Hubble And Extragalactic Nebulae

Many people were impressed by Curtis's arguments in the debate with Shapley, and opponents of the island-universe theory were increasingly put on the defensive. Still, there was no definite winner, and the question of the island-universe hypothesis was an open one in the early 1920s. As an advocate of the big-galaxy model, Shapley continued to question evidence for the hypothesis. Van Maanen's work on the rotation of the spirals also found an influential supporter in the British astrophysicist James Jeans (1877-1946). Although Jeans was skeptical of Shapley's big-galaxy model, he was even more critical of island universes and attempted to describe the dynamical motion of spiral nebulae using van Maanen's measurements. For a time Jeans developed an evolutionary theory of nebulae, suggesting that the spirals, as they evolved, turned into globular clusters. The English astronomer F. H. Reynolds (18741949) was another vigorous opponent of the island-universe hypothesis, as was the young Canadian researcher Harry Plaskett (1893-1980).

Support for the island-universe theory was stimulated by growing evidence for the very large distances to the spirals and, in particular, for the distance to the brightest and most prominent spiral, the Andromeda nebula M 31. On the basis of observations of novae in M 31, several authorities concluded that it was much too distant to be part of our galaxy. The Swedish astronomer Knut Lundmark (1889-1958) emerged as a vigorous advocate of island universes, using his study at Mount Wilson of bright stars in the nebula M 33 (a neighbor to M 31 in the constellation Triangulum) to obtain a distance for it of over

of observation. gress.

Figure 7.2: The Hooker 100-inch telescope at Mount Wilson. Courtesy of Library of Con-

Figure 7.2: The Hooker 100-inch telescope at Mount Wilson. Courtesy of Library of Con-

one million light-years. Lundmark was also dismissive of Shapley's big-galaxy conception, an attitude that resulted in some tension between the two men.

Edwin Hubble (1889-1953) was born into a middle-class American family, the son of a lawyer who worked in the insurance business. Upon completing high school in Chicago he entered the University of Chicago, where he studied mathematics and astronomy. One of his professors was Hale, whose efforts had led, in 1897, to the establishment of the Yerkes Observatory at Williams Bay, Wisconsin. Upon graduation Hubble went to Oxford as a Rhodes scholar, where he studied law, excelling as well as a heavyweight boxer and track-and-field athlete. He returned to the United States in 1913, and after a brief period as a practicing lawyer, entered graduate studies in astronomy at the University of Chicago, where he carried out research at Yerkes Observatory. His doctoral thesis in 1917 was titled "Photographic Investigations of Faint Nebulae."

After service in World War I Hubble, in 1919, was offered a position as staff astronomer at Mount Wilson. In the years that followed he trained the 100-inch reflector on the Andromeda nebula (Messier 31) and was able to resolve a multitude of stars within it (see figure 7.3). Such was the power of this telescope that Hubble was able to identify a group of Cepheid variables within M 31 and accurately measure their periods of variation. In estimating their brightness he made use of photographic magnitude scales that had been prepared by Frederick Seares (1873-1964), a Mount Wilson astronomer whose work in this area had also been essential to Shap-ley's earlier investigation of Cepheid distance indicators. The stars under study at Mount Wilson were too faint to be included in any of the existing magnitude scales. Seares was an expert on stellar photometric methods and successfully applied them to the 60-inch and 100-inch reflectors. Hubble's Cepheid data immediately provided an indication of the distance of M 31 relative to nearby galactic Cepheid stars. By late 1924 Hubble had established that M 31 was some 285,000 parsecs distant, an object outside our Milky Way and undisputedly an extragalactic nebula. His discovery was reported by Henry Russell at a historic meeting of the American Astronomical Association in early January of 1925.

With Hubble's result the astronomical community was largely won over to the island-universe hypothesis. The galaxy was named from the Greek word for "milky way." With Hubble's discovery the word galaxy was extended to any

Figure 7.3: Andromeda nebula M 31. Credit: Bill Schoe-nign, Vanessa Harvey/REU program/NOAO/AURA/NSF.

of the large star systems external to the Milky Way system on the grounds that these were objects similar to the Milky Way. Throughout his career Hubble himself preferred to retain the theory-neutral term "extragalactic nebula" for such external star systems.

Continued opposition to the island-universe theory was largely based on the evidence supplied by van Maanen for the rotation of spiral nebulae. Van Maanen himself stubbornly continued to defend his measurements and to oppose the island-universe theory well into the 1930s. Hubble was forced to expend considerable effort in exposing the errors of his senior Mount Wilson colleague, well after the time when the reality of external galaxies was accepted by most astronomers. Today, van Maanen is regarded as an example of how an experienced scientist can sometimes allow preconceptions to distort his interpretation of the observational data.

There were two respects in which the conception of external galaxies in the 1920s and 1930s differed from the traditional island-universe theory and from the views that prevail today. The measurement of distances to the spirals implied that they were distinctly smaller than the Milky Way galaxy. This fact provided some support for a modified version of Shapley's big-galaxy theory, and even those who objected to this theory tended to believe in an anomalously large size for the galaxy. The solution to this riddle would emerge from the work of Robert Trumpler (1886—1956) and his study of the distances to objects within the galaxy known as open star clusters. For comparing such clusters the apparent angular diameter provided an estimate of distance, with smaller clusters being farther away and larger clusters being closer. On the other hand, by examining the spectral characteristics of the stars in the cluster one could use the Hertzsprung-Russell diagram to obtain a measure of the stars' absolute luminosity, and this datum could be used to calculate the cluster's distance. When the two measures were compared, it was found that the distance obtained using the H-R diagram was always larger than the distance based on angular size. Trumpler concluded that a significant amount of gas existed in the galaxy and that this led to a decrease in the brightness of star clusters. Because of this, they looked fainter and farther away than they actually were, and the H-R method was overestimating their distance.

The H-R method had been used extensively by earlier researchers. When allowance was taken for interstellar absorption, the dimensions of the galaxy were found to be only a fraction of the size proposed by Shapley. Moreover, in the 1950s it was shown that the distances to the spiral galaxies had been underestimated by at least a factor of two. (We consider this development in the next chapter.) When the revised distance scales were taken into account, it was clear that the galaxy was comparable in size to the spirals, being in fact approximately the same size as the Andromeda nebula.

The second difference concerned the structure of the spirals and the relative balance of stars and nebulous matter to be found in them. Hubble and Jeans believed that the center of a spiral consists of pure nebulosity, while the outer parts of the spiral are composed primarily of stars. Elliptical galaxies were believed to be composed entirely of nebulosity and were thought to be the earliest stage in the development of a spiral galaxy. As the nebula evolved, it expanded outward, resulting in the formation of spiral arms and regions of star formation that grew in size over time. Hubble's views about an evolutionary sequence and the composite structure of spiral nebulae are now believed to be mistaken. However, his scheme for the classification of galaxies using the idea of evolutionary development is maintained today, largely for historical reasons.

One hypothesis that was confirmed during the period through a detailed study of star motions was Shapley's supposition that the Sun occupies a position offset from the center of the galaxy. The statistical studies of Jan Oort (1900-1992) and Bertil Lindblad (1895-1965) in the late 1920s indicated certain systematic patterns of stellar motion implying a differential axial rotation about a point some 5,000 parsecs from the Sun in the direction of the constellation Sagittarius. This center of rotation coincided with the center inferred by Shapley from the conjectured symmetrical distribution of globular clusters around the galaxy. Much later, in the 1950s, Oort would be one of the researchers who identified the spiral structure of the galaxy through a radio astronomical study of interstellar hydrogen.

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