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Lecture 7

PHYS 183 Lecture 7: Stars

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PHYS 183
Victoria Kaspi

1. Stars  Stars o Sun just an example o Factory of nuclear fusion  Creates heavy elements  Produces heat and light  Takes hydrogen and fuses it to helium in proton proton chain o Many types o Well defined path o Born and die (not eternal)  But much longer than human’s  Known thanks to surveys of many stars  Census shows maximum age o Cant study large lifespan but can test many different ones o Very far away (#shocker)  Closest star is 260k times distance to Sun  Proxima centauri 4.3 ly away = 1.36 PC o Distance to sun = 1AU = 1.5*10^11m o 1 parsec = 1 pc = 3*10^16 m o 1 light year = 1 ly = 9.5*10^15 m = 3.1 pc  Apparent brightness o Distance is reason other stars more faint  Luminosity: amount of energy star radiates per second  Equivelent to power  Eg 100 W light bulb: 100 Joules per second  Apparent brightness decreases with distance  Relationship between luminosity and apparent brightness is inverse square law o Assumes that no matter what direction you’re looking at it emits same light  Isotropic  Total energy is conserved, so it gets spread out to larger area  Each area gets less o Area = 4πD^2 o Basically just square the distance to find factor by which it’s fainter/brighter o BUT some stars are brighter than others!  Stars have different luminosities  Cannot use apparent brightness to determine distance  Must determine intrinsic luminosity (Absolute Brightness)  Or compare brightnesses by imagining all at same distance  Apparent brightness/Intrinsic luminosity  Luminosity is the amount of energy a star radiates per second  b= intrinsic luminosity/4π(Distance^2) o Distance found by parallax  ONLY FOR ISOTROPIC RADIATOR o Intrinsic luminosity = J/s o But the APPARENT brightness = J/s PER M^2  Apparent brightness is what you measure from earth  Distance you measure with other way  Can solve for luminosity  Star Temperature o Intrinsic luminosity determined mainly by temperature  Red stars cooler than blue stars o Measured by star’s spectrum  Use Wien’s law (???) to determine temperature  Emit black body spectrum where peak relates to the temperature
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