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How Bright is that star? Part 2 Luminosity And Radius 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. Absolute Magnitude Approximate Luminosity -5 10,000 Sols 0 100 Sols 5 1 Sol 10 .01 Sol 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 = Al 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 = σT⁴ (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 = 4πR²σT⁴ Luminosity and Radius Thus If we know the luminosity and temperature of a star we can… Use S-B’s law, solved for radius, To find its radius. Thus it is we know the radii of all nearby (<250 Parsecs) stars