Shapleys Second Copernican Revolution

The American astronomer Harlow Shapley (1885-1972) moved the Sun from the central position in the Milky Way where star counts had put it. Shapley's path to science was not straightforward. In his memoirs Shapley tells that he went to University of Missouri to study journalism, but the beginning of the course had been shifted till the following year. He decided to study something in the mean time and thumbed through the university syllabus. The first subject in the syllabus the name of which he was able to pronounce was astronomy. So that is how it was decided.

In 1914 Shapley was employed at Mount Wilson observatory which had the world's biggest (1.5m mirror) telescope. He started to study the Cepheids in globular star clusters, and their use in distance determinations. What are globular star clusters? Mostly the clusters are loose collections of stars, with hundreds of members, like Pleiades in the constellation Taurus. Globular star clusters are clearly different: they are spherical in shape, and the number of stars could be over a million. In their centers, the images of stars appear to merge together to form a smooth luminous nebula (Fig. 20.8).

The globular clusters are rather rare in the Milky Way, only a little over a hundred systems are known. But they are important objects of research for several reasons. Since they contain so many stars, they can be seen from far away and it is possible to find even rare stars in them. At Mount Wilson observatory, Shapley

3 For example, if the period is 10 days, the Cepheid is 2,000 times brighter than the Sun. A simple calculation shows that the same Cepheid, if it has magnitude 6 (just visible by plain eye), is at the distance of 800 parsecs (2,600 light years) from us.

Fig. 20.8 There are two kinds of star clusters. The open star clusters are more common. They are loosely bound gatherings of often young stars. We show the open cluster of Pleiades ("The Seven Sisters") in Taurus (a) and the globular cluster Omega Centauri (b) (photos by Harry Lehto and Tapio Korhonen, respectively)

Fig. 20.8 There are two kinds of star clusters. The open star clusters are more common. They are loosely bound gatherings of often young stars. We show the open cluster of Pleiades ("The Seven Sisters") in Taurus (a) and the globular cluster Omega Centauri (b) (photos by Harry Lehto and Tapio Korhonen, respectively)

discovered variable stars in globular clusters and used them to measure distances. After measuring the distances of a dozen of them, he realized that the cluster diameters were all almost the same size. He could then calculate the distances of the rest of the globular clusters using the apparent diameter in the sky as an indicator of distance.

In this way Shapley determined the distances of several dozen clusters, and then marked their positions in a plot with their known distances and directions. He found that the globular clusters were distributed in almost spherical manner around the Milky Way (Fig. 20.9). Shapley announced his conclusion in 1919: the Milky Way is much bigger than had been previously thought on the basis of star counts. Its center is not near the Sun, but far away in the direction of Sagittarius. "Kapteyn's universe" is only a small part of the much bigger Milky Way.

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-100 0 100 200 300 400 500

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Fig. 20.9 Harlow Shapley made use of globular star clusters in charting the structure of the Milky Way. The chart shows clearly that the Sun is far from the center

Fig. 20.9 Harlow Shapley made use of globular star clusters in charting the structure of the Milky Way. The chart shows clearly that the Sun is far from the center

It was a brave deduction, based on one class of celestial bodies. Also, Cepheids were a new distance indicator and people were suspicious of Shapley's great distances. Now we know the distances were too big for various reasons (e.g., the variable stars in globular clusters differ from "ordinary" Cepheids), but his general idea survived. A modern estimate of the diameter is 100,000 light years, near to one-third of Shapley's value.

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