Slipher, Vesto Melvin (1875–1969)

US astronomer who established that spiral nebulae lie beyond our Galaxy. He also discovered the existence of particles of matter in interstellar space. His work in spectroscopy increased our knowledge of planetary and nebular rotation, planetary and stellar atmospheres, and diffuse and spiral nebulae.

Slipher was born in Mulberry, Indiana, attended Indiana University, and in 1902 joined the Lowell Observatory in Arizona. He was director of the observatory 1926–52.

Slipher measured the period of rotation for Venus, Mars, Jupiter, Saturn, and Uranus. His work on Jupiter first showed the existence of bands in the planet's spectrum, and he and his colleagues were able to identify the bands as belonging to metallic elements, including iron and copper. He also showed that the diffuse nebula of the Pleiades had a spectrum similar to that of the stars surrounding it and concluded that the nebula's brightness was the result of light reflected from the stars.

Slipher's measurements of the radial velocities of spiral nebulae 1912–25 suggested that they must be external to our Galaxy. This paved the way for an understanding of the motion of galaxies and for cosmological theories that explained the expansion of the universe.