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

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

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 -2 K -4 )

7. Calculating with Stefan’s Law The Sun has surface temperature of 5800K. It’s radius is 6.96 x 10 8 m. Calculate its power output. ( the Stefan constant is 5.7 x 10 -8 Wm -2 K -4 ) The Sun like all stars is an almost spherical black body so: P= 4π (6.96 x 10 8 ) 2 x 5.7 x 10 -8 x 5800 4 P = 3.96 x 10 26 W

8. Calculating with Stefan’s Law A star has a surface temperature of 4800K. Its power output is known to be 3.2 x 10 28 Watts. Calculate its radius r= 3.25 x 10 10 m Compare this with the radius of the Sun