1. The Stefan-Boltzmann Law

2. Power is measured in Watts
Power is energy produced in 1 second
Joseph Stefan thought about the power produced per metre squared of a black body

3. The Stefan-Boltzmann Law
Power output per square metre
Wm-2
Temperature K
Joseph Stefan (Austria 1879) found the relationship between the temperature of a black body and the power output per square metre

4. The Stefan-Boltzmann Law
PowerOutput Per square metre (Wm-2)
This is a straight line graph the gradient is a constant (σ) called the Stefan-Boltzmann constant.
(Temperature)4

5. The Power of a STAR is called its luminosity

6. Stars are just about spherical.
The formula for the surface area of a sphere
Where σ is Stefan’s constant (5.7 x 10-8 Wm-2K-4)

7. Calculating with Stefan’s Law
The Sun has surface temperature of 5800K. It’s radius is 6.96 x 108m. Calculate its power output.
( the Stefan constant is 5.7 x 10-8 Wm-2K-4)
The Sun like all stars is an almost spherical black body so:
P= 4π (6.96 x 108)2 x 5.7 x 10-8 x 58004
P = 3.91 x 1026W

8. Calculating with Stefan’s Law
A star has a surface temperature of 4800K.
Its power output is known to be 3.2 x 1028 Watts.
Calculate its radius
Compare this with the radius of the Sun
r= 9.2 x 109m