Astronomers love to do surveys. It doesn’t really matter what we’re looking for: quasars, pulsars, cataclysmic variables, supernovae . . . the thrill is largely in the chase. The really valuable surveys, the ones that impact science for a long time (and, not incidentally, enhance the reputation of the surveyors) are often all-sky and complete. Searching for every single object of a given class over all 41,254 square degrees of the sky qualifies a survey as all-sky. Successfully locating all the sought-for objects to some well-defined brightness limit makes a survey complete.
Why bother? Aside from satisfying an obsessive-compulsive desire to count, categorize, and catalog everything in the sky, why should any sane astronomer devote years of her life to something as tedious as a complete, all-sky survey? The answer is twofold. First, a survey of an entire class of objects can reveal truths about the Universe that unlimited study of a few class members would never achieve. Second, a well-crafted survey sometimes uncovers rare or previously unknown astronomical bodies.
A beautiful example of the discovery of a new astronomical truth came from a series of galaxy recession-speed surveys, carried out in the 1980s by Margaret Geller, John Huchra and their colleagues at the Harvard Center for Astrophysics. These surveys demonstrated that galaxies cluster in filamentary structures that stretch for tens of millions of light years. Just as remarkably, enormous voids containing virtually no galaxies occupy the spaces between the galaxy filaments. The simple picture of a smooth, homogeneous spatial distribution of galaxies that cosmologists had long assumed to be true was flat-out wrong.
Jocelyn Bell and Anthony Hewish struck astronomical gold in 1967 when their novel radio survey detected the first four pulsars—rapidly spinning neutron stars. The existence of such exotic objects had been predicted in 1939 by J. Robert Oppenheimer and George Volkoff, but it’s safe to say that few astrophysicists remembered that prediction 28 years later. We recognize pulsars today as the collapsed remnants of supernova explosions. Hewish won the Nobel prize (his student Bell didn’t, but should have) because his radio survey unveiled an entirely new class of astronomical object. Serendipity played a major role in this discovery, but the sensitive radio survey tilted the odds in Hewish and Bell’s favor.
One of the most recently discovered astronomical species is the brown dwarf. Astronomers fully appreciated by the early 1960s that no star could ever get hot enough for hydrogen to fuse if its mass is less than about 7% the mass of our Sun (or 70 Jupiter masses). Such “failed stars” can still fuse deuterium if they’re at least 13 times the mass of Jupiter. Glowing dimly for a billion or so years, they become increasingly difficult to find as they age, cool and fade. The first survey that succeeded in demonstrating the existence of brown dwarfs (in 1995) found exactly one example! This was a major triumph of modern astrophysics.
Astronomers have now catalogued hundreds of brown dwarfs, but a very sensitive, all-sky survey is still decades away. Unless we live in a special place in the Milky Way—a conjecture that is surely wrong—the number of brown dwarfs inhabiting the Milky Way must be in the tens of billions. An extrapolation from the local density of brown dwarfs and the known volume of our Milky Way yields this guesstimate.
We finally come to Kristi’s audacious PhD thesis project: an all-sky survey to locate and characterize every brown dwarf in our Milky Way galaxy. What were she and Greg thinking when they planned her survey? The primary science drivers, as Kristi’s National Science Foundation Graduate Studentship application emphasized, were to map out the 3-D spatial distribution and velocity of every Galactic brown dwarf. She proposed to compare brown dwarfs’ locations and velocities with those of ordinary hydrogen-burning stars like the Sun and cooling stellar corpses (“white dwarfs”) that occupy the spiral arms of our Galaxy. The survey method outlined in the story is how many brown dwarfs are found today. Scaling the discovery rate up a million-fold is no small challenge, and I’m certain that it will require highly sophisticated space telescopes.
Even more interesting to Kristi were the nearly metal-free, ancient stars that spend most of their lives looping far above and below the plane of the Milky Way. Are there brown dwarf analogs to these nomads? Such brown dwarfs must have been born before the Milky Way was enriched in heavy elements by multiple generations of supernovae. Locating these fossil brown dwarfs would be as valuable to astronomers as new species of dinosaurs are to paleontologists.
Finally, a word about the chimeras. I don’t think that artificial brown dwarfs really exist, but I can’t prove that they don’t, either. That’s why I obsessively participate in massive astronomical surveys . . . you just never know what stunning tricks nature has up her sleeve until you go and look.
