Slipher Obtains the Spectrum of a Distant Galaxy

Vesto Melvin Slipher obtained the spectra of the Andromeda nebula and showed that it has a large radial velocity, suggesting that it, and other spiral nebulas, belong to different galaxies.

Summary of Event

Stars were not very satisfactory study subjects for early astronomers. In contrast to the planets, they did not appear to show motion relative to one another and appeared as mere points of light when viewed through a telescope. The snowy band known as the Milky Way was an exception. Ancient peoples had seen it as a reflection of their own worlds, but in the mid-eighteenth century, investigators assumed that its nebulous glow was produced by faint, distant stars. A different kind of “nebulous glow” was noted by Immanuel Kant, who suggested that these so-called nebulous stars were actually other universes outside the Milky Way. Astronomy;galaxies
Andromeda nebula
Spiral nebulas
[kw]Slipher Obtains the Spectrum of a Distant Galaxy (1912)
[kw]Spectrum of a Distant Galaxy, Slipher Obtains the (1912)
[kw]Galaxy, Slipher Obtains the Spectrum of a Distant (1912)
Andromeda nebula
Spiral nebulas
[g]United States;1912: Slipher Obtains the Spectrum of a Distant Galaxy[02960]
[c]Science and technology;1912: Slipher Obtains the Spectrum of a Distant Galaxy[02960]
[c]Astronomy;1912: Slipher Obtains the Spectrum of a Distant Galaxy[02960]
Slipher, Vesto Melvin
Lowell, Percival
Curtis, Heber Doust
Shapley, Harlow
Maanen, Adriaan van
Hubble, Edwin Powell

Pierre-Simon Laplace suggested that some kind of nebulous material might have been the formative material for Earth’s solar system, but it was William Herschel, Herschel, William one of the first astronomers to concentrate on stellar astronomy, who found that many of the nebulous clouds could actually be resolved into individual stars. Herschel concluded that if sufficiently powerful telescopes were available, scientists could see that all the nebulous clouds are star systems. Later, however, he recognized that this hypothesis would not explain all the phenomena, reversed himself, and suggested what became known as the “nebular hypothesis.” Nebular hypothesis He proposed that the unresolvable nebulas were the “parents” of the stars; first came nebulous matter, then nebulas, and finally stars. After enjoying a brief period of acceptance, the hypothesis fell into disrepute after William Parsons, Parsons, William third Earl of Rosse, reported to the British Association for the Advancement of Science in 1844 that he had constructed a reflecting telescope that could resolve the apparently formless nebular material into discrete stars. He discovered that certain nebulas had spiral structures and was able to distinguish individual stars in several of these “spirals” that Herschel’s less powerful instrument could not. Even with Parson’s telescope, however, all the nebulas could not be resolved into stars.

Advances in photographic techniques and the development of spectroscopy Spectroscopy
Astronomy;spectroscopy (analysis of light spectra) at the end of the nineteenth century made possible a more detailed examination of Parsons’ spiral nebulas. Some astronomers were convinced that they were a part of our galaxy (relatively small, nearby objects) and others thought they represented different universes (large and very distant).

Although the idea of “island universes” Island universes —other worlds similar to our galaxy—had been proposed earlier on philosophical, not observational, grounds, and Kant in the eighteenth century had suggested that the spiral nebulas might represent such island universes, the question was not completely resolved until 1935. Vesto Melvin Slipher’s observations in 1912 with improved spectrographic tools (instruments used to disperse and photograph light) provided data vital to determining whether the spirals were part of the Milky Way galaxy or part of distant, hitherto unknown worlds.

Slipher responded to a request by Percival Lowell to join the staff of the Lowell Observatory Lowell Observatory in Flagstaff, Arizona. His skill at perfecting spectroscopic equipment, his meticulous care in obtaining observations, and his cautious interpretations made it possible for him to make many important fundamental discoveries. Using spectrographic techniques, Slipher measured the rotational periods of Mars, Venus, Jupiter, Saturn, and Uranus, identified bands in the spectra of Jupiter and Saturn as caused by ammonia and methanem, and found evidence that interstellar space contained both dust and gas. He made his most profound contribution, however, when he turned his attention to the elusive spiral nebulas. Slipher’s part in early twentieth century astronomical controversy over the location of the spirals was indirect. Using an improved camera, he investigated not only the spectra of the spirals but also their velocities. Beginning with his work on the Andromeda nebula, he determined that the spirals had exceptionally large radial velocities. He found that “the great Andromeda spiral had the . . . exceptional velocity of –300 kilometers” per second, the greatest radial velocity ever observed.

Radial velocities, those measured only along the line of sight, can be either approaching or receding. For an object moving toward a wave source, the wave frequency appears to increase and to become shorter, but moving away from the source, the waves appear less frequent and the wavelength longer. This phenomenon, called the Doppler shift Doppler shifts (shift in wavelength of light or sound caused by the relative motion of the source and receiver), can be illustrated by sound waves. When a truck approaches with its horn blasting, its pitch is higher than when the truck is at rest. Just as the truck passes, its pitch is unshifted. Then as it moves away, its horn produces a lower pitch. The same phenomenon occurs with the light waves Slipher measured with his spectrograph. The Doppler shift (produced by the radial velocity) was measured by the change in the wavelength of lines in a spectrum. The emission, or absorption, lines are shifted to the blue end (short wavelengths) of the spectrum for an approaching object, and to the red (long wavelengths) for one that is receding.

After his initial work on the Andromeda nebula, Slipher observed other spirals. When he published his results in 1914, he had obtained Doppler shifts for fourteen, all showing large radial velocities. Although he was given a standing ovation when he announced his discovery at the 1914 meeting of the American Astronomical Society and won gold medals of the Paris Academy, the Royal Astronomical Society, and the Astronomical Society of the Pacific, the interpretation of his findings became part of the island universe controversy. Some astronomers, such as Ejnar Hertzsprung, were convinced that as the spirals had such remarkably large radial velocities, they could not be a part of the Milky Way. Slipher, in a lecture to the American Philosophical Society in April, 1917, recognized that his observations supported the hypothesis that spiral nebulas are very distant stellar systems.

Other astronomers would not be convinced until the actual distances of the spirals could be computed. Heber Doust Curtis, one of the first to suggest a much greater distance for the spirals, immediately accepted the implications of Slipher’s data. He had already reached the conclusion that the Andromeda nebula was 500,000 light-years away, and Slipher’s data provided support. Curtis publicly debated the point with Harlow Shapley, who (relying on measurements by Adriaan van Maanen) insisted that the spiral nebulas were nearby nebulous objects.

After 1925, van Maanen’s measurements were less influential, for Slipher’s work on the direction of rotation of the spirals directly contradicted van Maanen’s conclusions. Edwin Powell Hubble’s demonstration from the behavior of Cepheids Cepheid variable stars (pulsating variable stars) in spirals indicated that they must be remote. Nevertheless, given that van Maanen’s measurements seemed to represent observational facts, they were difficult to abandon even with contradictory data. Van Maanen’s results eventually were ignored, not so much because they were proved wrong, but because new evidence seemed to prove incontrovertibly that the spirals were galaxies. Hubble’s 1935 paper in the Astrophysical Journal finally brought the controversy over the distance of the spirals to closure.


Slipher’s meticulous spectroscopic measurements demonstrating the immense radial velocities of the spiral nebulas proved to be one of the most significant discoveries in modern astronomy. By finally succeeding at the very difficult task of obtaining the faint Andromeda nebula’s spectrum, he provided evidence that this nebula was much farther away than scientists had previously assumed. After perfecting his technique, he confirmed his results on Andromeda by measuring Doppler shifts in other spirals. Although the implications of extremely large radial velocities for the spirals were that they could not be contained within the Milky Way, the island universe dispute between major protagonists Curtis and Shapley retarded the acceptance of the significance of his discovery.

Although Slipher’s work on radial velocity was consistent with an extragalactic position for Andromeda and the other spirals, his findings did not prove the island universe theory, because they did not provide the distance to the spirals. Slipher also confirmed that the spirals rotated. This discovery meant that through determining proper motions (apparent angular rates of motion of a star) within the spirals, the distance to them might be obtained and the dispute settled conclusively. Agreeing on the procedure in principle, van Maanen’s systematic error led to a mistaken interpretation, indicating that the spirals (showing considerable proper motion) could not be at any great distance, seemingly disproving the island universe idea.

In 1924, Hubble discovered Cepheid variables and derived a distance of 490,000 light-years for this galaxy. This distance is far beyond the farthest globular clusters of the outer limits of the Milky Way. (This estimate has now been revised to 2.2 million light-years.) Although his distance determinations might have settled the problem in 1925, doubts still lingered, because Hubble’s conclusions were based on the controversial period-luminosity law and because van Maanen’s contradictory results were exceptionally consistent. The problem was not resolved until 1935, when a new set of measurements indicated that van Maanen was incorrect.

The most all-encompassing result from Slipher’s work on the spirals was the preparation that he made for investigations of the motions of galaxies and for cosmological theories based on an expanding universe. By 1928, Slipher’s measurement of the radial velocities of more than forty galaxies indicated that most galaxies apparently were moving away from the Milky Way. At about the same time, Hubble had noted that more distant galaxies had greater radial velocities. In fact, there seemed to be a direct relationship between radial velocity and distance. This relationship, known as Hubble’s law Hubble’s law[Hubbles law] (with a number connecting distance and velocity called Hubble’s constant), indicates that the universe is expanding. This conclusion is based on Slipher’s measurements, which showed that almost all spiral nebulas are moving away from the Milky Way (as well as from one another) with large velocities. These observations suggested not only that the spirals were galaxies like the Milky Way but also that the entire universe was expanding. Astronomy;galaxies
Andromeda nebula
Spiral nebulas

Further Reading

  • Berendzen, Richard, Richard Hart, and Daniel Seeley. Man Discovers the Galaxies. New York: Columbia University Press, 1984. Excellent volume focuses on pivotal events in the history of galactic astronomy, provides biographical information on the key players, and includes archival source materials. Both readable and scholarly. Includes a useful bibliography.
  • Clark, David H., and Matthew D. H. Clark. Measuring the Cosmos: How Scientists Discovered the Dimensions of the Universe. New Brunswick, N.J.: Rutgers University Press, 2004. Relates the stories of the scientists who have contributed to current knowledge about the size, mass, and age of the universe. Chapters 4 and 5 include discussion of the work of Hubble and Slipher. Features glossary, bibliography, and index.
  • Macpherson, Hector. Modern Cosmologies: A Historical Sketch of Researches and Theories Concerning the Structure of the Universe. London: Oxford University Press, 1929. Useful collection of eight lectures delivered at the Royal Technical College, Glasgow, during the winter of 1928-1929 includes one lecture on the island universe idea.
  • North, J. D. The Measure of the Universe: A History of Modern Cosmology. 1965. Reprint. Mineola, N.Y.: Dover, 1991. Focuses on the development of cosmology in the first half of the twentieth century and includes a section on the expanding universe. First part consists of a straightforward historical and theoretical narrative; second part includes a discussion of the conceptual problems introduced in the first part. Includes selected bibliography.
  • Slipher, Vesto M. “The Radial Velocity of the Andromeda Nebula.” Lowell Observatory Bulletin 2 (1913): 56-57. Slipher’s original paper presents his figures.
  • Zeilik, Michael, and Stephen A. Gregory. Introductory Astronomy and Astrophysics. 4th ed. Monterey, Calif.: Brooks/Cole, 1997. This introductory text provides a useful overview of general astronomy, including basic spectral issues.

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Hertzsprung Uses Cepheid Variables to Calculate Distances to the Stars

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Slipher Presents Evidence of Redshifts in Galactic Spectra

Hubble Confirms the Expanding Universe