Vesto Melvin Slipher (November 11, 1875 – November 8, 1969) was an American astronomer who made foundational observations in the early 20th century that contributed significantly to the development of modern cosmology. He is best known for his pioneering measurements of the radial velocities of spiral nebulae (now known to be galaxies), which provided the first empirical evidence for the expansion of the universe.
Early Life and Education Slipher was born in Mulberry, Indiana. He attended Indiana University, where he earned his Bachelor of Arts degree in 1901, his Master of Arts degree in 1903, and his Doctor of Philosophy degree in 1909. His doctoral research focused on spectrographic studies.
Career at Lowell Observatory In 1901, Slipher joined the Lowell Observatory in Flagstaff, Arizona, under the directorship of Percival Lowell. He remained at the observatory for his entire career, serving as acting director from 1916 and as director from 1926 until his retirement in 1952. His younger brother, Earl C. Slipher, also became a prominent astronomer at Lowell.
Key Contributions
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Radial Velocity Measurements of Spiral Nebulae: Slipher began using spectroscopy to measure the radial velocities of spiral nebulae in 1912. By analyzing the Doppler shift in their spectral lines, he could determine if they were moving towards (blueshift) or away from (redshift) Earth.
- In 1912, he reported the first such measurement for the Andromeda Nebula (M31), finding it to be approaching the Milky Way at a then-unprecedented speed of about 300 km/s.
- By 1914, he had measured the velocities of 15 spiral nebulae, finding that 13 of them exhibited significant redshifts, indicating they were receding from Earth.
- By 1921, he had measured 41 such nebulae, and all but a few showed redshifts, with increasing velocities for fainter (and presumably more distant) objects.
- These observations were crucial for Edwin Hubble and Milton Humason in the late 1920s, who combined Slipher's velocity data with their own measurements of galactic distances to formulate Hubble's Law and provide the first compelling evidence for the expansion of the universe. Slipher's data points formed the foundation of Hubble's velocity-distance relation.
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Atmospheric and Planetary Studies: Slipher also made significant contributions to the study of planetary atmospheres.
- He obtained the first spectra of the major planets, providing insights into their atmospheric compositions.
- He was the first to observe the rotation of Uranus and Neptune using spectroscopic methods.
- His work on Pluto (discovered by Clyde Tombaugh at Lowell Observatory in 1930) included initial spectroscopic studies of its atmosphere and surface properties.
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Instrumentation: Slipher designed and built much of the sophisticated spectroscopic equipment used at Lowell Observatory, demonstrating considerable skill in astronomical instrumentation.
Legacy Vesto Slipher's meticulous and groundbreaking spectroscopic work laid the empirical groundwork for modern cosmology. His accurate measurements of galactic redshifts were an essential component of the observational evidence for the expanding universe, a discovery that profoundly changed humanity's understanding of the cosmos. Despite the critical importance of his data, his contributions are sometimes overshadowed by those who later interpreted and expanded upon them.
Awards and Honors
- Lalande Prize, French Academy of Sciences (1919)
- Henry Draper Medal, National Academy of Sciences (1932)
- Gold Medal of the Royal Astronomical Society (1933)
- The crater Slipher on the Moon is named in his honor.
- The crater Slipher on Mars is named in his honor.
- Asteroid 1766 Slipher is named in his honor.