The Honor of Being Forgotten

One of the ways in which astronomers differ from physicists is that we know an absurd number of small facts about the Universe. As you may have surmised, the Universe is a fairly complicated place, and astronomers revel in collecting details about the various bits of stuff within it. While this process is frequently dismissed as "stamp collecting" (the lowest blow that one can level at an astronomer, no offense to the philatelists among the CV readership), it's actually an essential feature. At its best, astronomy is a search for underlying patterns and missed opportunities that can be used to contstrain fundamental physical processes. In the past, I've likened astronomy to gathering an enormous bag of puzzle pieces, but maybe there's multiple puzzles in the bag, and probably some of the pieces are lost, or chewed on, and perhaps someone threw some extra pieces from some other puzzle in there. But, if you're very persistent, perceptive, and have collected a lot of pieces, you might notice that some of the pieces would look a bit like a cat if you put them together just so. A by-product of the need to gather puzzle pieces is that to make any headway, there are lots of facts one just has to know, without going back to first sources. When I first started teaching, I was shocked to discover what a large fraction of my astronomical knowledge was based upon "Jim Gunn told me so". Now, Jim is well known as a clever fellow, so there are worse foundations to build from. However, I spent quite a bit of time those first few years trying to track down exactly why Jim told me various things were true. In most cases, the truths were established recently enough that tracking down sources was straightforward. Correlations and important processes were named after specific people, important calculations were cited in reference lists, all leaving a clear paper trail. Once in a while, though, I run across a piece of astronomical knowledge that is so fundamental to our thinking that we've lost all trace of who said it first. The puzzle piece seems so obvious that it's hard to imagine that there was a time when we didn't have it in our bag. Most recently, I spent several days tracking down the genesis of the Milky Way's stellar halo. The stellar halo, like the dark matter halo, is extensive and elusive. It consists of a diffuse smattering of very old stars, dating from the earliest assembly of the Galaxy. While halo stars are only a miniscule fraction of the stars in the Milky Way, their motions and their distribution around the Galaxy are superb tracers of the details of how structure first assembled, which in turn constrains the behavior of dark matter. Moreover, because they are some of the earliest stars formed, halo stars contain nearly pristine gas that has experienced little processing through other stars. The Universe forms mostly hydrogen and helium during Big Bang nucleosynthesis, and the paucity of all other elements (known as "metals" to astronomers -- yes, we consider oxygen to be a metal. sue me.) in halo stars therefore indicates that most of their gas is close to primordial. Astronomers have even found individual halo stars where what little "pollution" there is seems to have come from a single supernova, allowing one to shake out a fair bit of tasty nuclear physics. The upshot is that the stellar halo contains a pretty useful set of puzzle pieces to toss in the bag. However, how did anyone figure out that it was there? An electronic search finds that the term "halo" pops into the literature in the early- to mid-50's, right before a flurry of papers establishing that the halo stars were "metal poor". (Actually, "halo" appeared plenty of times before, but mostly in the context of coronae surrounding images of stars or the moon.) However, one of the papers from the mid-50's came from an entire symposium on how best to map out the structure of the Galaxy, including the stellar halo. So, if astronomers were already planning a full assault on the problem, they must have been long convinced that it existed. Indeed, they were so convinced that they didn't bother to reference any of the papers that established its existence. I transferred my sleuthing to the human abstract service, namely Extremely Sharp Professors in their 70's. One of the ESP's said "I think it was Oort". And indeed, in 1924 J. H. Oort published a note in the Procedings of the National Academy of Sciences titled "On a Possible Relation between Globular Clusters and Stars of High Velocity". What Oort did was note that if one took the orbits of nearby faint stars with unusual velocities and traced them backwards, then the high velocity stars could not be orbiting in a disk like most Milky Way stars, but instead would be travelling in a cloud surrounding the Galaxy, much like the globular clusters (which Harlow Shapley had earlier used to deduce that the position of the Sun with respect to the center of the Galaxy). So the ESP was right. It was Oort. Now, the point of this post is not to drag Oort out of obscurity. Oort doesn't need it, as the guy even has important astronomical constants named after him. The point is that if you do really good work, and discover something fundamental about how the Universe is put together, you might be forgotten. And that's ok. Indeed, it's a strange form of respect, to be forgotten. It means that there was something so obviously right about what you said that it was quickly absorbed by the scientific hive-mind, until it seemed like we knew it all along.