This short essay describes the work of Vesto Slipher, a rather unsung astronomer who at the turn of the 20th century made many of the fundamental observations that led to Edwin Hubble's discovery of an expanding universe.
Vesto Melvin Slipher – Astronomical Hero
W. Keith Fisher
Vesto Melvin Slipher was born in Mulberry, Indiana in 1875 and educated in Astronomy at the University of Indiana, where he received his doctorate in 1909. Although Slipher received many honors during his scientifically productive years as the world’s foremost astronomical spectroscopist, he was overshadowed in his most important contribution to science, the measurement of radial velocities of spiral nebulae (now known as spiral galaxies), by the flamboyant Edwin Hubble (General Notes 1969). Slipher being a humble, modest man never demanded his share of the glory. In this essay I will first discuss Slipher’s contributions that make him an astronomical hero then propose some reasons why his name and fame have faded over time.
In 1900 Percival Lowell ordered a custom-built spectrograph built by the John A Brashear, Co. to couple to the 24 inch Clark refractor at his observatory (appropriately named the Lowell Observatory) in Flagstaff Arizona. In 1901 he hired Slipher, who arrived at Flagstaff that summer (Bartusiak 2009).
It turned out that Lowell chose the right man. During the first two decades of the twentieth century Slipher made many important contributions to astronomy through spectroscopy. He took this spectrograph, intended for planetary work, and with great skill modified it to measure spectra of galaxies. As we will see Slipher found himself confronting results that gave the first indication that we live in an expanding universe (Bartusiak 2009; Slipher 1917).
II. Sliphers Accomplishments
When Slipher first arrived at Lowell his emphasis was on developing his expertise in using the 24” refractor plus spectroscope, which he did very successfully, eventually becoming a world leading expert on planetary spectroscopy. He was painstaking in his attention to detail regarding calibration and precise measurement of the wavelength of spectral bands (Slipher 1917). This emphasis on planetary spectroscopy was not surprising since his financial support came from Percival Lowell, whose interest was in planetary science, especially observations of Mars in his quest for canals.
In 1903 Slipher undertook a spectroscopic investigation of the rotational period of Venus. In this work he developed the inclined line method of measuring rotational period. For this measurement the slit of the spectroscope was oriented perpendicular to the rotation axis of Venus so that part of the light coming through the spectroscope slit was from the approaching side of Venus while the remainder was from the receding side. The result was that the spectral lines were inclined, and with the proper mathematics the velocity of rotation could be derived. He went on to use this method to measure the rotation rate of Mars, Jupiter, Saturn and Uranus (Slipher 1903; Slipher 1903a).
Slipher followed up this work with a photographic study of the spectra of Neptune and Uranus. Exposure times were long, 14 hours and 21 hours respectively. Using his careful calibration techniques he was able to determine the wavelength of observed spectral bands with good accuracy (Slipher 1904).
Then in 1905 and 1906 he continued working toward his goal to determine the condition and substance existing in the atmospheres of planets, with photographic studies of Jupiter and Saturn. After experimentation with red sensitive photographic plates, He obtained photographs of Jupiter and Saturn in the red-orange-yellow spectral region (Slipher 1905; Slipher 1906a).
During 1906 and 1907 he did extensive experimentation on methods to sensitize the plates to red light. He developed a method of stretching the spectral response of plates all the way out to 700 nm. He used plates sensitized in this manner to again photograph the spectra of Jupiter, Saturn, Uranus and Neptune. Many spectral lines were identified and differences in the spectra of these planets noted in detail but almost nothing was concluded regarding the chemical identity of the bands. Hydrogen was identified in all four planets with perhaps a low concentration of water vapor (Slipher 1906). In later work some of these lines were identified by Slipher and others to be from ammonia and methane (Abrahams).
In addition to his groundbreaking work on planetary spectroscopy he found that nebulae such as the Merope nebula in the Pleiades had the same spectral signature as that of the other nearby bright stars. He concluded that the nebula was “pulverized matter” that was shining by reflected light. Slipher had just discovered reflection nebula (Lowell & Slipher 1914). In addition he discovered interstellar sodium and found the first evidence of interstellar calcium, when in the spectrum of a binary system, the calcium line showed no oscillation (Bartusiak 2009; Abrahams). Slipher studied the radial velocities of globular clusters, the spectra of comets and the aurora. However it was his spectroscopic studies of spiral nebulae, the work where he failed to garner his fair share of credit that was his most significant contribution to astronomy.
His studies on spiral galaxies were motivated by a letter from Percival Lowell in 1909 asking him to use his red sensitive plates to photograph the spectra of spiral galaxies. At first Slipher balked at this request. Because of the slow speeds of photographic emulsions in 1909 he knew that the exposure times would be at least 30 hours with the long focus refractor. This telescope did not have sophisticated tracking capability, so keeping the slit of the spectrograph trained on a spiral nebula over the long exposures would be a formidable task. Slipher cleverly modified the spectrograph to operate 200 times faster than the original specifications and he carried on with the work (Bartusiak 2009).
The first glimmer of success came on September 17, 1912 when he was able to photograph a spectrum of the Andromeda Galaxy using an exposure time of nearly 7 hours. He showed for the first time that the Andromeda Nebula was approaching the Sun with the extraordinary velocity of 300 km/s, more than ten times the rate of average stars. He also measured its rotation rate using the inclined line technique and found it to be very high. Further work though 1912-1913 showed that spiral nebula as a class have a much higher order of velocity than have the stars (Lowell & Slipher 1914; Slipher 1917).
Of the twenty five radial velocities measured, only four were approaching, the remainder were receding and at extremely high velocity. In 1917 Slipher did not appreciate the meaning of the preponderance of receding spirals. His hypothesis at the time was that they were indicative of motion of our stellar system relative to the other spiral nebula. He did realize that, based on the star-like characteristics of their spectra, the spirals contained stars. But it was the high radial velocities of the spirals that led him to believe that they were an entirely different class of objects from stars, globular clusters and diffuse nebula. His observations eventually led him to embrace the “island universe” theory, which regards our stellar system and the Milky Way as a great spiral nebula seen from within and that the spiral nebula were at great distance relative to the stars (Slipher 1917).
Slipher presented his work on radial velocities of spirals at Northwestern University in 1914. In the audience was an ambitious young astronomer, Edwin Hubble, who realized that the only way to confirm the island universe theory was to measure the distances of these spirals. This occurred in 1923-24 when Hubble, using the 100 inch telescope on Mt. Wilson, identified Cepheid variables within the Andromeda Nebula and used their pulsation period and their period-luminosity relation to establish that it was indeed at very great distance and was a separate island universe. About five years later, working with Milton Humason, Hubble identified the proportionality between radial velocity and distance, the constant of proportionality became know as the Hubble constant (Hubble 1929).
III. Why Slipher Isn’t Famous
At issue here is that Hubble failed to give Slipher the credit he deserved for the radial velocity data in his famous 1929 paper where the relation between distance and radial velocity was first reported. To put in perspective the magnitude of this nefarious deed, by 1925 Slipher had measured the radial velocity of about 40 spiral nebula while only four radial velocity measurements had been done by astronomers in systems that Slipher had not studied first (Gribbin 2002). It was not until a lecture given in 1953 did Hubble give credit where credit was due. In this lecture he professed that his discovery of the distance-velocity relation in 1929, which was later realized to be indicative of an expanding universe, “emerged from a combination of radial velocities measured by Slipher at Flagstaff with distances derived at Mt. Wilson” (Bartusiak 2009).
So Hubble’s failure to recognize Slipher's accomplishment is one reason for his relative obscurity among astronomy’s elite, but it is not the only reason. Another reason is Slipher chose to publish most of his results in brief accounts in the Lowell Observatory Bulletin instead of a major astronomical journal (Bartusiak 2009). He did publicize his radial velocity measurements more widely but he was steamrolled by Hubble.
A third reason is that Slipher did very little original astronomical work after 1933. He became more interested in business pursuits and financial security, which was quite understandable as he was living through the depths of the Great Depression. However he remained director of the Lowell Observatory and let the facility slide into stagnation. It was not until the 1950’s, after Slipher resigned, that the Lowell Observatory revamped and was brought into the twentieth century (Tenn 2007).
So that’s it, Hubble gets a fundamental physical law named after him, credit for the discovery of the expansion of the universe and a large space telescope in his name. Slipher gets some belated credit. But it should be realized that, despite his unproductive later years, his measurements provided half of the critical data set that led to the discovery of the expansion of the universe and that his methods and the equipment he developed during the first two decades of the twentieth century were at the very cutting edge of technology for its time.
Abrahams, Peter, “Early Instruments of Astronomical Spectroscopy,” web site www.europa.com/~telscope/histspec.txt.
Bartusiak, Marcia, 2009, Sky and Telescope, Vol. 118, No. 3, p. 30-35.
Hubble, Edwin, 1929, “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae,” in The Early Universe: Reprints, ed. E. W. Kolb and M. S Turner, (Redwood City, CA., Addison-Wesley Publishing Company), p. 9.
General Notes, 1969, Publications of the Astronomical Society of the Pacific, Vol. 81, No. 483, p. 922.
Gribbin, John, 2002, The Scientists, Radom House, New York, 591.
Lowell, Percival and V.M. Slipher, 1914, “Epitome of Results at the Lowell Observatory, April 1913 to April 1914,” Lowell Observatory Bulletin, Vol. II, No. 59.
Slipher, V.M., 1903, Lowell Observatory Bulletin, Vol. 1, No. 3.
Slipher, V.M., 1903a, Lowell Observatory Bulletin, Vol. 1, No. 4.
Slipher, V.M., 1904, Lowell Observatory Bulletin, Vol. 1, No. 13.
Slipher, V.M., 1905, Lowell Observatory Bulletin, Vol. 1, No. 16.
Slipher, V.M, 1906, Lowell Observatory Bulletin, Vol. 1, No. 42.
Slipher, V.M, 1906a, Lowell Observatory Bulletin, Vol. 1, No 27.
Slipher, V.M., 1917, “Nebulae,” Proceedings of the American Philosophical Society, Vol. 57, p. 403-409. Reference provided by the NASA Astrophysics Data System.
Tenn, Joseph S., 2007m Journal of Astronomical History and Heritage, Vol. 10, No.1, 65.