Wien's Law and Black-body Radiation
The brightness (or luminosity) of a star depends upon its temperature, which in turn determines the star's colour. This can be inferred by using photometry to calculate a colour index. This is because stars produce the majority of their light as perfect thermal radiators (known as black-bodies).

Credit: National Schools' Observatory
λmax Teff = 2.898 × 10-3 m K
Note that the m and K (metres and kelvin) are included in the equation to make the units balance.
An example of the use of this equation is the Sun, whose peak wavelength is at around 500 nm (5 x 10-7 m)
So Teff (in kelvin) = 2.898 × 10-3 / 5 x 10-7 = 5800 K
The HyperPhysics website contains an online calculator to convert temperature to peak wavelength, frequency, photon energy.
So measuring the colour of a star allows us to produce a reliable estimate of its temperature. The simplest way of doing this is by use of colour filters.
Read more about filters.
Find out about the electromagnetic spectrum.
- A star's radius if we know its mass
- No, Wien's Law relates a star's temperature to its peak emission
- A star's mass if we know its peak wavelength
- No, Wien's Law relates a star's temperature to its peak emission
- A star's "surface" temperature if we know its peak wavelength
- Yes, this is correct - if we know either its temperature or its peak emission wavelength, we can calculate the other value
- A star's distance if we know its brightness
- No, Wien's Law relates a star's temperature to its peak emission