How Bright is that star?

1. How Bright is that star? Part 2 Luminosity And Radius

2. Luminosity Luminosity is the amount of energy a star gives off as E-M Radiation. Measured in Watts or Solar Units or “Sols” However for all practical purposes Absolute magnitude and Luminosity of a star measure the same thing. Absolute MagnitudeApproximate Luminosity -5 10,000 Sols 0 100 Sols 5 1 Sol 10 .01 Sol

3. The Luminosity (L) of a star depends on two things 1. The surface area (A) of the Star… bigger stars are brighter because there is more area to shine. And 2. The luminosity (l ) of a square meter of surface area. Thus L = Al

4. 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 Luminosity of star becomes L = 4πR²σT⁴ Thus Luminosity depends entirely on Radius and Temperature

5. 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