Elucidating the sidereal structure

In the fall of 1917, Shapley sought to put his results on the distances and arrangement of the globular clusters on a firmer footing, and for this he needed to re-calibrate the period-luminosity relationship of Cepheid variables that he had borrowed from Hertzsprung. In other words, the Cepheids' periods of variability were proportional to their absolute luminosities, and he sought the constant of proportionality.

Unfortunately, no Cepheids are close enough that their distances can be determined directly by trigonometric parallax. If he could get an independent distance measurement to a few local Cepheids, the general constant of proportionality would be known. With characteristic panache, Shapley made a series of assumptions that would allow him to obtain an answer. First he borrowed from Kapteyn's technique of statistical parallaxes. He collected data on the proper motions and radial velocities of 11 nearby Cepheid type variables—those not in globular clusters, but in our own system—to find their three-dimensional motion through space. He determined the average speed of these

Cepheids as a function of their luminosity, and, like Kapteyn, assumed that the average speed was related to a star's distance. In this way, he related the luminosity and absolute distance of nearby Cepheids, and established the proportionality between the Cepheid period of variability and distance.

Next, Shapley applied the period-luminosity relationship to calculate the distances of Cepheids in globular clusters. He had to assume, as astronomers frequently must, that the same physical principles governed the nearby and more distant objects, so that the nearby variable stars he had used to calibrate the relationship behaved the same way as the variables in globular clusters. This assumption turned out to be ill-advised, but was not shown to be somewhat in error until the 1950s. Shapley published his new calibration in a paper he submitted in November 1917.

The Cepheid variables could be used in about a dozen of the 69 known globular clusters. How to proceed with those clusters so far away that even the bright Cepheid variables could not be discerned? Again, Shapley made some bold assumptions to pierce deeper into space than anyone had before. The next link in the chain of distances was provided by the highly luminous stars. Shapley contended that the most luminous stars were intrinsically similar from one cluster to another, and used the principle of ''faintness means farness'' to gauge the distances to the clusters. But then, some clusters were so distant that he could not even isolate their most luminous stars; in this case, Shapley assumed that globular clusters come in a single size. The apparent diameter of the cluster then yielded an estimate of the distance.

Shapley revealed the distances and locations of the globular clusters he had mapped out in this way in a paper he submitted for publication in November 1917. He plotted the positions of the globular clusters on a set of concentric circles centered on the location of the Sun (see figure 8.3). The plot clearly showed that the globulars are clustered in the direction of Sagittarius, at galactic longitude 325°. Shapley also drew arrows on his diagram (not shown here) to indicate the distances of the globular clusters above or below the plane of the Galaxy. The arrows showed (though not very legibly) that the globular clusters tended to avoid the plane of the galactic system, and hovered at distances of thousands of light-years from the plane.

Figure 8.3 Distribution of globular clusters mapped by Shapley. The diagram is a kind of bird's-eye-view of the Galaxy, showing the globular clusters asymmetrically located in galactic longitude with respect to the sun. Shapley found that the center of the globular cluster distribution (marked by a + symbol), which he correctly assumed matched the center of the galaxy, lay tens of thousands of light-years from the Sun. (Note that the longitude system Shapley used is no longer the standard.) (Credit: Layne Lundstrom.)

Figure 8.3 Distribution of globular clusters mapped by Shapley. The diagram is a kind of bird's-eye-view of the Galaxy, showing the globular clusters asymmetrically located in galactic longitude with respect to the sun. Shapley found that the center of the globular cluster distribution (marked by a + symbol), which he correctly assumed matched the center of the galaxy, lay tens of thousands of light-years from the Sun. (Note that the longitude system Shapley used is no longer the standard.) (Credit: Layne Lundstrom.)

Late in 1917 or very early in 1918, Shapley had an epiphany of sorts. In a letter to Eddington dated 8 January 1918 he wrote, ''I have had in mind from the first that results more important to the problem of the galactic system than to any other question might be contributed by the cluster studies. Now, with startling suddenness and definiteness, they seem to have elucidated the whole sidereal structure.''35 Indeed, the elucidation of the structure of our system, which Shapley accomplished at the age of 32, proved to be a high point of his career and the most significant contribution he made to our understanding of the Milky Way galaxy. But it would be decades before his insights could be fully appreciated, for his hypothesis was not without difficulties.

Shapley had plotted the distances and positions of all 69 known globular clusters. He had known they were asymmetrically arranged in the sky; what leaped out at him from his plot was the picture of a swarm of globular clusters centered on a point in the plane of the Galaxy in the direction of Sagittarius, which must be the center of the whole system (see figure 8.2). In effect, he resurrected Bohlin's hypothesis, which he had explicitly dismissed in 1915, and placed it on a firm foundation. The Sun could no longer be considered the center of the sidereal system. It lay about 60 000 light-years from the center.

Shapley had found a way to fit the pieces together: abandon the island universe theory. The Milky Way system was very big— a "continent" among islands, and the Sun occupied a position about one-fifth of the way from the center to the edge. The Milky Way galaxy included the globular clusters, as shown by Shapley's distances. It included the spirals as well, as shown by van Maanen's data. The novae in spirals had always yielded ambiguous results; one must either equate the fainter, more numerous novae in spirals with their galactic counterparts and infer large distances, or one must assume that the bright flaring of S Andromedae in 1885 was an outburst comparable to that of a galactic nova. Once he had concluded that the spirals were near, Shapley placed his faith in the latter interpretation.

Within a week or two of writing to Eddington, Shapley reported with equal ardor to his chief, Hale. He described the Milky Way as no one had envisioned it before, an "enormous, all-comprehending galactic system.'' The plane of the Galaxy extended to a diameter of some 300000 light-years — at least ten times farther than most of Shapley's fellow astronomers would have it. Strikingly, to those accustomed to thinking of the Milky Way as a flattened or lens-shaped system, Shapley asserted that the galactic system's boundaries reached far above and below the plane as well: Its diameter "may be much the same at right angles [to the plane], but except for the first few thousand light-years from the plane there is little evidence of isolated stars — only clusters, spiral nebulae, the Magellanic Clouds.'' After stating the facts as he knew them, he allowed himself to hypothesize:

''There is no plurality of universes of which we have evidence at present.''36

Characteristically, Hale responded with both encouragement and a note of caution. He liked Shapley's ''daring hypotheses,'' but he urged Shapley to ''substitute new hypotheses for old ones as rapidly as the evidence may demand.''37

In the meantime, Shapley was buffeted by feelings of satisfaction at having sorted out the sidereal system and vexation over some petty issues with Adams, the acting director of the observatory. Late in 1917, Adams had written to Hale, complaining that Shapley had been rather cavalier with the evidence for island universes, and, more seriously, that Shapley had ''never given the credit where it belongs'' for some aspects of his distance methods. Shapley in turn wrote to a correspondent, ''If I did not take great joy in the actual learning of things, I would feel that scientific labors are after all quite futile, for the body suffers through the necessary privations and the spirit through clashes of professional jealousies.''38

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