A dopplergram is a map that shows the way in which different parts of an object that’s producing light are moving towards or away from us. They’re most commonly used for mapping the surface of the Sun. The idea is to measure the way the Doppler effect alters the wavelength of light waves or other radiation coming from different parts of the surface; when the source of waves is moving away from us, the wavelength is stretched, or ‘red shifted’, and when it’s moving towards us, it’s compressed, or ‘blue shifted’. By measuring the red and blue shifts on different parts of the Sun’s surface, we can tell which ones are moving towards or away from Earth, revealing patterns of granulation caused by currents of moving hot gas emanating from our star.
Answered by Giles Sparrow