Supernovae, Pulsars, 15 November 2013!When a red giant ejects its outer shells (which become planetary nebula and 1.dissipate), the iron core that is left behind becomes a white dwarf. !The cores of stars in planetary nebula fall below the main sequence of an A.HR diagram.!They are in the process of cooling off to become white dwarves, but 1.you can just call them white dwarfs.!This is an end point in stellar evolution: !B.This core has a lot of old energy, but it doesn't generate any new.!a.White dwarfs are not stars, but rather the corpses of stars. !C.They are taught in the stars unit because they look like dim, weak, stars. !a.If you measure a white dwarf's spectrum, you find that they are very hot D.despite their small size. !Since they no longer generate energy, they cool over time.!a.Observing white dwarfs is difficult because they are so small and dim.!E.Sirius's companion white dwarf was discovered by its gravitational pull a.on the much more visible star.!White dwarfs have a very strong, concentrated gravitational pull. !F.They have about the mass of the sun, but only the diameter of the earth. !a.They are so concentrated that a teaspoon full of white dwarf would 1.weigh tons. !Gravity is a response to mass–"if mass is concentrated, the gravity is too. !b.The gravitational field near a white dwarf is extremely intense.!1.If white dwarfs suddenly give off a lot of energy, it means they are either novae 2.or bright variable stars. !When a red giant and a white dwarf are very close to each other, the A.gravity of the white dwarf is strong enough to pull the outer shell of the red giant to itself. !This gas will spiral into the white dwarf, form an accretion disk around it, a.and possibly hit the white dwarf. When the mass hits the white dwarf, it explodes. !This process is called mass transfer!1.There are sometimes repeated episodes of mass transfer in closer 2.star systems where mass goes back and forth between members. !The numbers of this process are calculated in flawed computer A.programs. !Any star with more mass added to it will behave like a high mass star, 3.blowing hotter, running through its fuel faster, and dying sooner.!When a white dwarf dies, it explodes. !A.White dwarfs are only discussed when they are in double star systems b.with a red giant like these. !White dwarfs can have white dwarf companions, but nothing 1.spectacular happens!Nova occur when hydrogen hits the surface of a white dwarf and fuses B.right away, creating a surface explosion.!The white dwarf can brighten up fifty thousand times over night, stay at a.max brightness for a few days, decline sharply, and then fade more gradually.!The name "nova" comes from the ancient name "novum stellarum," b.meaning "new star." These stars were so named because they were new to astronomers, although nova are really just the final gasps of very ancient stars. !This is another example of a phenomenon in astronomy with a name 1.based on a wrong first impression! This is always a surprise to astronomers when it happens. !c.Years later, you can still see material around a white dwarf from a novae. !d.Some white dwarfs which are close to companion stars are dwarf novae, C.which is a type of variable star. !Due to a continual drip of material between a white dwarf and a a.companion star, the white dwarf can periodically have a surface explosion for thousands of years.!For example, SS Cygni spends most of its time at minimum brightness 1.until it gets enough mass from its companion to set off a surface explosion. Over night, it brightens and then dims almost as fast. !These variable stars can be measured with the unaided eye. !2.Note: There is another type of variable star called a flare star that b.astronomers do not know much about: !It is dim almost all the time, but then flares brightly for a few minutes 1.and never again. !It has been captured on film several times. !A.No one know why it happens or what the physical process is. !2.Any student who solves this problem to the satisfaction of experts A.in the specialty, gets an instant A for the entire course, regardless of anything else. !Supernova are split into type 1 and type 2 supernovae, the second of which is 3.far more important. !Type 1 supernovae are novae of white dwarfs which received new fuel from A.a companion and completely exploded down to their core (see above). !Type 2 supernovae come from a red giant exploding and creating new B.elements (see below). !Whenever this course mentions "supernovae", it means type 2 a.supernovae.!NOTE: One of the most recent supernovae had a completely new b.brightness curve that didn't match type 1 or type 2 curves. !It appeared to be a type 2, and seems to have come from a blue giant 1.(which is theoretically impossible).!When red giants have finished fusing silicon into iron 56, but can still condense 4.further, they have no choice but to go supernova. !You cannot get any energy from iron 56 by fission or fusion. !A.Any nuclear reaction involving in must be endothermic (takes in heat) a.rather than exothermic (gives off heat). !The mass of the star is still there, so gravity continues to exert inward B.pressure. !Since the star only has iron 56, outward radiation from fusion can no longer C.counteract the gravitational pressure. !This pressure causes the star to suddenly collapse: all the material above D.the core thunders into the core. !Different computations say that this could happen in a day, minute, hour, a.or second. !The material is all of the previous shells of fusion – the outer layers b.where it is still fusing helium into carbon for example. !As the stars' outer layers become compressed in the core, its density, E.pressure, and temperature increases. !This is the perfect environment for everything to fuse with everything. !F.This is the place all the chemicals not mentioned in the last section a.(carbon, neon, oxygen, magnesium, and silicon) are formed. !Every element on the periodic table is formed in these explosions. !b.All of the elements in humans besides hydrogen are supernova 1.material flung around billions of years ago.!These supernovae leave remnants which can be traced by astronomers G.later: an outbound nebula, and an inbound neutron star (or black hole). !The outbound remnant of this supernova is a nebula rich in heavy a.elements.!It is possible to see these without first seeing the supernova 1.explosion.!One of the prettiest outbound remnants is the veil nebula: !A.It is filled with