The Realm of the Nebulae

Edwin Hubble never really liked the word “galaxy.” He was the one, of course, who was most responsible for making the word an important one, by showing that (at least some of) the fuzzy patches in the sky called “nebulae” were actually collections of billions of stars in their own right, far outside our Milky Way. (That was his second-most important discovery, after the distance-redshift relationship that reveals the expansion of the universe.) It’s possible that Hubble didn’t want to do any favors for Harlow Shapley, his rival, who coined the term “galaxy.” But for whatever reason, when in the 1930’s he gave a series of prestigious lectures at Yale which he later turned into a book, Hubble’s chosen title was The Realm of the Nebulae. Near the end of his introductory chapter, he sniffs, “The term nebulae offers the values of tradition; the term galaxies, the glamour of romance.”

In the court of popular opinion, romance will usually be a heavy favorite over tradition, and these days we use “galaxies” to refer to large collections of stars, gas, dust, and dark matter. But Hubble’s book became a classic, and is a great treat to read these many decades later. Cosmology has marched on quite a bit, of course, but the insights Hubble offers into the practice of doing science are timeless. The guy was a smart cookie, and a better-than-decent writer, to boot.

So it’s great to have a new edition of the book recently published by Yale University Press. Precisely because science has been advancing in the intervening years, publishers have found it useful to commission new prefaces to keep the reader updated on cosmological progress, and these prefaces (or Forewords, I can never tell the difference) have been accumulating over time, all of them contained in the new version. There’s one by Allan Sandage, from 1958; another by James Gunn, from 1981; and now our Golden Age of Cosmology requires not one but two new contributions, one by Robert Kirshner and one by me. Given the extraordinarily high quality of my companion contributors to the front of the volume, I tried hard to make my offering both interesting and useful. Readers can judge that for themselves, but it’s certainly an honor to be in such esteemed company.

Hubble was an unforgiving empiricist; he didn’t worry too much about the theoretical implications of his discoveries, preferring to leave that to others. But he knew about them, and his last chapter discusses the different world models to emerge from Einstein’s general relativity, and the implication that we will only ever be able to observe a small part of the much larger universe.

Thus the explorations of space end on a note of uncertainty. And necessarily so. We are, by definition, at the very center of the observable region. We know our immediate neighborhood rather intimately. With increasing distance, our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary — the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial.

The search will continue. Not until the empirical resources are exhausted, need we pass onto the dreamy realms of speculation.

Fortunately, and contrary to the metaphorical implication, the dreamy realms of speculation aren’t a location where we have to remain once we arrive. Progress in science requires cooperation between speculation and observation. The dreamy realms are an important place to visit, even if Hubble wouldn’t have wanted to live there.