f10_nov02 - Luminosity Distance and Flux Luminosity is the...

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Ast 307 - Nov. 2, 2010 ch. 17: Taking the Measure of the Stars Luminosity is the total amount of power that a star radiates Its units are energy per second = Watts The intensity of the star’s light reaching us is called the brightness b , or more commonly, flux f . The units are energy per unit time per unit surface area. Luminosity, Distance, and Flux The apparent brightness of a star depends on its luminosity & distance: Luminosity b = flux = 4 ! (distance) 2 The Inverse-Square Law of Light Why? Because light spreads out in all directions - in a spherical way - and the surface area of a sphere is 4 ! d 2 . (Divide a fixed luminosity evenly over a larger surface area, and there will be less energy per unit surface area.) A star’s parallax is measured by comparing images taken at different times, and measuring the shift of the nearby star relative to more distant stars. Measuring Stellar Distances • The nearest stars have angular shifts of less than 1 arcsecond. • More distant stars have even smaller parallaxes. • The angular shift is inversely proportional to the distance.
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Parallax and the Parsec If p is in arcsec, then d is in parsecs, so that a star with a p arallax of 1 arcsec is 1 parsec distant We choose to define a new unit of length: 1 parsec " 206,265 A.U. = 3.26 light years d ( in pc ) = 1 p ( in ") The parallax angle, p, is defined as half the total annual shift The Scaled Small-Angle Formula
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