Today's guest post comes from Paul Green, an astrophyscist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. Dr. Green studies quasars, stars and helps to manage the science operations of NASA's Chandra X-ray Observatory. In his other life he is also the bassist for The Wicked Pickers, a jazz/folk band in the Boston area.
The sky is alive with the sound of music. The Sun rings like a bell. RR Lyrae stars throb like a disco. Eclipsing binary stars revolve in rhythmic pulsations. Supermassive black holes in the cores of galaxies thrum in crescendos of rumbling bass. If we could hear all this music, it might have driven us mad with its bewildering complexity. But empty space transmits no sound, so all we heard was silence. Until now.
Traditionally, the sky has been studied from static images. Images that offer incredible beauty and detail — the colors of stars tell us their size and age and history, as do the shapes and colors of galaxies and their jewel-like clusterings across the sky. But without ever leaving our home planet Earth, astronomers are poised to enter the next dimension of exploration. Astronomy is on the brink of a revolution. Astronomy is entering the time domain.
In the 1960s, the entire sky was photographed, and this huge "all-sky" survey was reprinted for astronomical libraries across the world. In the 1990s, sky surveys entered the digital age, when the Sloan Digital Sky Survey, or SDSS, mapped the Northern Hemisphere using five visual color filters. A Southern Hemisphere version, called SkyMapper, is in the works. Such surveys are incredibly important. From them, astronomers come to understand the life cycles of stars and galaxies the way a paleobotanist might understand a past ecosystem from a layer in the fossil record. She might never see an animal move, give birth or die, but she could infer much about their histories by studying the imprints left by populations.
There is still a huge amount left to learn from the digitally-stored public multicolor images of the sky. But astronomers have decided that it's time to listen in on the music. It's time to watch the changes as they happen, and see what we can learn from them.
But movement in the sky is rare. If you take a picture of the sky, wait a few days or weeks, and take another similar picture, it will look pretty much the same. Thus you have to use special software to align and subtract the images, to detect objects that vary over time the.
But taking many images of the entire sky across time requires huge cameras for rapid mapping, ingenious algorithms for scheduling the observations, and prodigious computers that can store the massive influx of digital data. Right now, the first large-scale time domain sky survey is underway atop a dormant volcano in Hawaii.
PanSTARRS-1 is quietly mapping the sky in a grand 5 color mosaic, and, if funding can be assured, it will do so more than a dozen times over the next several years.
PanSTARRS will be a key pathfinder for a much larger project called the Large Synoptic Survey Telescope (LSST) being built in the southern hemisphere, that will be many times more powerful still when it comes to light late in this decade. Time domain surveys like PanSTARRS can find nearby objects that move across the sky, like near-Earth asteroids, and even faint nearby brown dwarf stars. But like a massive ocean trawler, their multi-epoch images will haul in millions of objects for astronomers to study that do not move, but rather vary in their brightness.
Variable objects in astronomy are rare but real. Brightness graphed against time is called their lightcurve, and lightcurves reveal some of the most bizarre and interesting phenomena in physics. Some types of variable stars are well-known, like supernovae, the spectacular cosmic explosions that mark the end of life for a massive star. The dimming light curves of distant supernovae are what first revealed that long after the Big Bang, the universal expansion is not slowing down at all, but is instead accelerating.
Closer at hand, another more common variable, the RR Lyrae stars, pulsate rhythmically in the outer reaches of our own Milky Way. Their periodic expansion and contraction is understood as a fine and funky dance between the inward pull of gravity and the outward pressure of radiation, but RR Lyrae stars play different beats that can tell us about the history of how our galaxy grew as an agglomeration of smaller "dwarf" galaxies over cosmic time.
The sky is a vast wilderness of phenomena, some understood, but most mysterious. Black holes must fall in the latter category, because they can never be seen directly. But as black holes pull in material from their surroundings, they flash brightly in ways that reveal their deepest nature. New techniques to model the lightcurves of black holes, especially when used with multicolor information, promise to tell us how they pull in matter, and where and how the intense energy arises that allows us to detect them.
The new time domain surveys of the variable sky are like the first recordings of a cosmic symphony that has been playing since the beginning of time. This symphony will be new to our ears, replete with fireworks on a cosmic scale that are sure to surprise and delight us.