Hubble Shows That Other Galaxies Are Independent Systems

Date December, 1924

Hubble demonstrated that the Milky Way galaxy is only one of many in the universe and founded the astronomical study of galaxies external to the Milky Way.

Locale Mount Wilson Observatory, California

Key Figures

• Edwin Powell Hubble (1889-1953), American astronomer
• Heber Doust Curtis (1872-1942), American astronomer
• Harlow Shapley (1885-1972), American astronomer
• Adriaan van Maanen (1884-1946), Dutch astronomer

Summary of Event

At the beginning of the twentieth century, there were two theories about spiral nebulas—groups of stars that appear as spiraling streams flowing outward from a central core. One theory held that such nebulas were part of the Milky Way galaxy. The other held that they were “island universes,” large, distant, independent systems. To resolve this question, it was important to measure the distances of the nebulas. The island universe theory held that the nebulas were remote from the Milky Way; the other theory held that they were closer.

In 1914, the American astronomerVesto Melvin Slipher announced that the spiral nebulas were moving away from the Milky Way at high speeds. He had drawn his conclusions from his spectral analysis of Doppler shifts in these star groups. His announcement was taken as evidence that these nebulas could not possibly be part of the Milky Way. Slipher had not conclusively solved the problem, however, because he had not determined the distances of these nebulas.

Distance has always been a difficult question for astronomers. By the early twentieth century, astronomers had measured distances to some nearby stars using the parallax method, which compares the different angles of nearby stars in relation to Earth and the Sun as Earth moves around the Sun. Unfortunately, spiral nebulas are too far away for parallax to be useful.

Working at the Mount Wilson Observatory, the Dutch astronomer Adriaan van Maanen compared the positions of bright spots within spiral nebulas in photographs taken at different times in order to observe the motions of the spots. In 1916, he published his results, which suggested that the spirals were rotating and that the rotation was rapid. If large distances and sizes were assumed for the nebulas, the nebulas would have to be rotating at immensely fast speeds, in some cases exceeding the speed of light. As this was known to be impossible, van Maanen’s results were taken as evidence that the spiral nebulas must be nearby parts of the Milky Way.

In April, 1920, Harlow Shapley and Heber Doust Curtis debated their differing views before the National Academy of Sciences. Using a new distance determination method involving a type of star, a Cepheid variable, Shapley had arrived at a much larger size for our galaxy than had been previously deduced. Because of van Maanen’s studies, Shapley argued that the spiral nebulas were part of this large Milky Way.

Curtis agreed with previous studies that indicated a smaller Milky Way. He also believed in the “island universe” theory—that the spiral nebulas were other galaxies similar to and outside the Milky Way. As evidence, Curtis used Slipher’s measurements of the speed at which the nebulas were moving away from the Milky Way. Today it is recognized that Shapley’s results for the size and shape of the galaxy were substantially correct. Yet the island universe hypothesis was strongly supported by evidence presented by Edwin Powell Hubble to the American Association for the Advancement of Science in December, 1924.

In 1923, Hubble was working at Mount Wilson Observatory, studying photographs taken with the 100-inch (254-centimeter) Hooker telescope. He was the first to isolate Cepheid variables in the Andromeda nebula. Cepheid variables are stars whose brightnesses varies periodically and whose period of variation is related to their actual brightness. Once a star’s actual brightness is known and its apparent brightness as seen from Earth is measured, its distance can be determined. This was the same method used by Shapley to determine the size of the Milky Way galaxy.

Hubble used the Cepheids to calculate that the nebulas are, in fact, at great distances and must be huge, independent systems. The distance Hubble found for the Andromeda nebula is about 900,000 light-years and the diameter almost 33,000 light-years (a light-year is the distance light travels in a vacuum in one year, approximately 5.88 trillion miles, or 9.46 trillion kilometers). These results later required correction, as it was found that there are two types of Cepheid variables, and the ones that Hubble studied had a different period-luminosity relationship; the distance is closer to 2 million light-years. Yet Hubble’s results changed scientists’ idea of the scale of the universe and of Earth’s place in it.

Hubble was at first reluctant to publish his results, because he could not explain why van Maanen’s results would be incorrect. Although many influential astronomers were immediately convinced by Hubble’s results, controversy lingered for some years after these results were presented. Van Maanen’s results could not be duplicated by others, and all other evidence indicated that the galaxies were distant and separate from the Milky Way; therefore, his work was gradually forgotten.

Significance

The philosophical consequences of Hubble’s conclusions were immense. The great sizes and distances of the spirals meant not only that the Sun was only one of many in a huge galaxy but also that the Milky Way was merely one of many independent systems. This realization shifted humankind’s place in the cosmos, a shift that could be said to be equal to that which the Polish astronomer Nicolaus Copernicus inaugurated when he suggested that the Sun, not the Earth, was the center of the solar system.

Hubble’s work led to the beginning of the classification and study of galaxies. Once galaxies were identified as separate units of the cosmos, their shapes and sizes, their distances, and their distribution in space were studied. During the 1920’s, Hubble presented a classification scheme for galaxies that is still in use.

There were important follow-ups to Hubble’s work. Once Cepheids were found in other spirals and distances were known, Hubble was able to work out a plot of distance versus velocity; he found that the farther away a galaxy is, the faster it is moving away from Earth. This means that the universe is expanding. By using this plot to extrapolate backward in time to the so-called big bang, astronomers could estimate the age of the universe. Studies are still being conducted to determine the exact age, but data from the Wilkinson Microwave Anisotropy Probe have pinpointed it with incredible accuracy at 13.7 billion years. Hence the discoveries surrounding Cepheid variables created a drastically different picture of the universe: a universe in motion, rushing way from an energetic beginning, rather than the static and stable universe that scientists had previously assumed.

Bibliography

Berendzen, Richard, Richard Hart, and Daniel Seeley. Man Discovers the Galaxies. New York: Columbia University Press, 1984. Excellent history of early twentieth century work in studying the Milky Way and external galaxies, including much information on the process by which these galaxies were determined to be independent systems. Accessible to the nonscientist. Presents archival materials, including correspondence, and includes many photographs, charts, and graphs.

Gribbin, John. In Search of the Big Bang: The Life and Death of the Universe. Rev. ed. New York: Penguin Books, 1998. History of modern cosmology presents the story of the discovery of the nature of external galaxies. Puts Hubble’s work into the fuller context of the development of cosmological thought. Intended for the general reader. Includes illustrations, bibliography, and index.

Hetherington, Norriss S. “The Purported Rotation of Spiral Nebulas.” In Science and Objectivity: Episodes in the History of Astronomy. Ames: Iowa State University Press, 1988. Investigates the possible sources of van Maanen’s error when he measured rapid rotation in the spiral nebulas, with emphasis on how preconceptions and subjective ideas may have influenced the gathering of data. Discusses the influence of van Maanen’s work on Hubble.

Hubble, Edwin. The Realm of the Nebulae. 1936. Reprint. Mineola, N.Y.: Dover, 1991. Text of lectures given by Hubble at Yale in 1935 on spiral nebulas. Chapters 1 and 4 deal with the discovery of the nebulas’ distances; the remainder of the book describes then-current knowledge on types of galaxies, their distribution, and the overall view of the universe. Dated, but valuable for Hubble’s view of his studies.

Rowan-Robinson, Michael. Cosmology. 4th ed. New York: Oxford University Press, 2004. Comprehensive volume discusses advances in technology that have allowed scientists to gain increasing understanding of the origin and evolution of the universe. Topics include the formation and aging of various types of stars and galaxies, the big bang theory, quasars, and black holes. Also examines a number of controversies in the field. Includes illustrations and index.

Shapley, Harlow. Through Rugged Ways to the Stars. New York: Charles Scribner’s Sons, 1969. Autobiography contains a chapter on the Shapley-Curtis debate (chapter 6) and one that contains descriptions of Hubble and van Maanen (chapter 4). Written in a conversational style. Includes photographs.

Struve, Otto, and Velta Zebergs. “Galaxies.” In Astronomy of the Twentieth Century. New York: Macmillan, 1962. Cowritten by an astronomer who lived through some of the advances in astronomy described. Tells the story of the discovery of galaxies as independent systems and of the resulting work in classifying and understanding galaxies. Includes photographs, drawings, and a time line of twentieth century astronomy.

Whitney, Charles A. The Discovery of Our Galaxy. 1971. Reprint. Ames: Iowa State University Press, 1988. Presents the history of cosmological thought. Part 3 discusses the discovery and study of external galaxies as well as the study of our own galaxy. Describes actual observations and telescopes used to make discoveries. Includes photographs, drawings, glossary, and bibliography.