The whole of science is nothing more than a refinement of everyday thinking. Albert Einstein, Physics and Reality

Assignments: Read Part 6 (pp. 498-499) and Chapter 31, Sections 1-2 (pp. 500-505) in your textbook. Check out these images from NASA's Astronomy Picture of the Day: 1.) Massive star on the Verge of a Supernova Explosion 2.) Supernova Remnant and Neutron star 3.) Star shreds from a Supernova Explosion Have a good break.

In Class: Once a massive star fuses its core into iron, there's no more energy available from fusion - iron is the end of the line, the world's more stable nucleus Just as with the white dwarfs, it's now up to the electrons to hold things up against gravity -- however, for cores with mass greater than 1.4 solar masses - the "Chandrasekar limit" - electrons can't do it -- they literally get pushed into the protons and turned into neutrons -- PROBLEM! - neutrons have no charge - they can be stacked up right next to each other - super dense -- when the electrons are finally defeated, and pushed into the protons, the core collapses to a tiny fraction of its original size virtually disappears (NEUTRON STAR == NS) - where a WD might be the size of the Earth - a NS might be as small as 10 mi across (WITH THE SAME MASS!) Consider an analogy: You're standing on top of the World Trade Center (1000 ft up) The World Trade Center suddenly becomes 2.5 ft tall you're hanging out in space like the Coyote in a Road Runner cartoon -- you fall 1000 ft and splat onto the 2.5ft tall World Trade Center Same thing happens in stars -- core virtually disappears -- the rest of the overlying layers are no longer supported - come crashing down onto the NS - the ultimate in implosion - Because NS's are really solid, material literally "bounces" off the surface - creates a very big explosion - kaboom! - shock wave and nuclear fireball propagate outward at nearly the speed of light - shred the star and eject everything into space --> SUPERNOVA EXPLOSION -- luminosity reaches 10 billion L_sun -- more energy released in a few hours than during the star's entire lifetime -- for a little while, one star can be more luminous than an entire galaxy of 100 million stars This is also the time of heavy element nucleosynthesis - aka "explosive" nucleosytnthesis - production of elements heavier than iron - not energetically favorable - don't get any energy for it - that's why it doesn't happen in the core - but during SN, there's plenty of energy around - this is where you get all of the elements heavier than Fe - this is also why there aren't many of those In fact, the stellar evolution process explains the abundances of the elements pretty well - If the universe started as mostly H - produce a bunch of He - less C,N,O - a lot less Mg, Ne - a bit of Fe - almost none of the other stuff - corresponds pretty well to current cosmic abundance estimates --------------------------- Famous Supernovae - Crab Nebula progenitor 1054 - Tycho's 1572 - Kepler's 1604 - SN1987A - brightened within 3 hours - details in your text Supernova remnants - the ejecta - splatterred all over the interstellar medium - pretty hot - started hot - heated by explosion - density is low --> spectral line emission -- substantial enrichment of the interstellar medium - especially in metals --------------------------------------------------- Looking at the ashes of stars: WD, NS, BH WD: odd, incredibly dense form of matter - held up by electron degeneracy NS: odder still - electrons and protons have been pushed together - only neutrons - like a giant single nucleus - has nuclear densities - doesn't collapse any more becuase of neutron degeneracy BH: the oddest of all - gravity is too strong even for neutron degeneracy - what is it made out of: I dunno - densities even higher than nuclear - "what it is" is really beyond our understanding - "how it acts" we can talk about consider how regular gravity works - from Newton F = GMm/r^2 - force is proportional to masses and separation - we are pulled toward the center of the Earth because we have mass, and the Earth has mass - how hard we're pulled has to do with - our mass - Earth's mass - separation between us and all of the Earth's mass. - it's the combined pull of all of the different parts of the Earth - there are some nearby parts - but most parts of the Earth are far from us - their pull isn't too strong - now consider making the Earth more dense - put same mass in a smaller package (i.e., squeeze it down to a smaller size) - now each of those parts that used to be distant from us are nearer - can pull harder on us - net result: force of gravity on us is stronger - note that we have NOT INCREASED the mass only increased the density - to get off the Earth requires energy - lift mass off surface - push against force of gravity - in physics, this is formally called work - if the force of gravity is stronger (as with a squished Earth) - need more energy to get off the surface - we can think of this in terms of how fast we would have to shoot something in order to get it to leave the Earth - called "escape velocity" - making something move fast is the same thing as giving it lots of energy - it will "use" that energy to get off the surface - you can imagine a situation where the force of gravity is so great that you'd need to give an object an enormous amount of energy to get it to leave - escape velocity would be really high - for really strong gravity, escape velocity might be near the speed of light - but how can anything travel faster than light? -- It can't ---------------------- Enter Einstein - two important aspects of relativity theory 1) you can't go faster than the speed of light 2) mass and energy are just different forms of the same thing (i.e, E = mc^2) point #1 indicates that there are some places you can't get out of - no limit on how strong gravity can get - just put in more mass, or - get the density really high - but you can't go faster than the speed of light, so if gravity is really strong, you can't escape point #2 makes matters even worse - since energy is just another form of mass - energy feels the pull of gravity - that means photons feel the pull of gravity - they have to "work" to get out, too. - where does the energy to do this work come from? - from the photons themselves - after all, they're pure energy - if we use up some of the energy of a photon in getting out of a high gravity region - there will be less left when the photon gets out - the photon will have lower energy than when it started - lower energy = longer wavelegnth - the photon will actally change wavelength in getting out - seriously wierd - because it gets shifted to longer wavelengths by gravity this is called "gravitational redshift" - the stronger the gravity, the more energy the photon loses in getting out - don't notice the gravitational redshift on Earth - gravity is too wimpy - can just measure it from stars like the Sun - toward WDs and NS, it's a very strong effect - what if gravity is too strong? - ie., what if the energy required is greater than the energy of the photon? - doesn't get out - even photons can't leave - nothing gets out --> BLACK HOLE Not only can no matter even leave a black hole, but even light can't leave - i.e., no communication betweeen inside a BH and the outside universe - completely detached from our universe - very wierd