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lecture7

# lecture7 - Stellar Structure Main Sequence Stars X-ray UV...

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Stellar Structure -- Main Sequence Stars X-ray 1999 storm Visible UV H-R (Hertzsprung-Russell) diagram How do the stars work ? sun -- a typical star M sun <= earth orbits in P = 1 yr w/ R = 1 AU = 1.5 x 10 11 m v = 2 ! R / P = 30 km / s = 67,000 miles/hr ! M = Rv 2 / G = 2 x 10 30 kg R sun : 6.96 x 10 8 m density ( " ) = M / volume = 1.4 gr/cc (earth = 5.5 gr/cc) State of sun : solid, liquid, gas ?? must look at T and pressure ! gas following is meant as an appreciation of deductive powers of physics !! i.e. don’t get hung up on learning the equations , but appreciate the results ! Structure of stars : assume : sun in equilibrium (not shrinking or expanding -- evidence ?) => hydrostatic equilibrium gravity tends to collapse sun, what holds it up ? !! gas pressure -- (more precisely pressure gradient ) at center of sun, what is the weight of overlying matter ? => tells us what the central pressure must be Wt = # G " M interior dr = # G " (4 ! r 3 /3) " dr 0 R R 0 r 2 r 2 P center = GM " /R

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Central Press. = GM " /R M sun = 1.989 x 10 33 gr , R sun =6.96 x 10 10 cm ==> < " > = 1.41 gr / cm -3 , " center = 150 ==> P center ~ 10 17 dynes/cm 2 = 10 11 earth atmospheres !! (central pressure) Central Temp.
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