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Lecture 19 Review We have talked about our Sun. From a large gas cloud it collapses to a protostar, heating up, compressing, finally igniting. In the process planets are formed from some remaining high angular momentum solar gas and dust particles. A great deal of gas and dust is blown off in the process. This all happens in 100,000 years. The sun arrives on the main sequence of the H&R diagram where it will burns hydrogen stably for 10 billion years. Lacking hydrogen fuel in the core, the core collapses while a shell of hydrogen continues to burn around the core. The outer atmosphere cools, expands, and becomes more tenuous. A smaller core begins to burn helium. This red giant phase may last 2 billion years. Finally gravity wins and the Sun collapses to a white dwarf with a mass of M ~ .6MSun , the rest blown away. Degenerate electron pressure maintains the Sun at a size about R ~ 2RSun . With time the Sun will cool and evolve to a black dwarf. All stars, lighter or heavier, evolve through a protostar stage to the main sequence where they burn hydrogen. The more massive the star, the faster the hydrogen core fuel is consumed. In the end, all stars become unstable when their hydrogen fusion source is exhausted. Instability produces strong stellar winds in which considerable gas is expelled. Doppler shifts show expelled gas with speeds of 1500 to 1900 km/s. This gas produces planetary nebulae. In the end gravity wins. .08 to .25 MSun - These stars can only generate enough pressure to burn hydrogen. Their lifetime is greater than that of the Big Bang. They collapse to a helium white dwarf with R ~ RSun , density 105 to 108 g/cc (1 g/cc = water). .25 to 8 MSun - Similar to the Sun. For stars approaching 8 MSun greater temperatures and pressures are exerted on the core. Enough pressure can be exerted on an 8 MSun star to burn carbon, oxygen, neon, and magnesium. Up to 80% of the mass is expelled in the process. A degenerate white dwarf is left with mass from 1.2 MSun to 1.4 MSun . 1.4 MSun is called the Chandrasahkar Limit - Above this limit electron degeneracy pressure can no longer sustain the surface and further collapse occurs. M > 8MSun Further burning produces iron. The slow neutron process produces some heavy elements. There are no more fusion processes. Beyond this point it takes energy to make a heavier element. At this point gravitational collapse occurs followed by a catastrophic rebound. A fast neutron process produces heavy elements all the way up to Plutonium. There is a strong neutron burst and a supernova explosion. For a few days the supernova has a luminosity L = 10 billion Lsun and then begins to dim. A rapidly expanding nebula marks the spot of the event. The Type 1 supernova results from a binary star system where one of the stars is a very large white dwarf at the Chandrasahkar limit. If the companion star happens to be shedding large amounts of gas, then the white dwarf becomes a supernova. The Type 2 supernova is just a very large star. The question is, what happens after the supernova occurs? The answer depends on how much mass is left. If what remains is between 1.4 and 2.5 MSun , then under great pressure the electrons combine with protons to form neutrons. The density is ~ 1014 g/cc and the diameter is on the order of 20 km, the size of a fair sized city. This is a neutron star. Conservation of angular momentum produces a very high rotation rate. Escaping electrons flow out rotating magnetic field lines and produce synchrotron radiation. This radiation is directional and acts like a light house beacon. The result is radiation that oscillates. When observed these are called pulsars. If what remains is M > 2.5 MSun then gravity really wins big time, you end up with an object whose gravitational force is so large that even light cannot escape the attraction. This is a black hole, and you can’t see it, no matter how close or far you are from it. However, if the black hole is part of a binary system, infalling gasses from the other star heat up, ionize, and emit x-rays. While you can’t see the black hole, the x-rays are an indiction that the black hole is there.