Transcript How Bright is that star?

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”
However for all practical purposes 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 Luminosity of star becomes
L = 4πR²σT⁴
Thus luminosity depends entirely on Radius and
temperature
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