Asked by: Carwyn Smith, Peterborough
Stars emit light over a whole range of wavelengths (or colours). The wavelength where the amount of light peaks determines the colour we see, although they will also emit plenty of light at other wavelengths. In general, cooler stars appear red and hotter stars appear blue, with orange, yellow and white in-between.
There are no green stars because the ‘black-body spectrum’ of stars, which describes the amount of light at each wavelength and depends on temperature, doesn’t produce the same spectrum of colours as, for example, a rainbow. A star whose peak light emission is at a wavelength we might call ‘green’ actually produces almost as much red light, and our eyes interpret this combination as white, not green. For our eyes to see it as green, a star would have to emit only green light, which is not possible.