Slipher, Vesto Melvin
SLIPHER, VESTO MELVIN
(b. Mulberry, Indiana, 11 November 1875; d. Flagstaff, Arizona, 8 November 1969)
Slipher, a son of David Clarke and Hannah App Slipher, perfected techniques in spectroscopy and achieved great advances in galactic astronomy. He earned his B.A. (1901), his M.A. (1903), and his Ph.D. (1909) degrees at Indiana University, and received honorary degrees from the University of Arizona (1923), Indiana University (1929), the University of Toronto (1935), and Northern Arizona University (1965).
Soon after receiving his B.A., Slipher was asked by Percival Lowell to join the staff of the Lowell Observatory at Flagstaff, Arizona. Lowell had selected the site because its high altitude was conducive to good visibility. He had obtained a twenty-four-inch Alvan Clark refractor and a John Brashear spectrograph. Slipher installed the spectrographic equipment in 1902 and began work under Lowell’s enthusiastic, driving direction.
Slipher’s main contributions were to spectroscopy, in which he both pioneered instrumental techniques and made major discoveries. His research can be divided into three areas: planetary atmospheres and rotations, diffuse nebulae and the interstellar medium, and rotations and radial velocities of spiral nebulae.
Shortly after arriving at Lowell Observatory, Slipher began studying the rotations of the planets. Since Venus shows no surface markings, optical determinations of its rotation period proved difficult. Slipher oriented the slit of his spectrograph perpendicular to the terminator of Venus and measured the inclination of the spectral lines. In 1903, after taking twenty-six plates, he determined that the period was surprisingly long—certainly much greater than the twenty-four hours that it was commonly believed to be. And in the next issue of the Lowell Observatory Bulletin, Slipher announced his measurements for the rotation of Mars. These results, obtained in the same manner as for Venus, are close to presently accepted values. He continued spectrographic observations of planetary rotation periods, and by 1912 he had measured them for Jupiter, Saturn, and Uranus.
Slipher’s spectrograms also clearly showed, for the first time, bands in the spectra of the Jovian planets. In 1934 Rupert Wildt identified some of the spectral features as being caused by ammonia and methane, and Slipher and Arthur Adel identified many of the remaining bands. For his work on planetary spectroscopy, Slipher was awarded the gold medal of the Royal Astronomical Society in 1933.
In the area of diffuse nebulae and interstellar material, in 1912 Slipher noticed that the diffuse nebulosity in the Pleiades shows a dark-line spectrum similar to that of the stars surrounding the Pleiades: he therefore concluded that the nebula shines by reflected light. This discovery, one of the first to give incontrovertible evidence of particulate matter in interstellar space, paved the way for the work of Hertzsprung and Hubble on emission and absorption nebulae.
In 1908, while studying the spectrum of a binary star, Slipher discovered a sharp calcium line that did not exhibit the oscillatory motion of its companions. Recalling that J. F. Hartmann had found a similar line in 1904, he studied more spectra and found several other such lines. To explain the phenomenon, he correctly reasoned that there must be gas between the stars and the earth.
Thus Slipher’s research on interstellar space was extremely important, for he demonstrated the existence of both dust and gas. During the late 1920’s and early 1930’s, the studies of the interstellar medium by Eddington, Plaskett, Trumpler, and others were directly influenced by his work.
Probably the most significant aspect of Slipher’s research, however, dealt with spiral nebulae. During the fall and winter of 1912, he obtained a series of spectrograms indicating that the Andromeda Nebula is approaching the sun at a mean velocity of 300 kilometers per second, the greatest radial velocity that had been observed.
Slipher continued such observations; and by 1914, when he released his results, he had obtained Doppler shifts for fourteen spirals. Despite the initially enthusiastic response of the astronomical community, many questioned Slipher’s findings. For over a decade—until others began to believe and understand the implications of his findings—he was virtually the only observer investigating the velocities of extragalactic nebulae.
By 1925 Slipher had measured thirty-nine of the forty-four known radial velocities of spirals, the majority of which showed large velocities of recession, as much as 1,125 kilometers per second. Although the nature of spirals was not definitely known until Hubble proved in 1924 that they are external galaxies similar to the Milky Way, Slipher’s early results suggested to a few perceptive astronomers that spirals are exterior to our system. Since the radial velocities of the spirals are so extraordinarily great, they probably could not be contained within the Milky Way. On 14 March 1914, just weeks after Slipher’s original announcement, Hertzsprung wrote to him:
My harty [sic] congratulations to your beautiful discovery of the great radial velocity of some spiral nebulae.
It seems to me, that with this discovery the great question, if the spirals belong to the system of the Milky Way or not, is answered with great certainty to the end, that they do not....
Moreover, H. D. Curtis, the chief proponent of the revival of the “island universe” theory (before Hubble’s discovery of Cepheids in spirals), appears to have been influenced by Slipher’s findings.
Hubble’s velocity-distance relationship, first presented in 1929, was made possible by Slipher’s velocity measurements. To construct the relationship, Hubble used these velocities and the distance measurements available for eighteen isolated nebulae and four objects in the Virgo cluster. The relationship also was used to compute distances for the nebulae on Slipher’s list in which no stars could be detected.
The possibility of a relationship between distances and velocities of galaxies had been considered for years; C. Wirtz, K. Lundmark, and others had attempted unsuccessfully to construct such a relationship. Reliable distances were needed, but they were unobtainable without large instruments (like Mount Wilson’s Hooker telescope) and ingenious techniques (like Shapley’s period-luminosity law). Credit deservedly belongs to Hubble for his work on measuring these distances and in recognizing their relationship to the velocities; nevertheless, Slipher’s findings were crucial to the discovery. His work prepared the way for investigations of the motions of galaxies and for cosmological theories based on an expanding universe.
Slipher also measured rotations of spirals, using the technique he had developed in his studies of planetary rotation. He found rotational velocities on the order of a few hundred kilometers per second and the direction of motion to be such as to “wind up” the spirals. These results contradicted the controversial proper motion measurements of Adriaan van Maanen. This discrepancy was not entirely resolved until the 1930’s, when it was demonstrated conclusively that van Maanen’s measurements had been subject to systematic errors.
Other areas of Slipher’s research included the determination of radial velocities of globular clusters, spectroscopic studies of comets and aurorae, and observations of bright lines and bands in night sky spectra.
Slipher was also an unusually competent administrator: indeed, in recognition of that ability, as well as for his research, the Astronomical Society of the Pacific awarded him the Bruce Medal in 1935. He received his first experience in administration in 1915, when Lowell made him assistant director of the observatory. He became acting director upon Lowell’s death in 1916 and continued in that capacity until he was made director in 1926, a post he held until 1952. During his directorship he supervised the trans-Neptunian planet search, which culminated in 1930 in the discovery of Pluto by Clyde Tombaugh, a staff member at Lowell.
Slipher’s other administrative experience included serving as president of the Commission on Nebulae (no. 28) of the International Astronomical Union (1925 and 1928), vice-president of the American Astronomical Society (1931), and vice-president of the American Association for the Advancement of Science (1933).
In his work with the I.A.U. commission, Slipher made another important contribution to astronomy. As its president he became the center for all information concerning nebulae, serving as coordinator and organizer during the mid- and late 1920’s, when the nature of galaxies and their relationship to the universe as a whole were being discovered by Hubble, Landmark, and others.
Slipher was a member of the American Academy of Arts and Sciences, the American Philosophcal Society, the Astronomical Society of France, Phi Beta Kappa, and Sigma Xi. He received Lalande Prize of the Paris Academy of Sciences in 1919 and the Draper Gold Medal of the National Academy of Sciences in 1922.
I. Original Works. Slipher’s most important publiccations include “A Spectrographic Investigation on the Rotational Velocity of Venus.” in Lowell Qbservatory Bulletin, no. 3 (1903), 9–18: “On the Efficiency of the Spectrograph for Investigating Planetary Rotations and on the Accuracy of the Inclination Method of Measurement: Tests on the Rotation of the Planet Mars,” ibid., no. 4 (1903), 19–33; “The Lowell Spectrograph,” in Astrophysicat Journal, 28 (1908), 397–404; “Peculiar Star Spectra Suggestive of Selective Absorption of Light in Space.” in Lowell Observatory Bulletin, no. 51(1909), 1–2: “The Radial Velocity of the Andromeda Nebula,” ibid., no. 58 (1913), 56–57; “Spectrographic Observations of Nebulae,” in Popular Astronomy, 23 (1915), 21–24; and “Spectroscopic Studies of the Planets,” in Monthly Notices of the Royal Astronomical Society, 93 (1933}, 657–668.
II. Secondary Literature. Obituaries are in Publications of the Astronomical Society of the Pacific, 81 (1969), 922–923; and New York Times (10 Nov. 1969). 47. Two excellent biographies of Slipher have been prepared by John S, Hall: “V. M. Slipher’s Trailblazing Career,” in Sky and Telescope, 39 (1970), 84–86; and “Vesto Melvin Slipher,” in Yearbook. American Philosophical Society (1970), 161–166. Comments on Slipher’s research were published on the occasion of several of his awards: S. Einarsson, “The Award of the Bruce Gold Medal to Dr. Vesto Melvin Slipher,” in Publications of the Astronomical Society of the Pacific,47 (1935), 5–10; and “Gold Medal Award, President’s Speech,” in Monthly Notices of the Royal Astronomical Society, 93 (1933), 476–477. For additional, related information, see Otto Struve and Velta Zebergs, Astronomy of the 20th Century (New York, 1962); A. Pannekoek, A History of Astronomy London, 1961); and J. D. Fernie, “The Historical Quest for the Nature of the Spiral Nebulae,” in Publication of the Astronomical Society of the Pacific, 82 (1970), 1189–1230. Slipher’s private papers and correspondence (including the Hertzsprung letter cited above) are cataloged and are on microfilm at Lowell Observatory, Flagstaff, Arizona.