And the Planck Distribution THERMAL RADIATION And the Planck Distribution

Star Colours When we look at the stars on a dark night we can quite easily see that stars are coloured. This is an effect produced by the surface temperature of a star.

Radiating Heat Standard bodies are black Measure the radiation given off by a black body at every frequency of the spectrum at a fixed temperature

Changing the temperature Think of a black cooker hot plate. It does not remain black but glows when heated Getting Hotter

12000 K 6000 K Intensity W/m2 3000 K Wavelength (m)

Intensity W/m2 λmax Wavelength (m)

Intensity W/m2 Wavelength (m)

The Properties Of The Curves the curve is flatter for lower temperature As the temperature increases the wavelength of maximum intensity (λmax) for that temperature increases in promenance As the temperature increases λmax moves to the left towards higher frequency. At higher temperatures there is a sharp falling off of radiation at values greater than λmax towards a limiting value in the ultraviolet range which is of very short wavelength but not zero. This is referred to as the ultra violet catastrophe.

Wavelength (m) λmaxT = 2.898 x 10-3 In 1893 Wihelm Wien dicovered a simple relationship between λmax and the absolute temperature of a body λmaxT = 2.898 x 10-3 Intensity W/m2 Ultraviolet Catastrophe Wavelength (m) As the temperature of the star rises the λmax moves more towards the violet

Using Wien’s Law λmaxT = 2.898 x 10-3 The spectrum of star  Andromedae is found to be at maximum intensity at a wavelength of 380 nanometres. What is the surface temperature of the star. What colour would you expect this star to be?

Wavelength (m) λmaxT = 2.898 x 10-3 Intensity W/m2 T=7626K λmax = 380 x10-9m Wavelength (m) As the star’s maximum wavelength is beyond the violet end of the spectrum most of the visible light produced will be blue. It is a blue star.

wavelength of maximum intensity(nm) Work out the surface temperature of each of the star’s here using Wien's law Suggest the observed colour of each star star wavelength of maximum intensity(nm) α draconis 430 ε Ursa Minoris 520  cephei 740