stars depend on their masses so the stars are not internal so when you look at the sky you see hundreds of points of light and amazingly enough each one like the sun is a tremendous nuclear fusion reactor held together by its own gravity so the stars you see tonight are the same stars your parents grandparents and great-grandparents saw stars don't change perceptibly in a person's lifetime or even during all human history but they do not last forever stars are born and stars die so i showed this animation before so when we think about the diff of stars it's not like the diff star it is more something like this oh so you guys are correct i am sharing the wrong slides let me open up the right ones so my own notes are opened up and the slides i'm sharing are different slides so this should be the right slides now sorry about that can you all see these slides okay cool so for this lecture we're going to look at giant stars lower main sequence stars the evolution of binary systems and then finally death of massive stars so this lecture describes how stars are born and how they resist their own gravity by fusing nuclear fields in the centers to keep their internal pressure high but it can last only as long as they feel so there are five important questions we must answer in this lecture so firstly what happens to a star when it uses up the last of the hydrogen in its score secondly what evidence shows that stars really evolve thirdly how will the sun die forcefully what happens if an evolving star is in a binary system and then lastly how do massive stars die so the death of stars is not the death star blowing something up but it is rather something like this so now i can swap over to the proper slides so the depth of stars are important because life on earth depends on the sun but also because the death of massive stars create the atomic elements of which you are made if stars didn't die you would not exist so this is quite an interesting sketch here you can see where these different elements are formed so the yellowish ones is the merging of neutron stars the brownish ones or dark oranges dying of low mass stars the dark blue is exploding massive stars light blues exploding white dwarfs the red ones is the big bang and then the purple ones is the cosmic ray fusion so when we are talking about what happens in star life cycles there are free paths that we are going to discuss and what happens so the rainbow the star forms in stellar nebula then it can become an average star so the size for example island sun is an average star they've become a red giant then a planetary nebula and in off as a white dwarf but if we had a star that is eight solar masses or more that will form a massive start in a super red giant then a supernova and the painting we're going to discuss that in deep as well depending on a few things it will either become a neutron star or a black hole so gravity is patient stars generate tremendous energy resisting the in gravity but no star has an infinite supply of fuel for its nuclear reactions so when the fuel runs out the star dies so to always keep in mind when we talk about the life cycle of stars the gravity pulling inwards of pushing down needs to be counteracted by the pressure of the nuclear reactions pushing outward that keeps the stars stable and in the previous lecture we've talked about the temperature pressure thermostat and that is what keeps the star stable and until that thermostat is not working anymore then we starting to reach the end of a star's life cycle so occasionally astronomers see a formerly faint star suddenly appear much brighter than the sky then fade away after a few weeks or a month you will learn that what seemingly a new star in the sky is either nova an eruption on the surface of a stellar england was supernova the violent explosion explosive diff of an aging massive star so few nove plural of nova are discovered each year in supernova or supernovae plural are so rare that only one or two occur each century and our entire galaxy but astronomers surveying thousands of galaxies can see many supernova flay up per year the mass of a star is critical in determining its fate massive stars can die in violent supernova explosions but lower mass stars dark white deaths to follow the evolution of stars to their graves you can start by following the life story of a sunlight medium a star as it becomes a giant star after that you will learn how stars are different masses in their lives so i don't know have you guys been following the big super red giant beetlejuice last year and half so in the last year and a half beetlejuice has become exponentially brighter so usually when we see something like that we know the star is about to go supernova but it looks it's brighter but it looks like it has stabilized for now but all indications showed that it will go supernova so we don't know exactly when it's going to go boom but we are still trying to figure that out so eyes are on beetlejuice at a moment because it increased its brightness and then it stabilized so it all depends on how much fuel is left that it's fusing but we are safe to say that beetlejuice are at the end of its life so the massive red super giant star v838 flared up in 2002 the expanding shell of light from the flare illuminated surrounding gas and dust that was probably ejected from the star in earlier outbursts so bright spikes are produced by the fraction in telescopes so this spikes are just a diffraction pattern in telescope planetary nebula ic1295 consists of shell shells of gas expelled by an aging red star dread giant star the star's remnant can be seen in the center of the nebula contracting to become a white dwarf contains uv radiation from a central star is making the nebula grow the prominent green color is emissions from ionized oxygen plan a pla plan a near infrared image of galaxy m82 and supernova 201 aj observed from the stratospheric observatory for infrared astronomy so sophia the other two labeled objects are foreground stars in our galaxy so this is the galaxy infrared image of the galaxy and this is just stars between us and that galaxy so in our galaxy is these two stars the main sequence star generates energy by nuclear fusion reactions that combine hydrogen to make helium the period during which the star's fuses hydrogen lasts a long time and the star remains on the main sequence for 90 percent of its total existence as an energy generating star so when the hydrogen is exhausted however the star begins to evolve rapidly so first thing is a average sized star will first expand into a giant star so nuclear reactions in the main sequence star's core fuse hydrogen to produce helium because main sequence stellar cores are cooler than a hundred million kelvin the helium can't overcome the coulomb barrier to fuse in nuclear reactions so it accumulates at a star's center like ashes in a fireplace initially this helium ash has little effect on a star but as hydrogen is exhausted and the stellar core becomes pure helium the star loses the ability to generate the nuclear energy that opposes gravity as soon as the energy generation starts to die down gravity begins making the core contract so this making the core more dense although the core of helium ash can't generate nuclear energy it does grow hotter as it contracts because it is converting gravitational energy into thermal imagery the rise in temperatures heat the unprocessed hydrogen just outside the core oxygen that was never before hot enough to fuse soon hydrogen fusion begins in a spherical layer or shell around the exhausted core of the star like a grass fire burning outward from exhausted campfire the hydrogen fusion shell creeps inward leaving helium ash behind and increasing the mass of the helium core the flow of energy produced by origin fusion shell pushes toward the surface heating the outer layers of the star and forcing them to expand dramatically so in this image when a star runs out of hydrogen at its center the core contracts to a small size becomes very hot and begins nuclear fusion in a shell so the blue part of the shell over here the outer layers of the star start to expand and cool so the red giant star shown here has an average density much lower than the air at earth's surface so here the solar mass stands for the mass of the sun and the r0 symbol stands for the radius of the sun stars like the sun becomes dried stars of 10 200 solar radar and the most massive stars become super giants of a thousand times larger than the sun this explains the large diameters and low densities of the giant lenses the expansion of its envelope it changes location in the hr diagram just as contraction heats a star expansion cools it as the outer layers of gas expand energy is absorbed in lifting and expanding the gas the loss of that energy lowers the temperature of the gas consequently the point that represents the star in an hr diagram moves to the right relatively quickly so in less than a million years four started as five solar masses the massive star moves to the right across the top of the hr diagram and becomes a supergiant while a medium mass-like star like the sun becomes array driven so here we will see massive stars evolve from the main sequence into the supergiant region so a star that has 15 cellular masses like beetlejuice is a become a super retrial so five solar mass super rare giant so yeah at three cellar masses so less massive thin stars evolve into the giant region so here we can see the giant region and the sun will become a giant star end of its lifetime these beetlejuice can become a black hole so yes sir the current math and mathematical models we have suggest that beetlejuice will become a black hole so as the radius of a giant star continues to increase its enlarging surface area makes the star more luminous moving its point upward in a hr diagram so famous star aldebaran the glowing red eye of the taurus the bull is such a red giant with diameter more than 40 times that of the sun but a much cooler surface temperature so if you look at the orion constellation we have the ladies and here we have aldebaran so what happens when the star expands into a giant star to understand that process we need to understand degenerate matter so although the hydrogen fusion shell can force the envelope of the star to expand it can't stop the contraction of the helium core because the core is not hot enough to fuse helium gravity squeezes it tighter and becomes very small so while the stars becoming a giant star the outer layers start to expand and the core starts to shrink so when gas is compressed to such high densities it begins to behave in surprising ways that can affect the evolution of a star to continue the theory of still evolution you need to consider the behavior of gas at extremely high densities normally the pressure in the gas depends on its temperature the hotter a gas is the faster its particle moves and the more pressure it exerts the gas inside the star is ionized so there are two kinds of particles atomic nuclei and three electrons under normal conditions the gas and the star follows the same laws relating pressure and temperature as gases do on earth but if the gas is just one of two possible directions two electrons can occupy a single energy level only if they spin in opposite directions that day at that level is then completely filled and a third electron can't enter because whichever way it spins it will be identical to one or the other of the two electrons already in that level a low density gas has fewer electrons per cubic centimeter so there are plenty of energy levels available if a gas becomes very dense however nearly all of the lower energy levels are occupied in such a gas a moving electron can't slow down slowing down would decrease its energy and there are no open energy levels for it to drop down to it can speed up only if it can absorb enough energy to lead to the top of the energy ladder where they are empty energy levels so question since beetlejuice is 600 light years away what are seeing is 600 years old so surely the effects of beetlejuice as a black hole can already be felt or does gravitational waves travel at the same speed so i'm not quite exactly sure at the speed of gravitational waves travels but yes it will take more least longer so yes we're actually looking back in the past so we are seeing beetlejuice as was 600 years ago so here we will see this is the energy levels of a low density gas and here is the energy levels of a high density gas without two electron pairs so when the gas is so dense that electrons are not free to change the energy astronomers call it degenerate matter although it is a gas it has two peculiar properties that can affect the star first the degenerate gas resists compression to compress the gas requires pushing against the moving electrons and changing their motion means changing their energy that requires tremendous effort because you must boost them to the top of the energy ladder that is why degenerate matter though still a gas is harder to compress than the toughest hardened steel secondly the pressure of degenerate gas does not depend on temperature to see why note that the pressure depends on the speed of the electrons which can't be changed without tremendous effort the temperature however depends on the motion of all the particles in a gas both electrons and nuclei if you add heat to the gas most of that energy goes to speed up the motions of the nuclei which move slowly and don't contribute much to the pressure only a few electrons can absorb enough energy to reach the empty energy levels at the top of the energy ladder this means that changing the temperature of gas has almost no effect on its pressure these two properties of degenerate matter becomes important when stars end the main sequence lives eventually many stars collapse into white dwarfs and you will discover that these tiny stars are made up of degenerate matter but long before that the cause of many giant stars becomes so dense that they are degenerate a situation that can produce a cosmic bomb so are you guys happy with the explanation of degenerate matter okay cool so this because continuing on now after lecture this will become very important so now we're going to look at the helium fusion so remember in the star like our sun we start with hydrogen our origin gets fused so we have our helium ash the helium ash is making the core contract so what happens next cyto infusion main sequence stars leaves behind helium ash three helium nuclei can collide to make a carbon nucleus in what is called the triple alpha process the nuclear physicists refer to helium nuclei as alpha particles the helium ash in the core of main sequence stars is too cool to fuse because helium nuclei have positive charges twice that hydrogen nuclei so the helium nuclei need to move very fast to overcome that higher coulomb barrier as a giant star fuses hydrogen to helium in an expanding shell its inert core of helium contracts and grows hotter when the temperature of the core finally is reaches hundred million kelvin it begins to fuse helium nuclei to make carbon our star begins he infusion depends on its mass star more massive than about three solar masses contract rapidly the helium reach cores heat up and helium fusion begins gradually less massive stars evolve more slowly and the cores contract so much that a gas becomes degenerate on earth a teaspoon of the gas would weigh as much as a large truck in this degenerate method pressure does not depend on temperature and that means the pressure temperature thermostat does not regulate energy production when the temperature becomes hot enough helium fusion begins to make energy and the temperature rises but pressure does not increase because the gas is degenerate dire temperatures increases the helium fusion even further and the result is a runaway explosion called the helium flash in which for a few minutes the core of the star can generate more energy per second than does an entire galaxy although the alien flash is sudden and powerful it does not destroy the star in fact if you were observing a giant star as it experienced the alien flash you probably see no outward evidence of this eruption the helium core is quite small and all of the energy of the explosion is absorbed by the distant envelope in addition the helium flash is a very short-lived event in the life of a star in a matter of minutes to hours the core of the star becomes so hot that a significant number of electrons get boosted in the energy levels at the top of the energy ladder that increases the pressure and ends the degenerate conditions and a pressure temperature thermostat brings the helium fusion under control from that point on the star proceeds to fuse helium steadily in its core there are two reasons why the indian flash is important firstly it is so violent and so sudden that it makes it difficult to compute models of stars and be sure how they evolve astronomers have to exercise ingenuity to get past the indian flash and follow the further evolution of stars secondly the helium flash is a good illustration of how science reveals eden universe astronomers would never have known about the alien flash were not for the theoretical calculation of stellar models the sun will experience helium flash in six to eight billion years but stars less massive than about 0.4 solar masses never get odd enough to ignite helium stars more massive than free solar masses ignite helium before they contracting cores become degenerate so here we can see when a main sequence star exhaust origin in its score it evolves rapidly to the right an hr diagram as it expands to become a cool giant it then follows a looping path as it fuses helium in its score and then fuses helium in a shell so compared with figure 10 free so evolutionary track adapted from the artwork beginning of this chapter so here we can see we have our song leaving becomes super giant then we have the helium flash and it begins to fuse helium so helium fusion produces carbon and some of the carbon nuclei absorb helium nuclei to form oxygen if you have the oxygen in the nuclei i can absorb helium nuclei and form neon and then magnesium some of these reactions release neutrinos which have no charge are more easily absorbed by nuclei to gradually build even heavier nuclei these reactions are not important as energy producers but there are slow cooker process that forms small traces of heavy elements right up to bismuth with atomic weight of 209 nearly four times heavier than iron so many of the atoms in your body were produced this way as the helium fuel is used up the accumulation of carbon and oxygen atoms creates an inner core to cool diffuse once again the core contracts and heats up and soon a helium fusion shell ignites below the hydrogen fusion shell now that a star makes energy in two fusion shells it quickly expands and its surface cools once again the point that represents the stone hr diagram moves back to the right completing a loop what happens to the star after helium fusion depends on its mass but no matter what tricks the star plays to delay its end it can't survive long it must eventually collapse and in its career as the star so now we're going to start discussing star clusters and we use star clusters as evidence of evolution so just as sherlock holmes studies peculiar dust on a lampshade as evidence that will solve a mystery astronomers look at star clusters and say aha a photo of a star cluster freezes a moment of the evolution of the cluster and makes the evolution of stars visible to human observers because they so in a star cluster because all stars in the cluster formed nearly simultaneously from the same gas cloud the stars in the cluster have about the same age and composition so any differences you see among them are due to the differences in mass that means that when you look at a cluster you can see the effects of stellar evolution as it acts on otherwise similar stars of different mass so there are three important aspects of star clusters firstly there are two kinds of star clusters open clusters and globular clusters they look different but are similar in the way these stars evolve you can estimate the age of a star cluster by observing the turn off point in a distribution of the points that represents stars in hr diagram and then lastly finally the shape of star cluster's hr diagram is governed by the evolutionary paths for the star's take the hr diagrams of all the clusters are especially clear in outlining how stars evolve away from the main sequence to the giant region then move left along the horizontal branch before evolving back into the giant region by comparing clusters of different ages you can visualize how stars evolved almost as if you were watching a film of stock evolving over a billion years contracting stars heat up by converting gravitational energy to thermal energy low mass stars have little gravitational energy so when they contract they don't get very hot this limits the fields they can ignite in a previous chapter you saw that protostars less massive than 0.08 cellular mass can't get out enough to ignite hydrogen this section will concentrate on stars more massive than 0.08 solar mass but no more than a few times the mass of the sun structural differences divide the lower main sequence stars into two subgroups very low mass red wolves and medium mass stars such as the sun the critical difference between the two groups is the extent of interior convection if the star is convective fuel is constantly mix and its resulting evolution is drastically altered so let's talk about red dwarfs so stars between 0.08 and about 0.4 solar mass the red dwarfs have two advantages over more massive stars so firstly they have very small masses and that means they have very little weight to support so their pressure temperature thermostats are safe to low and they consume the oxygen feel very slowly the red dwarfs has a second advantage in that they are totally convective that is they are stirred by circulating currents of hot gas rising from interior and cool gas sinking inward this means the star is mixed like a pot of soup that is constantly stirred as it cooks origin is consumed uniformly throughout the star which means the star is not limited to the fuel in its core it can use all of its origin prolonging its life on a main sequence because a red wharf is mixed by convection it can't develop an inherit helium core surrounded by unprocessed origin therefore it can never ignite hydrogen shell and can't become a giant star what astronomers know about stellar evolution indicates that these red dwarfs should use up nearly all of the hydrogen and live very long lives on the lower main sequence surviving 400 billion years or more of course astronomers can't test the spot of their theories because the universe is only 13.8 billion years old so not yet a single red dwarf has died of old age anywhere in the universe every red dwarf that has ever been born is still shining today so as we discussed in previous lectures we can classify our red dwarfs and free subcategories el wolves t dwarfs and wide wolves stars like the sun eventually become hot enough to ignite helium as they pass through the giant phase but if they contain less than four solar massive they do not get hot enough to ignite carbon the next fuel of the alien when they reach that in-phase they collapse and become white dwarfs there are two keys to evolution of the sun-like stars that lack of complete mixing and mass loss the interiors of medium mass stars are not completely mixed stars of 1.1 solar masses or less have no conviction near the centers so they are not mixed at all so stars of masses created in 1.1 cell masses have small zones of convection at the centers but this mixes no more than about 12 percent of the star's mass so medium stars whether they have convective cause or not are not thoroughly mixed and the helium ash accumulates in a in an inner helium core surrounded by unprocessed origin so inside main sequence stars the more massive stars have small convective interiors and radiative envelopes so stars like the sun have radiated interiors and convective envelopes the lowest mass stars are convicted throughout the cause of the stars when nuclear fusion occurs and not shown here are small proportions of the interior so now it's starting to become a planetary nebula so with a medium-mass star like the sun becomes a distant giant its atmosphere schools as it cools it becomes more opaque and light has to push against it to escape at the same time the fusion shell becomes so thin they are unstable and begin to flare which also pushes the atmosphere outward because of this outward pressure an aging giant can expel its outer atmosphere in repeated surges to form one of the most intriguing objects in astronomy at planetary nebula so-called because through a small telescope it looks like the greenish blue disk of a planet like uranus or neptune in fact a planetary nebula has nothing to do with a planet it is composed of ionized gases expelled by dying star so here we can see our white dwarf in the center and our outer layers of the sauce atmosphere that has been pushed away here again we can see the white dwarf in the center and ionized gas that has been pushed away from the star so there are four important points on planetary nebula firstly you can understand what planetary nebula are like by using simple observation principles such as kirchhoff's laws and a doppler effect and secondly notice the model that astronomers have developed to explain planetary nebula the real nebula are more complex than a simple model of a slow wind and a fast wind but a model provides a way to organize the observed phenomena thirdly oppositely directed jets so much like wi-fi polar flows from podestars produce many of the asymmetric symmetry seen in planetary nebula and then lastly that star itself must contract into a white dwarf so what exactly are white dwarfs so when you survey the stars you discovered that white dwarfs are the second most common kind of star only red dwarfs are more abundant now you can recognize the millions the billions of white dwarfs in a galaxy as the remains of median mass stars the first white wolf discovered was the feigned companion to famous star sirius in that visual binary system the bright star sirius a the white star white dwarf series b is nine thousand times fainter than the famous star series a the orbital motions of the stars reveal that a white dwarf contains 0.98 solar mass and its blue white color tells you its surface is hot about 45 000 kelvin so when you survey the stars you discover that white wolves are the second most uh starfish carbons they're so the orbital motions of the stars reveal that a white dwarf contains that so it's about 45 000 kelvin because it is both very hot and very low luminosity must have a small surface area in fact it's about the size of earth dividing its mass by its volume reveals that it's very dense about 2 times 10 to the power 6 grams per cubic centimeter on earth a teaspoon of serious b material weigh more than 10 tons a normal star is supported by energy flowing outward from its core but a white dwarf can't generate energy by nuclear fusion it has exhausted its hydrogen and medium fuels and converted them into carbon and oxygen when a star collapses into a white dwarf it converts gravitational energy into thermal energy its interior becomes very hot but it can't get hot enough to fuse its carbon oxygen interior instead the star contracts until its score becomes degenerate although a tremendous amount of energy flows out of dark interior it is not the energy flow that supports the star the white dwarf is supported against its own gravity by the pressure of its degenerate electrons a white dwarf's future is bleak acid radiates energy into space its temperature gradually falls but it can't shrink any smaller because its degenerate electrons resist getting closer together degenerate matter is a very good thermal conductor so as heat flows to the surface and escapes into space the white dwarf gets fainter and cooler moving downward and to the right of the hr diagram because a white dwarf contains a tremendous amount of heat it needs billions of years to radiate that heat through its small surface area eventually such objects may become cold and dark so-called black dwarfs our galaxy is not old enough to contain a black dwarf the coolest white wolves in our galaxy are about the temperature of the sun currently so perhaps the most interesting thing astronomers have learned about white dwarfs has come from mathematical models so the equations predicted if you add mass to a white dwarf the radius will shrink because added mass will increase its gravity and squeeze it tighter if you add enough to raise its total mass to about 1.4 solar masses the equations predict that its radius will shrink to zero this is called the chandra scar limit of the shara s sub prime on chandrascra the astronomer who discovered it so here we will see our normal star with radius and mass so if we add more mass onto that degenerate material and we get to about 1.4 solar mass something weird happens and you guys know where this is leading up to yes two black holes so here we can see remnants of these black holes so here in this image this is a false helmet so stars in a binary system can evolve independently of each other if their orbits are large in this situation one of the stars can swell into a giant and collapse without disturbing its companion but some binary stars orbit as close to each other as 0.1 au in one of these stars begin to swell into a giant its companion can be strongly affected these interacting binary stars are interesting in their own right the stars share a complicated history and can experience strange and violent phenomena as they evolve but such systems are also important because they can help astronomers understand the ultimate fate of stars in the next chapter you will see how astronomers search for black holes in interacting binary systems so what happens is mass transfer so binary stars can sometimes interact by transferring mass from one star to the other the gravitational fields of these two stars combined with the rotation of the binary system define a dumbbell shaped volume around a pair of stars called the rush lobes the surface of this volume is called the rush surface here we have that two star so here we will have one rush load and here we will have the other rush loop everyone still following an understanding awesome so the size of the rush lobes depends on the masses of the two stars and the distance between the stars if the stars are far apart the lobes are very large and the stars can easily retain their own material if the stars are close together however the lobes are small and can interfere with evolution of the stars matter inside each star's right lobe is gravitationally bound to the star but matter that leaves a star's large lobe can fall into the other star or leave the binary system completely so looking at the sketch uh there's something interesting that i'm going to show you now so have you guys ever learned about lagrange points have you talked learned in one of your courses yet or haven't you touched on it before so yes so in essence somewhere in classical mechanics so other words newtonian mechanics but i prefer with classical mechanics you are going to learn about lagrangian mechanics have you learned about lacrosse mechanics okay so i thought so because you usually do it in second year when you have the appropriate calculus but what's lagrangian mechanics is nicer say for example you need to solve a system of equations the rule of thumb normally is for each variable you need an equation so say for example you have a large system and you have 30 variables that means you need 30 different equations to solve those variables so in lagrangian mechanics you look at your system and you look at different points where your system is moving so for example the base system usually is if you have a c source a seesaw only has one point where everything is moving so for each place in your model or your system that has a point where you can move around that is you create an equation for that differential equation and then you solve all the differential equations to points where those player points or places of movement will be stable so that means they will know if nothing will interact on that point and that points where the system is stable is called your lagrange points so that's why the james waves telescope is at l2 so that means lagrange point two so that means that's the second point in the system where it will be stable so meaning there won't be any interactions from gravity or if it will cancel each other out so the same principle here we have our different lagrange points so around our diagram here we will have our inner lagrange point or l1 l4 l2 l5 and l3 so the lagrange points are planes in the orbital plane of a binary star system where a bit of matter can reach stability for astronomers the most important of these points is the lagrange point where the two rush lobes meet so something interesting if i continue about lagrangian mechanics so lagrangian mechanics makes it very very very easy to solve for different different systems but it is very mathematically inclined so you usually do triple integrals or triple um differentiation but it is very powerful and makes it much nicer so i know in second year probably if you in the classical mechanics course on a calculus course or applied math calls they will ask you to solve a emotion problem a motion problem for example you throw a ball off a building and you have to solve it with classic mechanics and you have to solve it making use of lagrangian mechanics usually in that system where you use normal newtonian physics you would be able to solve it in about four or five lines of calculation but in lagrange mechanics you should be able to solve in two and three lines of mathematics were much less variables so yes multivariable calculus so in general there are only two ways mata can escape from a star and reach the inner lagrange point the first one is if a star has a strong stellar wind some of the gas blowing away from it can pass through the inner lagrange point and be captured by other star and secondly if an evolving star expands so far that it falls the rush lobe which can occur the star are close together and the lobes are small then mata can overflow through the lagrange point onto the other mass so mass transfers driven by a stellar wind tends to be slow but mass can be transferred rapidly by an expanding star so mass transfer between stars can affect the evolution of the stars in surprising ways in fact it is the solution to a problem that puzzled astronomers for many years in some binary systems the less mass of star becomes a giant while the more massive star is still one main sequence if biomass stars evolve faster than lower mass stars how do low mass stars in such a binary manage to leave the main sequence first so this is called the agile paradox of the binary system agile so yes the agile paradox explains so this paradox can be estimated by mass exchanging so the 0.8 solar mass sub giant star used to be the more massive of the two stars so when the agile binary formed it was a freestyle mass main sequence star with a 1.5 solar mass main sequence companion so as the free syllabus star evolved into a red giant tidal forces began to deform the star and the surface got close enough to the other stars so that gravity pulled matter from one on to the other so here we can see tomatoes being pulled from one on to another so as a result of mass exchange today the giant lost 2.2 solar mass and shrunk into a sub-giant star while the companion star is now a 3.7 solar mass star so mass transfer explains how this could happen imagine a binary system that contains a five solar mass star and one solar mass companion the two stars formed at the same time so the higher mass star evolving faster leaves the main sequence first when it expands into a dry diver it folds its rush lobe and transfers matters to the low mass companion the higher mass star loses mass and evolves into a lower mass star and the companion gains mass and becomes a high mass star that is still on the main sequence that is the reason why there could be a system such as agile that contains a five cellular mass main sequence star and a one solar mass giant so this is what happens in a evolution of a close binary system so here we have star a and star b so star b is more massive than star a so star b becomes to expand and becomes a giant and it falls as gradual and it loses its mass to star a so then star b loses more mass and star a gains more mass and then eventually star a is a massive main sequence star with a lower mass giant companion and this is what happens in the agile system and then we can see things what happens is from there on the star a becomes a giant star again and it fills a square slope and then it starts giving back material to star b so what point does this agile paradox reach equilibrium so it won't reach equilibrium so for this is it will start passing mass from one star to another until it reaches a point where none of these two stars can fuse fuel anymore because remember while the system is going on you are still in a fusion process so what's happening here is so either those two become neutron stars and the neutron stars will collide and that will cause gravitational waves or depending on um way they stop fusing and become neutral stars to become tall cells becoming black old but it all depends on the mass of these two stars and what happens at the end of the life when it stops fusing but this process of sharing mass back and forth can prolong the life of these two stars so now we're going to touch on accretion disks so matter flowing from one star to another can't fall directly onto the star rather because of conservation of angular momentum it must flow into a whirling disc around the star so this is an accretion disk so the mass will be four believe the rush lobes and will form a spinning disc around the other stall called an accretion disk so angular momentum refers to the tendency of a rotating object to continue rotating all rotating objects possesses some angular momentum and in the absence of an external force an object maintains its total angular momentum an ice skater takes advantage of conservation of angular momentum by starting a spin slowly with her arms extended and then drawing them in as the mass becomes concentrated closer to axis of rotation she spins faster so the same effect causes the slowly circulating water in a bathtub to spin in a whirlpool as it approaches the drain so here we can see a skater demonstrates conservation of angular momentum when she spins faster by drawing her arms and legs closer to axes of rotation so mass transferred through the inner lagrange point in a binary system towards a star must conserve its angular momentum if the star is small enough as in the case of a white dwarf the mass will form a rapidly rotating wool pool called an accretion disk two important things happen in an accretion disk first the gas in a disc grows very hot due to friction and tidal forces the disc also acts as a break shifting angular momentum outward in a disk and allowing the innermost matter to fall into the white dwarf the interior parts of an accretion disk around a white wolf of island places the temperature of the gas can exceed a million kelvin causing the gas to emit x-rays and a matter falling inward can produce a vast explosion when enough accumulates on a white dwarf the explosion of this material hardly disturbs the white dwarf and its companion star mass transfer quickly resumes and a new layer of fuel begins to accumulate how fast the fuel builds up depends on the rate of mass transfer you can expect novae to repeat each time an explosive layer accumulates many know they take thousands of years to build an explosive layer but some only take decades now we're going to start talking about novae so at the beginning of this chapter you read that the word nova refers to what seems to be a new star appearing in the sky for a while and then fading away modern astronomers know that anova is not a new star but an all-star flaring up after nova fades astronomers can photograph the spectrum of the remaining faint point of light invariably they find a short period of spectroscopic binary containing a normal star and a white dwarf so nova is evidently an explosion involving a white dwarf nova explosions occur when mass transfers from a normal star through the inner lagrange point into an accretion disk around the white dwarf as the matter loses its angular momentum in the inner excretion disc it settles onto the surface of the white wolf and forms a layer of unused nuclear fuel mostly origin as the layer deepens it becomes then certain hotter until that infuses in a southern explosion that blows the surface of the white dwarf although the expanding cloud of debris contains less than 0.001 solar mass it is hot and it is expanding surface area makes it very luminous now the explosions can become a thousand hundred thousand times more luminous than the sun as the debris cloud expands cools and thins over a period of weeks and months the nova fades from view so here we can see novati exidus europe erupted about two decades expergo spelling shells of gas into space that this hubble space telescope shows in detail the shell consists of knots of excited gas that presumably form when a new shell collides with a shell from a previous eruption now that this is the system depicted in figure 10 12 in the artist impression its name designated the third variable star discovered in the constellation pixadus so in other words translated the compass so what would happen to earth what will be the end of earth so astronomy is about us although this chapter has discussed the deaths of stars and also have been discussing the future of our planet the sun is a media mass star and must eventually die by becoming a giant possibly producing a planetary nebula and collapsing into a white dwarf that will spell the end of earth mathematical models of the sun suggest it may survive as an energy generating star for an additional 8 billion years but it is already growing more luminous as it fuses hydrogen to helium in a few billion years it will exhaust origin and has caused core and swells into a giant star about 100 times its present radius that giant sun will about will be about as large as the orbit of earth so that will mark the end of our world where the expanding sun becomes large enough to totally engulf earth its grain luminosity will certainly evaporate earth's oceans drive away the atmosphere and even vaporize much of earth's crust while it is a giant star the sun will lose mass into space this mass loss is a relatively gentle process so any cinder that might remain of earth will not be disturbed although its orbit will grow larger as the sun's mass decreases if anything is left off earth after the sun becomes a giant he will witness the sun's final collapse into a white dwarf so this is from ethical models that will predict what happens so in 7.588 billion years from now the sun have already lost 0.9 solar mass and it will grow more larger so involving venus in about 7.59 billion years the sun is a great giant and has 0.8 solar mass and its orbit will engulf earth so this is presumably what will be the end of earth so you have seen that low and medium mass stars die relatively quietly as they exhaust hydrogen helium and then drive away the surface layers to form planetary nebula in contrast massive stars live spectacular lives and then destroy themselves in violent explosions so right now we have discussed the style enabler how stars warm the average star the red giant the planetary nebula and a white dwarf and we've also discussed the massive star to become a red supergiant and that will now what we're going to discuss now is that will become a supernova instead of a planetary nebula they will either become a neutron star or a black hole you guys have any questions still following you guys still there you're sleeping okay cool your guys are still awake so now we're going to start discussing the nuclear fusion in massive stars so stars on the upper main sequence have too much mass to die as white wolves but the evolution begins much like that of their lower mass cousins the origin in their cores and ignite hydrogen shells as a result they expand into giants or for the most massive stars supergiants next they cause contract and fuse helium first in the core and then in a shell producing a carbon oxygen core unlike medium mass stars the mass of stars do become hot enough to ignite carbon fusion at a temperature of about 1 billion kelvin carbon fusion produces more oxygen and neon as soon as the carbon is exhausted in a pore the core contracts and carbon ignites in the shell this pattern of coordination and shell ignition continues with fuel after fuel and the star develops the layered structure as shown in the following figure so here we will have our core then outside a call we will have our different view so the elements in fusion silicon core until we reach iron the vitamin fusion the shell above the helium fusion shell above the carbon fusion shell above and above and above and so on so after carbon fuses oxygen neon and magnesium fuse to make silicon and sulfur and then the silicon fuses to make iron the fusion of these nuclear fields goes faster and faster as the mass of star evolves recall that massive stars must consume their fuels rapidly to support their great weight but other factors also causes the behavior fuels like carbon oxygen and silicon diffuse at increasing speeds so hydrogen core fusion can last 7 million years in a 25 cellular mass stop but that same star will fuse oxygen and its core in six months and silicon in a day so here we can see the heavy element fusion 25 solar mass star so helium will take about 300 years uh there's hydrogen helium so now this is a bad figure so i can't receive this zero four but this comes out of the textbook so here we can see for solar mass star helium to hydrogen to helium will take seven billion years helium to carbon will take about 700 000 years carbon to oxygen 600 years oxygen to silicon 6 months silicon to iron one day and then a core collapse in a quarter of a second so now we can talk about supernova explosions of massive stars so theoretical models of evolving stars combined with nuclear physics allow astronomers to describe what happens inside a massive star when the last of its nuclear fuels are exhausted the death of a massive star begins with iron nuclei and ends in cosmic violence so so iron will never fuse to form an heavier element no matter how much it is compressed or energy input there is so yes sometimes the gun happens so when we discuss now in supernova explosions heavier elements are formed so as a star develops iron core energy production begins to decline and the core contracts for nuclei less massive thin ions such contraction heats the gas and ignites new fusion fuels but nuclear reactions involving iron remove energy from the core in two ways so first higher nuclei begin capturing electrons and breaking into smaller nuclei the gas is so dense it is degenerate and the degenerate electrons help support the core the loss of some of the electrons allow the chord to contract even faster second temperatures are so high that average photon is high energy gamma ray and these gamma rays are absorbed by atomic nuclei causing them to break into smaller fragments the removal of the gamma rays heavy also also allows the core to contract even faster although the core of the star can't generate energy by nuclear fusion it can draw on a tremendous energy stored in its gravitational field as the core contracts the temperature shoots up but is not enough to stop the contraction and the core of the star collapses inward in less than a tenth of a second the collapse of the core produces an immense supernova explosion in which the star's outer layers are blasted outward the core of the star must quickly become a neutron star or black hole so the subjects of the next chapter so here we can see the core of massive supergiant has begun to collapse at the lower left corner mata continues to fall inward so the blue and green as the core expands an outward yellow creating a shock wave to show the entire star the scale the stage would have to be 30 kilometers in diameter so only about 0.4 seconds after beginning violent convection in expanding core so red pushes outward so the shock wave will blow the star apart as the neutron star forms at the extreme lower left corner and in this we are left with a supernova remnant so ladies german before we start to classify different supernovas i think we're going to continue with this lecture tomorrow night this is four minutes left of class so you had more you have no questions then you are more than welcome to log off just a bit of admin i will be on campus tomorrow for consultation so you are more than welcome to come for consultation sessions tomorrow otherwise have a wonderful evening if i don't see you tomorrow during consultation then i will see you tomorrow night in class i will remain active for another minute or two of your questions but otherwise have a great evening so questions so sir uh which spectroscopy spectroscopy do you quickly relate to study the characteristics of this robotic visual radio infrared i use radio okay so someone had a hand risen as well so where and what time are the consultations that will be in my office tomorrow so it's natural science building one fifth floor physics department office 68 says national science department floor 5 office 68 i will be there from around about 10 o'clock tomorrow morning to 3 o'clock tomorrow afternoon you are more than welcome to pop in anytime here in those times so let me type it in it's office fifth floor office 68. so it's a natural science building one office five six eight pleasure guys so if you have no further questions then i'm also going to log off so have a wonderful evening everyone and i will see you all tomorrow