Slipher Presents Evidence of Redshifts in Galactic Spectra

Date Early 1920’s

Vesto Melvin Slipher pioneered the spectroscopy of galaxies and discovered that most galaxies are receding at high velocities, demonstrating that the universe is composed of many galaxies expanding away from one another.

Locale Lowell Observatory, Flagstaff, Arizona

Key Figures

• Vesto Melvin Slipher (1875-1969), American astronomer
• Percival Lowell (1855-1916), American astronomer
• Edwin Powell Hubble (1889-1953), American astronomer
• Milton L. Humason (1891-1972), American astronomer

Summary of Event

The general view held by astronomers in the first two decades of the twentieth century was that the universe consisted of a single aggregation of stars, the Milky Way galaxy, which was a system of stars estimated to be about 10,000 light-years in diameter. All objects that were visible in the heavens through the largest telescopes were thought to be part of our galaxy; beyond, a trackless, undifferentiated void extended infinitely far.

By the early part of the twentieth century, astronomers had compiled extensive catalogs listing the visible constituents of the galaxy, which included numerous stars and various nebulas, misty patches of light in the sky whose complete nature was not understood then but were presumed to consist primarily of gases. Photographs of nebulas showed that some had irregular shapes but that many had a distinctly spiral structure.

Percival Lowell was keenly interested in the study of the spiral nebulas. A member of a prominent Boston family, Lowell was able to finance the construction of his own observatory in Flagstaff, Arizona, dedicated to the study of planets. Lowell’s interest in the spiral nebulas lay in the notion, held by nearly all astronomers at the beginning of the twentieth century, that they were planetary systems in the process of formation. The study of spiral nebulas, Lowell hoped, would disclose valuable clues to the origin of the solar system.

In 1901, Lowell hired Vesto Melvin Slipher as an observer at Lowell Observatory and assigned him to a project on spiral nebulas. Slipher was to take spectra of the brighter ones and look for Doppler shifts in their light, which would reveal motions taking place within them. Lowell expected the results to support the theory that the spiral nebulas were rotating, contracting clouds of gas that would eventually form a planetary system or a cluster of stars. Slipher thought this was unlikely; he believed the spirals were probably systems of stars outside our own galaxy.

By late 1912, and using the 24-inch (61-centimeter) refractor at the Lowell Observatory, Slipher had photographed four separate times the spectrum of the Andromeda nebula, one of the largest and brightest of the spirals. Because of the slowness of the photographic emulsions of the time, Slipher found it necessary to expose each photograph for twenty to forty hours, spread over several nights. When examined, the spectra were found to be similar to those of stars like the Sun, a band of colors from blue to red crossed by dark lines that are characteristic of the elements found in the stars. Slipher had provided the first hint that a spiral nebula was a system of stars rather than a collection of gases. Without knowing the distance to the Andromeda nebula, however, Slipher was not able to demonstrate conclusively that the nebula was external to the Milky Way. Slipher interpreted, as did many other astronomers who learned of his work, the failure to detect individual stars in the spirals as an indication of their large distances.

In the spectra of the Andromeda nebula, Slipher noticed that there was a systematic shift of all the dark lines toward the blue end of the spectrum. Such a Doppler shift of all the lines toward either the red or blue was attributed to a systematic motion of the emitting nebula as a whole. If the object is moving toward the observer, the shift is toward the blue end of the optical spectrum and is called a blueshift. If the object is moving away, there is a corresponding redshift. The size of the shift in the position of the dark lines provides a direct measure of the speed with which the object is moving toward or away from the observer. In the case of the Andromeda nebula’s blueshift, Slipher’s data indicated that it was approaching at the speed of 300 kilometers per second, a speed greater than that of any astronomical object measured at the time.

Slipher extended the work on spiral nebulas and by 1914 had analyzed the spectra of twelve additional spirals, making a total of thirteen spirals whose Doppler shifts had been measured. Two of the spectra displayed a blueshift (one was that of the Andromeda nebula), but the other eleven were all redshifts, indicating that these nebulas were receding. If the spirals were part of the Milky Way galaxy, astronomers expected that roughly half would be approaching and half would be receding. Moreover, the speeds of recession measured by Slipher ranged up to an astounding 1,100 kilometers per second. A pattern was beginning to emerge, and astronomers began to suggest openly that the spirals must be stellar systems outside of our galaxy. The speeds of the spiral nebulas seemed to be too great for them to be gravitationally bound to our galaxy. Slipher’s results helped direct attention to the spiral nebulas and to the theory that space is populated by visible galaxies.

Slipher was fastidious, methodical, and careful. Although his results until 1914 suggested that most spirals were in recession, he believed it necessary to continue this line of inquiry and extend the survey further; a sample of thirteen Doppler shifts was not convincing. By 1923, Slipher had measured Doppler shifts in forty-one different nebulas; thirty-six had redshifts, and the remaining five had blueshifts. Meanwhile, other astronomers had added four more to the list; these were all redshifts. When it was discovered in 1925 that the Milky Way galaxy as a whole was rotating and that the Sun, because of the rotation, was moving with a speed of 250 kilometers per second in a direction generally toward the Andromeda nebula, the Doppler speeds of the nebulas, as measured by Slipher, were corrected for the rotation of the galaxy. Thus the 300-kilometer-per-second approach of the Andromeda nebula is composed of a 250-kilometer-per-second motion of the Sun toward the spiral and an intrinsic 50-kilometer-per-second approach of the spiral. When so corrected, of the total of forty-five nebular Doppler shifts measured, forty-three were redshifts.

Slipher had demonstrated clearly that the general trend was for the spiral nebulas to exhibit redshifts in their spectra, indicating that they were receding from the Milky Way galaxy and from one another. The two exceptions were the largest spirals, and therefore probably the nearest. Nevertheless, Slipher’s work forcefully showed that spirals were probably galaxies in their own right, external to the Milky Way, and that the overwhelming preponderance of redshifts indicated that galaxies were all rushing away from one another.

Slipher’s pioneering work on the redshifts of the spiral nebulas provided the foundation for a coherent picture of the evolution of the universe. In the late 1920’s, Slipher’s evidence was juxtaposed, along with the cosmological arguments that were being discussed by astronomers. It provided the impetus for a major new project to determine the distances to the galaxies.

Following the construction of the giant 100-inch (254-centimeter) Hooker telescope at the Mount Wilson Observatory in California, Milton L. Humason and Edwin Powell Hubble were able to study the spiral nebulas in much greater detail than had been possible for Slipher. In fact, with the new telescope, individual stars could be discerned in some of the larger spirals, the Andromeda nebula in particular. Now, all doubt was removed regarding the spiral nebulas: They were galaxies, vast systems of stars similar to the Milky Way of which the Sun is a part.

With individual stars available for study, they could be compared with stars in the Milky Way, and the distance to the galaxy could be estimated. The Andromeda galaxy lay at a distance of 750,000 light-years, far outside the Milky Way. Other spirals had even greater distances. Using the redshift data inherited from Slipher, Hubble combined those results with the distance measurements based on his and Humason’s observations to arrive at a relationship between distance and recessional velocity: The greater the recessional velocity of a galaxy, the greater is its distance.

Significance

To astronomers of the time, the cause of the recession was now clear. To explain the rush of galaxies away from one another, the universe must be expanding. Such an idea had its roots in the theory of general relativity published by Albert Einstein in 1916. Thereafter, Einstein and other astronomers and physicists examined the consequences of the theory and found that the universe should generally be in a state of either expansion or contraction. Slipher’s redshift measurements and Hubble’s correlation with distance demonstrated that the universe is expanding, an observation of central importance to cosmology. Since as time progresses, the universe expands, it must have been the case that in earlier epochs the galaxies were very close together and, at one time, entirely coalesced. Astronomers reached the startling conclusion that the universe was born in a titanic explosion (now called the big bang) that hurled the material from which the galaxies eventually formed. The high-speed rush of galaxies away from one another is direct evidence of that genesis.

Humason extended the redshift work, and by 1935 had added 150 measurements to Slipher’s list. Recessional speeds now were up to 40,000 kilometers per second. When distances were determined for the galaxies, Hubble’s redshift-distance relation still held. By the mid-1930’s, astronomers pictured a universe full of galaxies that were rushing apart from one another as a result of a fiery birth in the distant past. This model of the universe has remained fundamentally unchanged to the present.

Bibliography

Barrow, John D., and Joseph Silk. The Left Hand of Creation: The Origin and Evolution of the Expanding Universe. 1983. Reprint. New York: Oxford University Press, 1994. This book, authored by internationally renowned astronomers, presents an account of the origins and evolution of the universe geared for the general reader. Focuses primarily on developments in cosmology after the 1930’s. Includes an extensive glossary, an index, and suggestions for further reading.

Ferris, Timothy. The Red Limit: The Search for the Edge of the Universe. Rev. ed. New York: Harper Perennial, 2002. Well-presented volume discusses the history of the major discoveries in astronomy, paying particular attention to the individuals who made these discoveries. A comprehensible, accurate discussion of astronomy written in an engaging style for readers who have no familiarity with modern cosmological ideas. Includes extensive glossary, selected bibliography, and index, as well as a small collection of rare photographs of the personalities involved in the major discoveries of the twentieth century.

Sagan, Carl. Cosmos. 1980. Reprint. New York: Random House, 2002. Visually stunning book, based on Sagan’s thirteen-part television series of the same name, includes more than 250 color illustrations. Addresses science in its broadest human context: how science and civilization grew up together. Makes scientific ideas both comprehensible and exciting, often focusing on the forces and individuals who helped shape modern science. Includes index and suggestions for further reading.

Silk, Joseph. The Big Bang. 3d ed. New York: W. H. Freeman, 2000. Presents a sweeping account of the formation and evolution of the universe. Recounts the history of astronomical speculation about the universe and examines evidence for the big bang theory. Includes glossary and index.

‗‗‗‗‗‗‗. On the Shores of the Unknown: A Short History of the Universe. New York: Cambridge University Press, 2005. A history of the universe and the development of humankind’s knowledge about it that is accessible to lay readers. Includes illustrations and index.

Wilson, Colin. Starseekers. Garden City, N.Y.: Doubleday, 1980. Charts the history of the discovery of the universe from ancient to modern times, including information on the lives of the seminal thinkers involved. Weaves in elements of history, religion, philosophy, and science. Imaginatively illustrated. Includes bibliography and index.