After coming to Lowell Observatory, Slipher developed expertise in spectroscopy. In 1904, he wrote a paper on the Lowell spectrograph, built by the Brashear company for planetary spectroscopy, to go on the 61 cm Clark refractor. Lowell requested Slipher to acquire spectra of nebulae, because Lowell believed that the nebulae may be solar systems in the process of formation. Nebular spectroscopy was a major observational challenge because of their low surface brightness, and Slipher modified the Brashear spectrograph for nebular spectroscopy. His modifications made it possible to acquire spectra of the nebulae and measure their redshifts and detect rotation (see talks by Smith and by Thompson at this meeting).
With the original optics, used for stellar and planetary spectroscopy, the linear dispersion of the spectrograph was about 11 Å mm−1 corresponding to a resolving power of about 22,000 (this is typical of the spectrographs of the time and is on the low side of high resolution spectroscopy today). After modification for nebular spectroscopy, the linear dispersion of the spectrograph was 140 tenth-meters per mm (140 Å mm−1) which is more than adequate to measure nebular velocities and detect their rotation. The nebular exposures were long: 20 to 40 hours.
For about a decade, Slipher provided most of the nebular velocities. He mentions confirmations by Wright (Lick), Wolf (Heidelberg) and Pease (Mt Wilson). Fath (Lick) and others had already acquired nebular spectra but, for technical reasons to do with wavelength calibration, had not measured their radial velocities.
Here are Slipher's most significant papers on nebular spectroscopy:
How accurate are Slipher's velocities relative to modern values ? The dispersion of Slipher's velocities about the modern values is 112 km s−1, close to his own estimate of the uncertainty.
The velocities of stars were known from the work of Boss, Campbell, Kapteyn and others to increase with spectral type, from about 6 km s−1 at B to 15 km s−1 at K and then to 27 km s−1 for the planetary nebulae (Smith 2008). (This effect is still not entirely understood.) Could the nebulae be Galactic objects, much further up this evolutionary chain ? By 1916 there were already some ideas about obscuring material in the Milky Way. It was long known that the nebulae avoided the Galactic plane, which again favored an interpretation putting them outside the Milky Way. Further support came from observations of extragalactic novae.
Slipher was well aware of the significance of his observations. The velocities of the nebulae are much larger than those of the Galactic stars. He inferred that they lie outside the Milky Way. In his 1917 paper, he wrote:
It has for a long time been suggested that the spiral nebulae are stellar systems seen at great distances. This is the so-called “island universe” theory, which regards our stellar system and the Milky Way as a great spiral nebula which we see from within. This theory, it seems to me, gains favor in the present observations.
A letter from Hertzsprung to Slipher in 1914 makes the same point. Hertzsprung wrote:
My … 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.
Although some (like J.H. Reynolds) had their doubts about the data, Slipher had a convincing response, with confirmation of the velocities from others. Slipher's discoveries were well known at the time. Why did they not settle the issue about the nature of the nebulae ?
Van Maanen's work on proper motions in nearby galaxies confused the issue. He was a respected figure and his results were taken seriously: they were not really discredited until about 1935 (see M. Way's paper in this volume). Van Maanen's papers on proper motions began to appear in 1916. His results may have been known to astronomers a year or two earlier, and it seems likely that his work was influencing the thinking at the time when Slipher was assembling his 1915 and 1917 catalogs of nebular velocities.
In the 1920 Shapley-Curtis debate, Shapley argued that the nebulae are just nearby clouds and the universe is one big Galaxy. Curtis argued that the nebulae are galaxies like our own, far outside the Milky Way. In the end, Hubble's 1925 cepheid study on M31 settled the question. I am interested in why the "great debate" ever took place, given Slipher's discoveries. Hale appears to have been the prime mover in setting up this debate (see Smith 2008). van Maanen's work would have been well known to the Mt Wilson astronomers, and it may be that Hale's view of the island universe controversy (and also Shapley's) was unduly influenced by van Maanen's results, in contrast to the view of Curtis from Lick.
There is some similarity of this controversy and the dark matter controversy of the 1970s, but in reverse. The development of the dark matter story was supported by theory which we now believe to be only partly relevant, while Slipher's island universe story was delayed by erroneous observations. Progress is not always linear.
I began preparation of this talk, believing that Slipher's nebular velocities did not have an impact in their time, similar to the lack of impact of Zwicky's discovery of dark matter in the Coma cluster. But that view is not correct: Slipher's situation is not at all comparable with Zwicky's. Slipher's work made an impact at the time, but his problem of recognition came later. I speculate that, without the confusion from van Maanen's incorrect proper motions, the nature of the nebulae would have been clearer. Maybe the Shapley-Curtis debate would not have happened and Slipher would have got the credit for identifying the nebulae as extragalactic, which I think he deserves.