The Magellanic Clouds and other metagalactic subjects

Shapley harbored a fascination for the Magellanic Clouds that his position as observatory director at Harvard allowed him to satisfy. Indeed, his colleague Bart Bok asserted that ''For thirty years, from 1922 to 1952, Harlow Shapley was 'Mr. Magellanic Clouds.' ''59

Shapley viewed the Large and Small Magellanic Clouds together as a ''gateway'' to the universe, or at least to the group of galaxies to which the Milky Way belongs.60 Their significance for research on galactic systems first became apparent in 1913 when Hertzsprung, seizing on the fact that the Clouds contain large numbers of Cepheid variables, calculated their distance from the Milky Way system using the Cepheid period-luminosity relation. By the mid-1950s, when Shapley wrote a review article on the Magellanic Clouds, it was clear that they constitute our nearest galactic neighbors, and so provide a close-up view of important features that are difficult to study in more remote systems.

Although the Clouds are less massive than our own galaxy, they are by no means sidereal lightweights, and are richly endowed with interesting and unusual stars, clusters, and nebulae. Shapley and his collaborators accumulated thousands of photographic plates and spectra in the course of charting these two objects alone. They found ''supergiant'' stars hundreds of thousands of times more luminous than the Sun, indicating the enormous possible range of stellar characteristics. They saw regions they took to be stellar nurseries, where the processes of stellar evolution might be studied, and they marveled at the extreme phenomenon of the Tarantula Nebula in the Large Magellanic Cloud. This gargantuan emission-line nebula is intrinsically more luminous than many small galaxies. These and other attractions make the Magellanic Clouds useful ''gateways'' in galactic astronomy even today.

Nor did Shapley ignore the more distant island universes whose existence he had at one time doubted. One of his best-known legacies is the so-called Shapley-Ames catalog of 1932, now much revised and expanded upon. Shapley and Ames prepared a survey of all of what he called ''external galaxies'' — Shapley consistently used the term ''galaxies,'' while Hubble insisted on calling them ''nebulae'' — down to a limiting brightness of 13th magnitude. Other extensive studies followed; some of his publications have titles such as, ''A Study of 7900 External Galaxies'' (1935) and ''A Survey of Thirty-Six Thousand Southern Galaxies.'' These surveys permitted a statistical analyses of the properties of galaxies, complementing Shapley's in-depth studies of the Magellanic Clouds. Sadly, Ames died in the summer of 1932, the year the first survey was published. She drowned in a canoeing accident.

In 1938, Shapley and his collaborators discovered two peculiar objects that, like the Magellanic Clouds, are cohorts of the Milky Way system. The objects resemble extremely faint and extended globular clusters — ''phantom'' systems that are so rarefied, they showed up on photographic plates as mere smudges. We know them now as ''dwarf galaxies,'' an important class of object. The two that Shapley found, in the southern constellations Sculptor and Fornax, are more distant than the Magellanic Clouds, at about 270 000 light-years and 630 000 light-years, respectively. Still, they form part of the so-called ''Local Group'' of galaxies, dominated by the Milky Way and Andromeda systems. (chapter 10 discusses dwarf galaxies and their hypothesized role in galaxy evolution.)

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