%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Lecture notes for 23 Oct 2003 (jrigby) WE WILL TALK ABOUT collapse of massive stars FIRST. When a star turns into a neutron star or a BH, there's an explosion. THIS IS THE TYPE OF EXPLOSION WE CALL A CORE COLLAPSE SUPERNOVA. Why? Well, it's triggered when the core collapses. How to make a core-collapse supernova: (20 Msol for example) core hydrogen burning --> He (10^7 years) core helium burning --> C + O (10^6 years) core carbon burning --> O, Ne, Na, Mg ( 300 years) (neutrino cooling is very important) core oxygen burning --> lots of stuff, mostly Si ( 200 days) core silicon burning -> Fe and iron-peak elements ( 2 days) You get the most energy fusing hydrogen --> Helium. You get less energy out of each successive fusion. Therefore, it's like the star's taking out credit cards to pay out other credit cards... what does the star look like now? Roughly, an onion skin (see fig 6.1 in wheeler) once iron core forms, the star will explode within a day! remember, this is a star that's been stable for 10^7 years! By the time an iron core forms, the core becomes so hot that the average photon has enough energy to destroy the heavy nuclei!!! remember how the star made the heavy nuclei in the first place: light elements ===> heavy elements + energy So if the star GOT energy out of making heavy nuclei, destroying the heavy nuclei TAKES energy. Energy goes into breaking apart the iron. Less energy in the core, so lower pressure. The rest of the star is still pushing in, so the core collapses a little. The collapse raises the temperature and pressure in the core, so there are more photons around to destroy the iron, so the pressure decreases. The core starts collapsing in free-fall. THIS IS A RUNAWAY TRAIN! ONCE THE IRON CORE STARTS COLLAPSING, IT'S DOOMED! As the core collapses, the density gets so high that the electrons get squished into the protons, forming neutrons. Now the core is mostly neutrons -- weird! the core collapses in free-fall in *1 second* there's no pressure support, so this happens really quickly if star isn't too massive, neutron degeneracy pressure starts supporting the core (neutron degeneracy pressure = the neutrons get packed so tightly together that they do NOT want to be any closer.) Now the core has pressure support, so it stops collapsing. The rest of the star is still falling inward. It slams into the now-stationary core at high speeds. You can think of the rest of the star as BOUNCING off the core. The bounce sends a shock wave through the star [did the demonstration of a tennis ball (part of the star, just outside the core) falling toward with the basketball (the core). Basketball stops falling (hits the ground) and the tennis ball rebounds off very fast.] SO FAR, THIS IS PRETTY DARN CERTAIN. we know stars burn all the way to iron we know that iron is the most stable element, so we know that you can't create by fusing iron. we know that stars blow up. As it turns out, it's hard, when making a model of the supernova on a (super)computer, to get the stupid star to actually blow up. The shock tends to "stall" partway through the shock. Temperatures, densities are very extreme -- pretty extreme physics. "An astronomer is someone who sees something working in practice, and wonders whether it will work in theory." somehow, the shock wave (or another one, formed soon after) passes through the star neutrinos may get trapped in the shock, heat it and restart it the star may "boil" (convenction) until the shock restarts. AN IMPORTANT POINT: the energy source for the explosion of a core-collapse supernova is the collapse of the core. As the shock wave passes through the star, the layers gets really hot --> fusion in the shells of the onion skin makes lots of heavy elements. it takes about an hour for the shock wave to reach the stellar surface HOW DO WE KNOW THAT??? answer: neutrinos! (computer models also predict it) 12 neutrinos detected in Japan (2.5 hrs before astronomers saw explosion) 8 neutrinos detected in Ohio BTW, this tells you neutrinos have very small mass Now, the supernova was on the far side of the world from Ohio then. HOW DETECTED??? (netrinos go through the earth) Explosive Nucleosynthesis throughout the star: (NOT in the core) fuse carbon in a supernova, make: Ne, Mg, Al, Na, Si (don't need to memorize all these!) fuse oxygen in a supernova, make: Si, S, Ar, Ca fuse silicon in a supernova, make: Fe and similar-mass elements rapid force-feeding of neutrons by nuclei, make: Pb, Au, Uranium what's made depends on how the shock moves through the star also what expands into space vs. what ends up in the remnant test against supernova remnants (sift through the debris) point: MANY/MOST of the heavy elements in the universe get made in supernovae! EXPANSION: the envelope of the star is launched into space at a few percent of the speed of light! ENERGETICS: drop 1 drop of water 1 cm: 10^-3 ergs = 1/1000 of a spider pushup Hiroshima nuclear detonation: 10^20 ergs largest nuclear weapon ever exploded: 10^25 ergs kinetic energy of a supernova: 10^51 ergs neutrino energy of a supernova: 10^53 ergs So a supernova is 10^28 times more powerful than the largest nuke ever built. The largest nuke ever built is 10^28 times more powerful than a drop of water falling 1cm. So to a supernova, a nuke looks smaller than a spider-pushup looks to a nuke. masticating mandibles, batman! a supernova has a lot of energy! 10^53 ergs --> 50 Jupiters turned *completely* into energy WHAT HAPPENS TO THE CORE of the star? If the star initially had a mass of 10-30 Msol, the core will become a neutron star. in some supernova remnants, we've found neutron stars (ex. Crab) but much rarer than we expect - a lingering puzzle. also see pulsars with no apparent SNR - another puzzle. If the star started out as a ~30-100 Msol star, the remnant (after supernova) will be a 2-3 Msol black hole, probably. Why a black hole rather than a neutron star? Well, there's a Chandrasekar (maximum) mass for neutron stars, too. If the star's big enough, the supernova makes a neutron star bigger than the Chandra mass, and it collapses to a black hole. Figuring out where the mass boundary lies is hard! - Can't exactly calculate the Chandra mass for a neutron star (because we don't know it's structure all that well.) - we also don't know how much mass rains down on the core, and how much gets blown outwards RADIATION (early=heat, late=fission) early on, see light b/c the supernova is hot (energy from the core collapse) once the supernova has expanded to twice pluto's distance from the sun, it emits lots of radiation: 10^49 ergs in photons so 100 times more energy in neutrinos than in energy-of-motion and 100 times more energy-of-motion than energy-in-photons after ~50-100 days or so, the brightness is dominated by radioactivity from unstable=radioactive nuclei created in the supernova this is how you make many radioactive elements with ~10^9 yr half-lives. also how you make lots of iron remember that fission stuff from earlier in the course? here it's useful lots of Ni56 created in the supernova equal numbers of protons & neutrons, so can make by combining Helium together (He has 2 p and 2 n) Ni56 decays to Co56, which decays to Fe56 Fe56 is the ultimate end-point of stellar evolution. can't get any more energy out SOME ANIMATIONS! http://zenith.as.arizona.edu/~burrows/movies/smv1.mpeg http://zenith.as.arizona.edu/~burrows/movies/smv2.mpeg (the first animation is the core exploding, and the second follows that explosion through the rest of the star.)