1865-1936

German astronomer whose observations provided the first clear evidence for interstellar matter. While observing the spectrum of the binary system Delta Orion, Hartmann noticed that the calcium lines did not exhibit the periodic Doppler-shifting expected from stars orbiting each other. Since these stationary lines could not be produced by calcium in the binary stars' atmospheres, Hartmann concluded they were the result of a mass of stationary gas between Delta Orion and Earth.