The Realm of the Nebulae

41CF7V31PmL._SY300_ 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.

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8 Responses to The Realm of the Nebulae

  1. Phil P. says:

    A “Foreword”, I believe, is written by someone other than the book’s author; whereas a “Preface” is written by the author. Forewords are “prepended”, meaning that the latest Foreword appears first. So your Foreword should be the first words people see in the new edition. How cool is that?

  2. Sean Carroll says:

    It is cool, although Sandage’s contribution is labeled as a “preface.” I think the distinction is function, not authorship:

  3. Harry says:

    Congratulations, that is definitely an honor.

    I always found Hubble to be an interesting character. His contributions to cosmology are undeniable. He had access to the right telescope at the right time; and, unlike Howard Shapley, was open-minded enough to pursue the right questions.

    However, I still feel that Vesto Slipher deserves much credit as well.

  4. Martin says:

    Something curious, Merriam-Webster lists “light-year” and “world” as synonyms of galaxy.

  5. Tony Rz says:

    Proponents of QM keep hoping to find cracks in General Relativity, maybe it’s QM that’s imperfect. Just a thought. Perhaps it has nothing to do with Hubble, but why is it that GR must be imperfect. Since most on this site are Physicists, you must hope as well that Einstein’s theory must be imperfect.

  6. Meh says:


    your comment is riddled with mistakes and assumptions.

  7. “prefaces (or Forewords, I can never tell the difference)”

    While there is a distinction in purpose, the link you cite confirms that someone other than the author writes the foreword.

  8. Jim Sweeney says:

    I read that book as a teenager in the 1960’s. The fact that it’s still in print is a testament to its author. Congratulations to our host for being selected to contribute a fresh introduction.

    ‘Galaxy’ has the problem that it means, literally, ‘Milky Way’, which is also the name we give our home, um, nebula. Perhaps this would only be an issue for a very few Greeks.