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How Bright is that star? Part 2 Luminosity And Radius

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Luminosity Luminosity is the amount of energy a star gives off as light. Measured in Watts or Solar Units or Sols The Absolute magnitude and Luminosity of a star measure the same thing. AbsoluteMagnitudeApproximate Luminosity -510,000 Sols 0 100 Sols 5 1 Sol 10.01 Sol

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The luminosity of a star depends on two things The surface area (A) of the Star… bigger stars are brighter because there is more area to shine. And The luminosity ( l ) of a square meter of surface area. L = A l

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The Stefan-Boltzmann Law Relates luminosity, temperature and Radius of a star. The luminosity/meter² ( l), is determined by the temperature (T) of that area ) l = σT (σ is a constant which if T is in °K, l comes out in Watts) Surface area is determined by radius(R): A = 4πR² So the total Lumnosity of star becomes L = 4πR²σT

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Luminosity and Radius Thus If we know the luminosity and temperature of a star we can… Use S-Bs law, solved for radius, To find its radius. Thus it is we know the radii of all nearby (<250 Parsecs) stars

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© 2011 Pearson Education, Inc. Chapter 17 Measuring the Stars.

© 2011 Pearson Education, Inc. Chapter 17 Measuring the Stars.

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