Jan Hendrik Oort provided convincing evidence of the correctness of Bertil Lindblad’s proposal that the Milky Way is a rotating spiral like many of the exterior galaxies.

In 1927, Jan Hendrik Oort published data that provided dynamical proof of the proper motion of the stars near Earth. This evidence established that the observed effects were the result of differential velocities that could best be explained as the result of movement in the spiral arm of a large galaxy. He found the galactic center in the direction of Sagittarius in opposition to the direction Jacobus Cornelius Kapteyn Kapteyn, Jacobus Cornelius had found, but in accord with the findings of Harlow Shapley. Shapley, Harlow Oort came to this study quite naturally, as he had been a student of Kapteyn, the famous Dutch astronomer who was active in the study of the structure of the Milky Way. At the age of seventeen, Oort went to the University of Gröningen to study with Kapteyn. In his elementary lectures, Kapteyn emphasized deriving interpretation from observation rather than hypotheses or conjectures; in many ways, Oort’s career followed this dictum. [kw]Oort Proves the Spiral Structure of the Milky Way (1927)
[kw]Spiral Structure of the Milky Way, Oort Proves the (1927)
[kw]Milky Way, Oort Proves the Spiral Structure of the (1927)
Milky Way galaxy
Astronomy;galaxies
Galaxies;Milky Way
[g]Netherlands;1927: Oort Proves the Spiral Structure of the Milky Way[06800]
[c]Science and technology;1927: Oort Proves the Spiral Structure of the Milky Way[06800]
[c]Astronomy;1927: Oort Proves the Spiral Structure of the Milky Way[06800]
Oort, Jan Hendrik
Lindblad, Bertil
Plaskett, John Stanley

After earning a degree at Gröningen, Oort studied at Yale University in the United States. He began work in 1924 at the Leiden Observatory, becoming a professor in 1935 and director of the observatory in 1945. Oort was always fascinated by the conflict between Kapteyn’s star counts and Shapley’s studies of the globular clusters. Globular clusters Oort hoped to resolve this conflict and applauded Bertil Lindblad’s rather bold proposal of the solution to the problem. Lindblad was a Swedish astronomer who had suggested that a rotating model of our galaxy could explain most of the observed phenomena. Oort was aware that no globular clusters appeared near the galactic plane and surmised that gas and dust were obscuring them. It did not occur to him until 1925 that the same obscuration was causing Kapteyn to propose a much too small stellar system as well, because his assumption of uniform luminosity was not valid. This was the key to unraveling the conflict between the two systems.

Prior to Lindblad and Oort’s work with the motion of the Milky Way, dynamical astronomy was almost solely the province of the solar system specialists. Oort recognized from Kapteyn’s discovery of the star streams, and Karl Schwarzschild’s interpretation of them as an ellipsoidal distribution of stellar motions, that there was potential for the dynamical study of our galaxy. Stars;motions He was also influenced by Sir Arthur Stanley Eddington and Sir James Jeans. This interest in dynamics led to his 1927 presentation and was a consistent theme throughout his varied career, extending to the dynamics of star clusters, stellar systems, galactic clusters, and finally superclusters. Oort consistently emphasized the lack of homogeneity in the distribution of galaxies, whereas Edwin Powell Hubble Hubble, Edwin Powell and others emphasized the large-scale homogeneity of the distribution of galaxies.

Oort began his research in the middle of the 1920’s with a study of high-velocity stars. Other astronomers had already found a strange phenomenon, namely, that stars with radial velocities of 150 kilometers (93.2 miles) per second or higher tended toward one direction in galactic longitude. Oort studied somewhat lower-velocity stars and found a surprisingly sharp limit at 65 kilometers (40.4 miles) per second, above which all the stars were on the same side of Earth and below which the stars were uniformly distributed over all longitudes. Oort’s 1926 doctoral dissertation contained a full description of the effect but an inadequate explanation, for he was still attempting to explain the phenomenon on the basis of the dynamics of a local system within Kapteyn’s perception of the shape and size of the Milky Way. He noted, however, the concentration of globular clusters in the galactic plane and in only one direction. He further noted other objects with the same kind of concentration. He stated that the globular clusters had the same type of motion as the high-velocity stars and that their average velocity was about 92 kilometers (57.2 miles) per second, well above the 65-kilometer-per-second limit. The data pointed toward the strength of Shapley’s contention that the universe was larger than Kapteyn perceived and that Earth was far from the center.

Within a year, however, rather than proposing a collection of swarms drifting in the large system of globular clusters—which had been Kapteyn’s compromise with Shapley’s evidence—Lindblad suggested that the Milky Way consisted of concentric subsystems at various velocities, with the high velocities the consequence of the faster rotation of inner stars around the galactic center far from the Sun in that direction. Oort then provided the evidence from his studies of the differential rotation of the galaxy, which conformed to Lindblad’s model. He demonstrated that Kapteyn’s streaming effect was caused by the inner stars catching up with the Sun while the outer stars lagged behind. This initial research could have implied concentric rings of a circular or elliptical galaxy, and his research continued seeking firmer evidence for the spiral nature of the Milky Way. He established in 1930 (allowing for Robert Julius Trumpler’s discovery of dust clouds that absorbed light and made distant star clouds appear fainter and more distant than they were) that Earth’s orbit of rotation was 30,000 light-years from the center of the galaxy, which was a reduction of Shapley’s 50,000 light-year estimate. He found the Sun to be following a fairly circular orbit at a rate of approximately 220 kilometers (136.7 miles) per second, a velocity that would lead to orbiting the galactic center in approximately 230 million years.

Next, Oort investigated the relationship between his previous velocity distribution studies and the decrease in density of star numbers with increasing distance from the galactic center and increasing distance from the galactic plane. He determined the density of matter in the vicinity of the Sun and made estimates of density at other points toward the center for the purpose of computing the gravitational forces and reaching a comprehensive dynamical model of the distribution of the mass. He noted that the high-velocity globular clusters could not be held by a system unless it was at least two hundred times more massive than Kapteyn’s universe.

By 1932, Oort had established that only about two-thirds of the mass of the Milky Way could be accounted for from the known stars and gas; he thus implied a considerable unseen mass in the galactic plane that was hidden by gas and dust. The distribution of the mass also implied the spiral nature of the galaxy.

The desire to penetrate the unseen core led Oort into radio astronomy. Radio astronomy During World War II, while the Netherlands was occupied and observation was denied, Oort turned to theoretical work on the structure of the Milky Way and encouraged his student Hendrik Christoffell van de Hulst Hulst, Hendrik Christoffell van de to study the hydrogen atom emission in radio wavelengths. In 1944, van de Hulst announced that hydrogen ought to emit at the 21-centimeter (8.3-inch) wavelength. After the war, Oort and C. A. Muller built a 21-centimeter receiver and hooked it to an 8-meter (26.2-foot) antenna owned by the Dutch Post and Telegraph Service. An unfortunate fire destroyed their experiment and before they could rebuild, Edward Mills Purcell Purcell, Edward Mills and Harold Ewen Ewen, Harold announced observations of hydrogen emissions. Oort duplicated their effort six weeks later. Oort and Muller then mapped a halo of hydrogen around the galaxy; in the following years, they went on to study the galactic structure in the optically obscured center, mapping the distribution of gas in the galactic disk. This gas distribution, as well as the earlier research on mass distribution, supported the spiral nature of the galaxy. These studies verified rotation rates near the center and found a concentrated mass.

The variety of Oort’s career interests led him to propose the existence of a comet cloud outside the planetary orbits as an explanation for the origin of comets and to conclude that the Crab nebula was the supernova of 1054 c.e. recorded by Chinese astronomers. He studied the origin and evaporation of solid particles in interstellar space with van de Hulst. His wide-ranging interests contributed to his remarkably long, active career in astronomy.

Jan Hendrik Oort’s work in seeking to resolve the issue of size, shape, and motion of the Milky Way was a natural consequence of his studies under Kapteyn. His primary concern was to resolve the conflict between the size of Kapteyn’s “universe” and that of Shapley, which was several times larger. Significantly, he was able to blend physics, mathematics, and astronomy into a dynamical interpretation of the observed phenomena that took into account gaseous absorption of light in the galactic plane and toward the center of the galaxy. He was able to provide evidence for a rotating spiral galaxy that resolved many issues and made Shapley’s view with modifications more acceptable than in its previous form.

Oort’s published observational confirmation of differential galactic rotation, built on the mathematical theoretical work of Lindblad, encouraged other astronomers to search for further evidence and to continue efforts to resolve the remaining problems of scale, which were not finally completed until Walter Baade’s work on the two stellar populations appeared in 1952. Oort had a great effect on John Stanley Plaskett, who spent much time studying hot blue stars (types O and B) at the Dominion Astrophysical Observatory in Victoria, British Columbia. Plaskett applied Oort’s analysis to the radial velocities of faint B-type stars, which, because they are extremely bright, can be detected at great distances. His analysis reduced the relative error of their radial velocities. His independent method produced results close to Oort’s value for distance and direction of the galactic center.

Oort established that Kapteyn’s galaxy was too small and Shapley’s too large. Plaskett’s verification of Oort’s results focused renewed efforts to resolve the problem of the distance scale. The problem of identifying the nature and magnitude of absorption of light by gas and dust continued into the 1930’s. Trumpler’s work provided the first definitive proof of an absorbing medium that Oort had insisted was present. Those who followed Trumpler were able to use the absorption he found to resolve the issues among Kapteyn’s, Oort’s, and Shapley’s models of the galaxy and the conflict between Oort’s and Shapley’s distances. Oort’s extremely long career allowed him to follow up his own work by moving into radio astronomy in the late 1940’s and 1950’s, when his research resulted in more complete confirmation of the spiral structure of the galaxy. Later studies have demonstrated that Trumpler’s uniform absorbing medium is really small clouds with irregular patterns of absorption in different parts of the sky.

Oort’s early work stimulated the development of better photography, faster films, shorter focal lengths, and the use of radio astronomy. Oort’s many honors were well deserved, and the fertility of his astronomical efforts can only be admired. His long career spans the full development of a comprehensive view of the nature, structure, and size of our galaxy. Milky Way galaxy
Astronomy;galaxies
Galaxies;Milky Way

• Berendzen, Richard, Richard Hart, and Daniel Seeley. Man Discovers the Galaxies. New York: Columbia University Press, 1984. Excellent cosmological history includes a somewhat technical description of Lindblad’s and Oort’s efforts to comprehend the dynamics of the Milky Way. Notes the importance of Oort’s evidence for later cosmological studies.
• Ferguson, Kitty. Measuring the Universe: Our Historic Quest to Chart the Horizons of Space and Time. New York: Walker, 1999. Examines the history of humankind’s efforts to measure and understand the size and structure of the universe. Chapter 7 includes discussion of research concerning the Milky Way, including Oort’s work. Features glossary and index.
• Motz, Lloyd, and Jefferson Hane Weaver. The Story of Astronomy. New York: Plenum, 1995. Presents the history of astronomy from ancient times to the end of the twentieth century. Chapter 17 includes discussion of Oort’s work and galactic research in general. Features bibliography and index.
• Oort, Jan Hendrik. “The Development of Our Insight into the Structure of the Galaxy Between 1920 and 1940.” In Education in and History of Modern Astronomy, edited by Richard Berendzen. New York: New York Academy of Sciences, 1972. Historical lecture delivered at the New York Academy in September, 1971, in which Oort discussed his work and its relation to that of several others active in this field.
• _______. “Some Notes on My Life as an Astronomer.” Annual Review of Astronomy and Astrophysics 19 (1981): 1-5. Brief survey of some of Oort’s early accomplishments, focusing especially on his galactic studies. Indicates the influences that led him into galactic studies.
• Stromgren, Bengt. “An Appreciation of Jan Hendrik Oort.” In Galaxies and the Universe, edited by Lodewijk Woltjer. New York: Columbia University Press, 1968. Tribute to Oort by an eminent Danish astronomer provides excellent commentary on Oort’s career to that point.
• Whitney, Charles A. The Discovery of Our Galaxy. 1971. Reprint. Ames: Iowa State University Press, 1988. Includes concise and clear exposition of Oort’s work. Also discusses the issue of whether the Milky Way’s spiral arms lead or trail as they rotate, which was an issue between Lindblad and Hubble, with Hubble eventually able to demonstrate that they trail. Includes illustrations, glossary, and bibliography.

Kapteyn Discovers Two Star Streams in the Galaxy

Slipher Obtains the Spectrum of a Distant Galaxy

Leavitt Discovers How to Measure Galactic Distances

Hertzsprung Uses Cepheid Variables to Calculate Distances to the Stars

Hooker Telescope Is Installed on Mount Wilson

Shapley Proves the Sun Is Distant from the Center of Our Galaxy

Slipher Presents Evidence of Redshifts in Galactic Spectra