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