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

2. Luminosity
Luminosity is the amount of energy a star gives off as light. 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 Magnitude Approximate Luminosity
,000 Sols
Sols
Sol
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