Nebulous categories: the many species of galaxies turn out to be close kin

Two and a half centuries ago, before the English astronomer Sir William Herschel built the world's first seriously large telescope, the known universe was little more than the stars, the planets, the Sun and Moon, and the Galaxy, forming a milky band across the night sky. Indeed, the word "galaxy" derives from the Greek galaktos, or milk. The sky also held the nebulae--fuzzy, indeterminate objects such as the Andromeda nebula, which lives among the stars of the constellation Andromeda.

Herschel's telescope was forty-eight inches across, an unprecedented size in 1789, the year it was built. It was an ungainly instrument, but when he aimed it at the heavens, Herschel could readily see the countless stars that make up the Milky Way. Using his forty-eight-incher as well as a smaller, more nimble telescope, Herschel and his sister Caroline compiled the first extensive "deep sky" catalog of northern nebulae. Sir John--Herschel's son--continued the Family tradition, adding to his father and aunt's list of northern objects and, during an extended stay at the Cape of Good Hope at the southern tip of Africa, cataloging some 1,700 fuzzy objects visible from the Southern Hemisphere. In 1864 Sir John produced a synthesis of the more than 5,000 known deep-sky objects: A General Catalogue of Nebulae and Clusters of Stars.

In spite of that large body of data, nobody at the time knew the true identity of the nebulae, their distances from Earth, or the differences among them. Nevertheless, the massive 1864 catalog made it possible to classify the nebulae morphologically--that is, according to shape. In the "we call 'em as we see 'em" tradition of both umpires and astronomers, the spiral-shaped nebulae were called spiral nebulae, those with a vaguely elliptical shape were called elliptical nebulae, and the various irregularly shaped nebulae--neither spiral nor elliptical--were called irregular nebulae. Finally, nebulae that looked small and round, like a telescope's view of the planets, were called planetary nebulae, thus permanently confusing newcomers to the subject.

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