Don't forget who you are You're a rock and roll star The Byrds, So You Want to be a Rock and Roll Star

Assignments: Read Chapter 26, Section 7, and all of Chapter 27 (pp. 433-449) for Monday's class Problem Set #4 due Thursday 4 March

In Class: ----------- review: nuclear reactions - involve only nuclei - protons and neutrons - electrons don't play much of a role - key is to get stability - not just any combination of p and n will work - some combinations will be unstable - might hold together for a little while - how long an unstable arrangement will last is often called a "half-life" - time required for half of the nuclei to fall apart (on average) - unstable combinations will "decay" - types of decay - fission - 92U235 + n --> 55Ce140 + 40Zr94 + n + n (92p,143n + n --> 55p85n+ 40p54n + n + n) - bop it with a neutron - proton-neutron interchange (beta decay) neutron can change to a proton - 1H3 --> 2He3 + e- (tritium to helium-3) - for charge balance emits an e- started neutral; have to end neutral neutron falls apart into p+ and e- proton can change to a neutron - 7N13 --> 6C13 + e+ - for charge balance emits an e+ (positron) started +; have to end + proton falls apart into a neutron and positron proton--> neutron process happens a lot in the Sun - lots of protons around - try to make nuclei with only protons too many protons; too much electronic repulsion - they either fall apart - or they convert protons to neutrons until stable ------------------- first step in Sun-like stars In the hot core T = 10-20 million K size = 200,000 km (~25% of the Sun's radius) mass = ~10-20% of the Sun's mass 1H1 + 1H1 --> 1H2 + e+ + neutrino conversion to deuterium involves proton-->neutron conversion positron doesn't get far hit e- and annihilates --> lots of photon energy photons are absorbed locally and HEAT THE CORE neutrino is a very wierd particle - doesn't really interact with anything - cruises right out of the Sun, right through the Earth through us, through basically everything - the only bit that comes straight from the solar core to us - very hard to detect - still can provide a window into the solar interior (you can read about it in your text) OK, so we make deuterium, what next? 1H2 + 1H1 --> 2He3 + energy(i.e., photon) no extra magic here just stick another proton on helium-3 is pretty stable get the energy out in the form of a photon quickly absorbed by nearby stuff heats the core (which is, after all what we're trying to do here) So we can make lots of helium-3 2He3 + 2He3 --> 2He4 + 1H1 + 1H1 helium-4 is the "standard" helium; completely stable liberates two protons; can participate in the next round Net result: take 4 protons --> make 2He4 (actually take 6 p --> 2He4 + 2 extra protons, but the net result is above) 4 protons weigh 4 * 1.6726 x 10^-27 kg = 6.6904 x 10^-27 kg 2He4 weighs 6.6429 x 10^-27 kg - we're missing a mass of 4.75 x 10^-29 kg - where did this mass go? - E = mc^2 - turned into energy - mass and energy are more closely related than you think - can convert mass into energy and, perhaps even more oddly, can convert energy into mass - can calculate the energy liberated by measuring the "lost" mass - E = mc^2 gives you the energy - example: for one PP reaction: mass lost = 4.75 x 10^-29 kg E = (4.75 x 10^-29 kg)(3 x 10^8 m/s)^2 = 4.28 x 10^-12 kg m^2 / s^ = 4.28 x 10^-12 Joules - the mass "lost" is the source of energy for the Sun - conversion from 4H's to He results in mass "loss" - so in some odd way, the Sun really is burning up The pp chain is the main process occurring in the Sun