Kapteyn Discovers Two Star Streams in the Galaxy Summary

  • Last updated on November 10, 2022

Jacobus Cornelius Kapteyn discovered that the proper motions of stars were not randomly distributed but tended in two opposite directions, implying the rotation of the Milky Way galaxy.

Summary of Event

Following several years of study of the structure of the Milky Way galaxy, Jacobus Cornelius Kapteyn concluded, on the basis of exhaustive star counts in sampled portions of the sky, that the proper motions Stars;proper motions (perpendicular to the line of sight) of the stars were not randomly distributed. To his own satisfaction, Kapteyn confirmed previous perceptions of the Milky Way as a flattened ellipsoid (a lenslike shape). However, because gas and dust toward the center of the galaxy distorted his counting in that direction, he incorrectly placed the solar system relatively near the center of the Milky Way. Astronomy;galaxies Stars;motions Milky Way galaxy;star streams Galaxies;Milky Way [kw]Kapteyn Discovers Two Star Streams in the Galaxy (1904) [kw]Star Streams in the Galaxy, Kapteyn Discovers Two (1904) [kw]Galaxy, Kapteyn Discovers Two Star Streams in the (1904) Astronomy;galaxies Stars;motions Milky Way galaxy;star streams Galaxies;Milky Way [g]Netherlands;1904: Kapteyn Discovers Two Star Streams in the Galaxy[00910] [c]Science and technology;1904: Kapteyn Discovers Two Star Streams in the Galaxy[00910] [c]Astronomy;1904: Kapteyn Discovers Two Star Streams in the Galaxy[00910] Kapteyn, Jacobus Cornelius Shapley, Harlow Curtis, Heber Doust

Having constructed what he believed was a correct view of Earth’s galaxy, Kapteyn then measured the proper motions of many stars for the purpose of verifying that these were randomly distributed, as theory indicated. Instead, he discovered systematic departures from randomness, with the stars showing movement in opposite directions in different parts of the sky. The two streams of movement of the stars implied that the Milky Way is not static, with the Sun near the center of random individual stellar motions. Kapteyn’s discovery had major cosmological implications, most obvious among them being that the Milky Way is a rotating galaxy.

The originator of this great discovery was well prepared, both in temperament and in education, to contribute to the development of modern astronomy. Kapteyn, one of fifteen children, was the son of a boarding school proprietor who stimulated his interest in physics. He studied at the University of Utrecht and was awarded a doctorate in physics in 1875. Following completion of his studies, he pursued a position at the Leiden Observatory, where he became a successful astronomer. By 1878, he had won a professorship in astronomy at the University of Groningen. When he arrived at Groningen, however, he was unable to raise funding to equip an observatory. Undaunted, he became a world leader in arranging collaboration with other astronomers, notably with Sir David Gill at the Cape of Good Hope in South Africa. From 1885 to 1899, he analyzed photographic plates taken by Gill and assisted in producing a star catalog with nearly a half million entries. He later continued studies as a visiting astronomer at the Mount Wilson Observatory near Pasadena, California.

While still at Leiden, Kapteyn became interested in the structure of the universe, a topic he pursued throughout his career. His primary method of study consisted of counting stars at the different levels of brightness, his underlying assumption being that the measured brightness of stars was a statistically reliable means of assessing their distance. Through this method he made the first major advance since the work of Sir William Herschel Herschel, William a century before. Herschel resolved some “nebulas” Nebulas into star clusters and consequently believed that all nebulas would eventually be resolved, with some located outside Earth’s galaxy. Kapteyn also accepted the nebulas or unresolved patches of light in the sky as “island universes” Island universes outside our own galaxy. The thinning of star counts away from the plane of the Milky Way implied to Kapteyn an ellipsoid-shaped galaxy. In the midst of these studies, he found the two star streams.

Jacobus Cornelius Kapteyn.

(Yerkes Observatory, University of Chicago)

If the Milky Way were essentially a static collection of stars, then theoretically their motions across the line of sight (proper motions) ought to be randomly distributed. Only those stars relatively close to the earth have measurable proper motions, but there were enough of these by the early part of the twentieth century that Kapteyn could analyze them statistically. The result was that he found consistent displacement in two opposite directions for neighboring stars. The motions, toward the constellations Orion and Sagittarius, implied what he called the “streaming” of stars or a pattern of movement that indicated that stars were moving in opposite directions. In 1904, Kapteyn announced his discovery at the International Congress of Science at the World’s Fair in St. Louis, Missouri.

Desiring to follow up these early discoveries, Kapteyn devoted much of his time after 1904 to attempts to organize the worldwide astronomical community in cooperative efforts to photograph and analyze 206 different portions of the night sky. These studies focused on magnitude, color, proper motion, radial velocity (along the line of sight), and spectral type. Before he was able to arrange for the full implementation of his plan, World War I largely ended international cooperation, but he did succeed in generating enough information to convince himself that he was correct about the ellipsoidal shape of the Milky Way. He estimated that the major axis and the minor axis of the ellipsoid were related by the ratio of five to one, the major axis measuring approximately 52,000 light years. His main concern about the accuracy of his model was the central location of the Sun, a point that also caused other astronomers to question the model because of this apparent favored position.

Kapteyn was also deeply concerned about the effects on his model of the absorption of light by gas and dust. He realized that appreciable absorption in the direction of the galactic center would have caused his central position for the Sun to be wrong and, more important, would have caused him to underestimate the size of the Milky Way. As a result, he repeatedly sought some means of measuring the amount of absorption. Having failed to measure any sizable amounts by 1918, he convinced himself that it was negligible and his model accurate.

Kapteyn’s inability to measure the effects of obscuration of light in the direction of the galactic plane did cause him to underestimate seriously the size of the Milky Way. Harlow Shapley, who was studying many of the same problems as Kapteyn at Mount Wilson in California, proposed a much larger universe on the basis of his study of clusters of stars (Globular clusters) Globular clusters and Cepheid variables Cepheid variable stars (stars of fluctuating brightness). The superior equipment that had resolved the clusters into individual stars led Shapley to conclude that all the nebulosities might eventually be resolved and that they could be a part of the galaxy. By 1914, Shapley had established that all the globular clusters were in one direction; thus he concluded that they surrounded the center of the Milky Way, and he necessarily placed the Sun at the edge. If the universe were the size Kapteyn indicated, the clusters would have been outside the Milky Way, but Shapley was confident in his distance measurements based on the brightest stars and the Cepheid variables. He was also confident in his interpretation of the cause of their asymmetrical location. The implication followed that the streaming of the stars, which was firmly established, would have to be reinterpreted in a new fashion to fit the data.

The divergence of Shapley’s views from Kapteyn’s, including Shapley’s much larger estimates of the scale of the universe, led to conflict between Shapley and astronomer Heber Doust Curtis and resulted in a public debate at the annual meeting of the National Academy of Sciences in Washington, D.C., on April 26, 1920. Shapley presented his views while Curtis defended the perspectives of Kapteyn’s followers. Both sides, later discoveries would show, were partially right and partially wrong. Kapteyn’s view of the “nebulas” as island universes was eventually established (even by 1914 Curtis had already noted the high radial velocity of some of the spirals, implying their distance and recession from the earth), and Shapley’s views of the dimensions of the Milky Way were found to be more accurate. This debate served to place Kapteyn at the center of the cosmological questions of the day. His interpretation of the physical cause of the streaming effect may have turned out to be only partially correct, but that does not minimize the significance of his achievement in recognizing the reality of the streaming.


Kapteyn’s efforts in the early part of the twentieth century demonstrate clearly that careful observation and experimentation have exceptional value, even if the interpretation is erroneous. On the basis of methodical observation and careful statistical analysis of star counts from sampled regions of the sky, Kapteyn constructed a model of the Milky Way in the shape of a lens. The galaxy was rotating, as demonstrated by his discovery of the two star streams, with the earth in a position near the center. The Kapteyn universe provided a framework for the explanation of astronomical observations that served scientists well at the beginning of the century.

Kapteyn’s view of the universe dominated the first two decades of the twentieth century. His work significantly affected the cosmology of the day, especially his view of the nebulas as “island universes” beyond the bounds of the Milky Way. When this perspective was combined with the high radial velocities of the nebulas, it eventually contributed to Georges Lemaître’s Lemaître, Georges cosmology of the mid-1920’s, which was, in turn, an antecedent of the currently accepted big bang cosmology. Big bang theory His discovery of the two star streams contributed to the establishment of the rotation of the Milky Way, although he incorrectly placed the Sun at the center and believed the streams were the result of passage of stars on either side of Earth.

Kapteyn’s model was inaccurate for a variety of reasons, the most important of which was the obscuring effect of gas and dust toward the center of the Milky Way. His failure to measure obscuration successfully caused him to be overly confident in his star counts in that direction, and he searched for alternative explanations for Shapley’s research findings and disputed those findings until his death. Shapley, in turn, erred in his interpretation of the nebulas because he depended on erroneous rotational measurements for the nebulas. In less than a decade, however, further research resolved the conflict of the early 1920’s.

By the late 1920’s, as the result of work by Bertil Lindblad, Lindblad, Bertil Jan Hendrik Oort, Oort, Jan Hendrik and Edwin Powell Hubble, Hubble, Edwin Powell a more accurate picture emerged of our galaxy as a spiral rotating in the same fashion as the other exterior galaxies. In 1927, Oort, a former student of Kapteyn, demonstrated (once the Sun’s position near the edge of the Milky Way was established) that the apparent motion in one direction was the result of the Sun’s lagging behind stars nearer the center. The motion in the opposite direction was the result of slower outer stars’ falling behind the Sun. Kapteyn’s universe was superseded by research it had stimulated.

At the beginning of the twentieth century, Kapteyn’s influence greatly enhanced interest in galactic astronomy, which had been somewhat eclipsed by emphasis on planetary studies at the end of the nineteenth century. Kapteyn also set a fine example of the advantages of cooperative efforts among astronomers. Although he was utterly confident in the value of his star counts and overly committed to his method (soon superseded by better methods and newer, more powerful equipment), he made an essential contribution to later cosmological studies. Astronomy;galaxies Stars;motions Milky Way galaxy;star streams Galaxies;Milky Way

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Berendzen, Richard, Richard Hart, and Daniel Seeley. Man Discovers the Galaxies. New York: Columbia University Press, 1984. A major survey of historical cosmology. The first five chapters present a fairly complete picture of Kapteyn’s work as it relates to the work of other astronomers of the period.
  • citation-type="booksimple"

    xlink:type="simple">Kapteyn, Jacobus Cornelius. “Discovery of the Two Star Streams.” In Source Book in Astronomy, 1900-1950, edited by Harlow Shapley. Cambridge, Mass.: Harvard University Press, 1960. This is an excerpt from a paper presented to the South African meeting of the British Association in 1905. It recounts briefly Kapteyn’s procedures for discovering the two star streams.
  • citation-type="booksimple"

    xlink:type="simple">_______. “First Attempt at a Theory of the Arrangement and Motion of the Sidereal System.” Astrophysical Journal 55 (1922): 302-327. Significant because it is the last and best statement of Kapteyn’s perspective on the significance of his discovery of star streaming. Unfortunately, given that Shapley turned out to be closer to the truth about the size of the Milky Way, it was already out of date when it was published. Somewhat technical, but accessible to the general reader.
  • citation-type="booksimple"

    xlink:type="simple">_______. “On the Absorption of Light in Space.” Astrophysical Journal 29 and 30 (1909): 47-54, 163-196. Kapteyn summarizes his efforts to establish the amount of absorption of light by gas and dust. Presents the evidence that encouraged him to maintain that the Sun was near the center of the Milky Way and that his interpretation of the two streams was correct. Moderately technical, but worth the effort to understand the seriousness with which he regarded the problem.
  • citation-type="booksimple"

    xlink:type="simple">_______. Plan of Selected Areas. Groningen, the Netherlands: Hoitsema Brothers, 1906. An explanation of what Kapteyn wished to accomplish by means of cooperative surveys of 206 regions of the sky by observatories in various parts of the world. The project was to form a database for his statistical studies, which have caused some to call Kapteyn the father of statistical astronomy.
  • citation-type="booksimple"

    xlink:type="simple">Kapteyn, Jacobus Cornelius, and Pieter J. Van Rhijn. “The Proper Motions of the Cepheid Stars and the Distances of the Globular Clusters.” Bulletin of the Astronomical Society of the Netherlands 1 (1922): 37. Kapteyn’s main effort, an attempt to establish two classes of Cepheids, one dimmer than the other, to defend his position against the newer evidence Shapley was marshaling against Kapteyn’s model of the size of the universe and the location of the Sun, which affected the interpretation of the two star streams.
  • citation-type="booksimple"

    xlink:type="simple">Paul, E. Robert. “J. C. Kapteyn and the Early Twentieth-Century Universe.” Journal for the History of Astronomy 17 (August, 1986): 155-182. A major historical study of the significance of Kapteyn’s work. Presents an almost wistful regret that one who contributed so much to the development of statistical astronomy could have been incorrect about the shape and size of the Milky Way.
  • citation-type="booksimple"

    xlink:type="simple">Van Der Kruit, P. C., and Klaas Van Berkel, eds. The Legacy of J. C. Kapteyn: Studies on Kapteyn and the Development of Modern Astronomy. New York: Springer, 2000. Contributors address Kapteyn’s influence on the development of modern astronomy, including his leadership in establishing collaboration among astronomers around the world. Includes an inventory of Kapteyn’s correspondence.

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