Hermann Carl Vogel
In 1882, Vogel became the Director at the Astrophysical Observatory Potsdam and he is best known for some work there in 1890. Vogel found that the colour of some stars changed over time, becoming red and then later becoming blue.  His idea was that the star was moving toward and then away from the Earth, and that the colour changes were the because of that movement. These stars seemed to be orbiting around a hidden star as if they were double-stars but the second star could never be seen, only known by the colour changing Doppler effect.