The Doppler effect explains why objects moving towards us or away from us at high speed appear to have their colors shifted either towards blue or red respectively.
When an object moves towards us, the crests of the light waves we see from it are compressed together, making the wavelength of the light shorter (and hence bluer), while for an object moving away the separation between crests is stretched, making the light’s wavelength longer (and hence redder). In the simulation above, the monochromatic source of light, as it moves right, would appear blue to an observer on the right-hand side, and red to an observer on the left.
Scientists use spectroscopy — a technique that breaks light up into its component wavelengths — to study the vicinity of supermassive black holes. As matter spins around the black hole, the Doppler effect kicks in, this means that one side appears slightly redder, and the other slightly bluer than it really is. Note that the effect is exaggerated in this computer simulation, which depicts the vicinity of the black hole in the galaxy Messier 87 — one of the first to be studied by Hubble.
Credit: ESA/Hubble (L. Calçada)