The Believing Brain - Michael Shermer [164]
In 1781, a comet chaser named Charles Messier published a catalog of the nebulae, primarily as a means of distinguishing these fixed blurry dots from the moving wispy comets he was searching for.4 This became the definitive compendium of nebulae and is still in use today because historical nomenclature holds precedence in science (in the same way that we still use Carl von Linné’s eighteenth-century pre-Darwinian binomial nomenclature for identifying organisms—for example, Homo sapiens). Messier’s catalog gave grist to the telescopic mill. The great astronomer William Herschel, after his remarkable discovery of Uranus, ramped up the search by turning his twenty-foot tube with its twelve-inch mirror to the objects Messier said were not moving. “I have looked farther into space than ever a human being did before me,” he boasted. He was able to resolve individual stars within the blotches, proving that there were island universes after all!5 Kant was right.
Not so fast. It turns out that Herschel was not imaging distant galaxies. He was looking at globular clusters—collections of stars in or near the Milky Way galaxy that astronomers differentiated from nebulae without discernible individual stars. Herschel correctly identified the Orion Nebula as an interstellar cloud of gas within our galaxy in the process of giving birth to new stars. As well, in 1790 Herschel imaged “a most singular Phaenomenon!”: “a star of about the eighth magnitude, with a faint luminous atmosphere” in which “the star is perfectly in the center and the atmosphere is so diluted, faint and equal throughout, that there can be no surmise of its consisting of stars; nor can there be a doubt of the evident connection between the atmosphere and the star.”6 It was a planetary nebula—a star within our galaxy that is shedding its outer gaseous layer. This was evidence against Kant’s island universe theory and in favor of the nebular hypothesis. By the 1790s Herschel had cataloged more than a thousand new nebulae and stellar clusters. Despite the wide variety of nebula types that he imaged, and over the voices of many skeptical colleagues, Herschel pronounced: “These curious objects, not only on account of their number, but also in consideration of their great consequence, are no less than whole sidereal systems” that “may well outvie our Milky-Way in grandeur.”7
Conflicting Patterns of Data
With the hindsight bias, of course, we know how the story turns out. It is easy to rummage around in the dustbin of history and pull out those who were ahead of their time, which is what I’ve been doing thus far, but with two centuries left in the story astronomers had obviously not solved the riddle of the nebulae. An additional problem arises at this point: in a sense both theories were correct. On the one hand, there are lots of local phenomena within our galaxy that appear as fuzzy patches in the night sky: comets, gaseous clouds, globular clusters of stars, open clusters of stars, planetary nebulae, ancient nova and supernova stars that blew up and left only shells of gas, and so on. On the other hand, the vast majority of Messier’s catalog objects labeled as nebulae are, in fact, island universes—galaxies of stars—enormous distances away from the Milky Way galaxy. The problem in distinguishing between the two categories of celestial objects comes down to better data and refined theory. The latter followed the former, and the former depended