Priority in the discovery of the velocity-distance relationship may be shifting from Edwin Hubble to Georges Lemaitre, but any reassessment of credit must also consider the contributions of Vesto Slipher. Not only were his spiral galaxy redshifts a necessary component in all of the original velocity-distance plots, but it was the quest to explain these unprecedentedly large Doppler velocities that pointed the way to a new physical interpretation.
In a simple succinct statement, Vesto Slipher was the first to photographically detect galaxy spectra with sufficient S/N to reliably measure their Doppler shifts. Although Milton Humason extended the galaxy redshift work to fainter galaxies on behalf of Edwin Hubble, the astronomers at Mt. Wilson would not have made rapid progress without Slipher's pioneering effort. Hubble was fully aware of the significance and priority of Slipher's early spectroscopy (Putnam 1994), but consistent with his style of claiming sole credit for most topics he worked on, Hubble never emphasized this point. In what follows, I will explain the early development of nebular spectrographs, briefly state Slipher's technological breakthroughs, and summarize how the Mt. Wilson group moved on to measure redshifts of even fainter galaxies.
Prior to the use of dry photographic emulsions, spectroscopy was a visual science. Starting at about the time of Sir William Huggins in the early 1860’s, spectral lines were identified in visual spectroscopes, and for nebulae, a distinction was made between emission line objects (eg. H II regions and planetary nebulae) and those that showed a faint continuous spectrum. By the early 1900’s, these were called, respectively, “green” and “white” nebulae. Visual Doppler velocities could not be measured with high accuracy for either stars or nebulae because the wavelength shifts were small. As astronomers learned which photographic emulsions were more light-sensitive, Doppler shifts became accessible. Because of the way the first astronomical spectrographs were designed, stars were the first to have their Doppler shifts measured. By the late 1800’s photographic spectroscopy of stars was commonplace and “green” nebulae had been detected and studied. It was at this point in time that Vesto Slipher began his career.
Excellent accounts of Slipher’s accomplishments, as well as the events that motivated his work, are discussed in a biographical memoir of Vesto Slipher by Hoyt (1980) and in the history of Lowell Observatory as told by Putnam (1994). Slipher arrived at Lowell Observatory in 1901 immediately after receiving his undergraduate degree in mechanics and astronomy at Indiana University and was put to work commissioning a new spectrograph. [He returned to Indiana University in 1909 to be granted his Ph.D. degree.] The new spectrograph had been purchased by Percival Lowell from the well-known instrument maker John Brashear, and Slipher was assigned the job of making it work on the Lowell 24inch refractor. As his first observing project, Percival Lowell asked Slipher to use the new spectrograph to measure the rotation velocity of Venus’ atmosphere, a project that Slipher completed by 1903. Because of frequent written correspondence between Lowell (from his Boston office) and Slipher (from the observatory in Flagstaff, AZ), a record of these developments is still available.
Believing that spiral nebulae were an early stage in the formation of other solar systems, Lowell set Slipher on a program to understand the spectra of spirals starting in 1906. The Brashear spectrograph had been designed to use either 1, 2 or 3 prisms in series (3 prisms for the highest dispersion) in conjunction with a long focal length spectrograph camera working at f/14.2. We know today that the speed of a spectrograph is different for point sources than it is for extended sources, and it so happened that Brashear spectrographs (also used at other major observatories in this era) could detect stellar spectra and high surface brightness “green” nebulae, but they were not of much use in the detection of galaxies because of the galaxies’ lower surface brightness.
Slipher soon came to several conclusions about how to photographically detect the light from spiral nebulae. His published papers, as well as his correspondence with Lowell, contain the following points. First, when a nebula is placed on the slit of a spectrograph, the brightness of the sit image at the photographic plate is greatest at the lowest spectral dispersion (using 1 prism), but if all 3 prisms are used, the slit width can be widened to compensate, so the speed is not compromised (for a given velocity accuracy). Second the speed of the spectrograph for a spiral of uniform surface brightness depends almost entirely on the f/ratio of the camera lens that focuses the slit image on the photographic plate. The spectrograph camera must be “fast” (a small f/ratio) to shorten the exposure time. Finally Slipher correctly concluded that for pure nebular spectroscopy, the speed of detection is independent of the telescope aperture and the telescope f/ratio. [This last statement was strictly true in the photographic era and is somewhat less so now.] Today, these three conclusions are well known. They were first expressed algebraically by Bowen (1952) when describing the spectrographs for the Palomar 200-inch telescope. Bowen (1962) extends these relations. Because galaxy nuclei
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