wavelength

Intensity of emitted e/m radiation

6000K

2000K

1500K

1000KVIBGYOR

Wien’s Displacement Law for black body radiation

peak wavelength 1 / Temp(wavelength of

maximum intensity)

Wein’s law

max . T = constant (Wein’s constant)

Where Wein’s constant = 0.0029 m K(metres Kelvin)

(not milli Kelvins !)Use it to estimate the surface temp of a star if we approximate the star to being a black body

E.g. peak wavelength of the sun is 490 nm effective surface temp of …

5920K

Stefan’s law

E = T4

Total energy radiated by a black body

- in unit time - per unit surface area - is proportional to the fourth power of the absolute temperature of the bodyWhere E is the energy radiated per second per square metre of surfaceT is the Temp of the black body is Stephan’s constant = 5.67 x 10-8 W m-2 K-4

Estimate the intensity of the radiation emitted per unit area from a star if it’s effective surface Temp is 6000K

Estimate the energy emitted from a star if its peak wavelength is 600nm.

Total power radiated

Power output = E x surface area of starSurface area of a star = 4 R 2

Where R is the radius of the starConclusion: a combination of Wein’s Law and Stephan’s Law can lead to an estimation of the temp of a star and it’s power output. This could allow us to estimate its absolute magnitude and using a rearrangement of m – M = 5 log (d/10) could lead to an estimation of its distance away from us in parsecs. Of course its power output depends not only on the temp of the star but also its size radius and therefore surface area … more techniques are required …