w8.sun - Stars Huge spheres of gas floating in space...

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Stars • Huge spheres of gas floating in space • Composed primarily of H, He. • Produce their own energy. • Our Galaxy: 10 11 (100 billion) stars. • The Sun: a typical star • Stars range from ~ 0.1 to ~ 20 M  • Individual stars evolve as they age… sun is in middle of its ~ 10 10 year lifespan. M  = solar mass The size of the Sun • Diameter: 1.4x10 6 km • 109 x Earth • Mass: 2x10 33 g • 333,000 x Earth • Distance from Earth • 93 million miles • 1.5x10 8 km • 8 light minutes • Luminosity (amount of energy emitted per unit time): 4x10 26 watts •1 0 15
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The Composition of the Sun • From spectroscopy of outer layers [Table 14.2] The Interior of the Sun • (almost) no observations of interior • Interior structure deduced from observing the outside • Luminosity •M
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What Powers the Sun? • Need to provide •4x10 26 watts < 2x10 33 grams (mass of Sun) > 4.5 billion years (age of Earth) but … each battery lasts only 43 hours • 43 hrs x 20,000 = 98 years. / • 139g / battery ==> 10 29 batteries total • room for 20,000 times more batteries in reserve. • Duracell batteries? • A single D-cell: • 0.5 watts / battery ==> 8x10 26 batteries required. . Gravitational contraction? • Converts gravitational potential energy into kinetic energy • Kinetic energy in a gas = heat • Collisions between atoms convert heat to light • To provide 4x10 26 watts • sun must contract by 40 meters per year • 40m x 2000 years of observations: undetectable! • But total available gravitational energy = 10 42 • 1 watt = 1 Joule/second •1 0 42 J / 4x10 26 J/s = 2.5x10 15 seconds = 79 million years. • 800,000 x better than Duracell batteries But not good enough (we need > 4.5 billion years). /
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What Powers the Sun? Hydrogen: 1 H Helium: 4 He • Need to provide •4x10 26 watts < 2x10 33 grams (mass of Sun) > 4.5 billion years (age of Earth) • How about H bombs? •4 x 1 H 4 He + neutrinos + energy 4 x Classical: v’ = (v - u) Special relativity: v’ = (v - u)/(1-uv/c 2 ) The Principal of Relativity. The laws of physics are the same in all inertial reference frames. The constancy of the speed of light. Light travels through a vacuum at a speed c which is independent of the light source. Special Relativity Î E = mc 2 100 mph 60 mph Î distance, time, velocity add up in funny ways Î Total energy of a particle moving at constant velocity v: Classical: E = 1/2 mv 2 Special relativity: E = mc 2 / 1-v 2 / c 2 ~ 1/2 mv 2 + mc 2 “Rest Energy” is there even when v = 0
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•4 x 1 H 4 He + neutrinos + energy • Atomic structure • Proton: + charge, 1.67265x10 -27 kg • Neutron: 0 charge, 1.67495x10 -27 kg • Electron: - charge, 9.11 x10 -31 kg • Neutrino: 0 charge, 0(?) mass How about H bombs?
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This note was uploaded on 12/02/2010 for the course PHYS 125 taught by Professor Cline during the Spring '10 term at McGill.

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w8.sun - Stars Huge spheres of gas floating in space...

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