Nebular velocities

Up to now, the Andromeda nebula remained a puzzle to astronomers, since no stars could be discerned within the cloud. By studying its spectrum, Slipher determined that it was approaching Earth at aspeed of 125 miles per second, and determined the radial velocities of other nebulae.
 Vesto Melvin Slipher United States 1912 Nebular velocities

Up to now, the Andromeda nebula remained a puzzle to astronomers, since no stars could be discerned within the cloud. By studying its spectrum, Slipher determined that it was approaching Earth at aspeed of 125 miles per second, and determined the radial velocities of other nebulae.