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.

For most of its history, astronomy has remained plainspoken and has employed descriptive methods of inquiry that greatly resembled the ones used in botany. From the ever-lengthening compendiums of stars and fuzzy things, astronomers searched for patterns and sorted objects accordingly. It's a sensible approach: most people, beginning in childhood, arrange things according to appearance and shape without even being told to do so. But it can carry you only so far. The Herschels always assumed, because many of their fuzzy objects span a patch of about the same size on the night sky, that all the nebulae lay at about the same distance from Earth. So to them it was simply good, even handed science to subject all the nebulae to the same rules of sorting.

Trouble is, the assumption that all nebulae lay at similar distances turned out to be badly mistaken. Nature can be elusive, even devious. Some of the nebulae classified by the Herschels are no farther away than the stars, and so they are relatively small (if a trillion miles across can be called "relatively small"). Others turn out to be much more distant, and so, to appear the same size on the sky, they've got to be much larger than the fuzzy objects nearby.

The take-home lesson is that at some point you've got to stop fixating on what something looks like and start asking what it is. Fortunately, by the late nineteenth century, advances in science and technology had empowered astronomers to do just that: to move beyond merely classifying the contents of" the universe. That shift led to the birth of astrophysics.

Just about the time Sir John Herschel published his vast catalog, a new scientific instrument, the spectroscope, joined the search for nebulae. The sole job of a spectroscope is to break light into a rainbow of its component colors. The fine details in that color spectrum hold information about not only the chemical composition of the light source but also, because of a phenomenon called the Doppler effect, the motion off the light source toward or away from Earth.

Spectroscopy showed that nearly all the spiral nebulae, a shape that happens to predominate outside the swath of the Milky Way, are moving away from Earth, and at very high speeds. In contrast, it showed that all planetary and most irregular nebulae are traveling at very low speeds--some toward us and some away from us. Had some catastrophic explosion taken place in the center of the Milky Way, kicking out only the spiral nebulae? If so, why weren't any of them falling back? The conundrum persisted, even though advances in photography brought forth faster emulsions, enabling astronomers to measure the spectra of ever dimmer nebulae.

By the 1920s, however, another key instrument had appeared on the scene: the formidable 100-inch Hooker Telescope at the Mount Wilson Observatory near Los Angeles. In 1923 the American astronomer Edwin P. Hubble, using this telescope--the largest in the world at that time--discovered a special breed of star, a Cepheid variable, in the Andromeda nebula. Variable stars of any species vary in brightness according to well-known patterns; the Cepheid variables are extremely luminous and therefore visible over vast distances. Hubble knew the distances to a few of them within the Milky Way, yet to his astonishment, the Cepheid he saw in Andromeda was much dimmer than any of them.