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  • Created on: 18-04-13 14:20


Recognise and use the Stefan Boltzmann Law for Black body radiators

A black body is a theoretical object that absorbs all of the radiation that hits it – none is reflected. Of course the radiation on is re emitted – a perfect absorber of radiation is also a perfect emitter, since if absorbs radiation of all wavelengths, if must also emit at all wavelengths.

Astronomers use a more precise method of determining brightness because we can only see six different levels of stellar brightness with the naked eye. This is called Luminosity.

Luminosity is defined as the rate at which energy of all types is radiated in all directions. This depends on the size and the temperature of the star.

A perfect radiator is called a black body and will give energy of the entire electromagnetic spectrum.

The Stefan-Boltzmann law

So for a sphere it is,


Assuming that a star is a black body, which is a good approximation.

Recognise and use Wien’s Law for Black Body Radiators

This is one method used to calculate the temperature of a star by using its peak wavelength.

where λmax is the peak wavelength, T is the absolute temperature of the black body, and b is a constant of proportionality called Wien's displacement constant, equal to 2.8977685(51)×10−3 m·K (

where λmax is the peak wavelength, T is the absolute temperature of the black body, and b is a constant of proportionality called Wien's displacement constant, equal to 2.8977685(51)×10−3 m·K

Absorption Lines tell us about the chemical composition of stars but also their Doppler shift, which tells us their velocity.

The Hertzsprung - Russell diagram

The Hertzsprung – Russell diagram is a plot of star temperature against luminosity. The general trend is that the temperature increases as the Luminosity increases. This looks like a negative correlation because the temperature axis (x-axis) goes from hot to cool.



The main sequence stars…


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