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Lecture 17

Lecture 17

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PHYS 183
Tracy Webb

PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 Lecture 17: February 13 , 2013h  3 main properties of star  luminosities, temperatures, masses  most important property of a star is its mass  it determines everything o luminosity & temperature come out of its mass  brightness of a star depends on distance & luminosity o a very faint star to us could be extremely bright, it is just far away from us  2 stars could be the same physical size, but one looks bigger to us because of how bright it is  luminosity = total energy/second o the total amount of power (energy per second) that a star radiates into space o the total amount of light coming out from something  apparent brightness/flux is the amount of starlight reaching earth (energy per second per square meter)  this is the brightness as we see it  the further away the earth is from the sun, the more spread out the sunlight will be, so dimmer  brightness is not luminosity  if you know how luminous the star is, then you can determine how far away it is  inverse-square law  brightness = luminosity / 4πr o means that if you put an object 3x farther away, it will be 9x fainter  can determine the distance by comparing how bright something looks  stellar parallax is a method of triangulation that we use to measure the luminosity of a star  1 parcsec is about 3 light years  parcsec = parallax and distance o a star with a parallax of 1 arcsecond is 1 parsec away from the sun o p = parallax angle o d (in parcsecs) = 1/p (in arcsecs) o d (in light years) = 3.26 x (1/p (in arcsecs)) 6  most luminous stars are 10 sun  least luminous stars are 10 L sun  can measure the temperature of stars through continuum or spectral lines (absorption) o stars of different luminosities are of different temperatures  cold things are redde
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