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Time Domain Astronomy

Most things in astronomy change too slowly for the human eye to notice. However, change is an important part of astronomical systems, whether it’s comets crossing the Solar System, explosions of stars, collisions of black holes, or exoplanets briefly eclipsing their host stars. Time domain astronomy is dedicated to studying systems that fluctuate measurably during observation. It’s a study that has become more important with the advent of large-scale surveys of the sky, where changes are measurable that were once too small to notice.

This X-ray image of the Crab Nebula from NASA's Chandra X-ray Observatory is one part of a long-running set of observations to measure how this supernova remnant fluctuates over time.

NASA/CXC/MSFC/M.Weisskopf et al

The Evolving Sky

Time domain astronomy has its roots in antiquity, when ancient astronomers mapped the motion of the planets on the sky. They also distinguished the predictable cycles of planetary motion and eclipses from the things they couldn’t anticipate: the passage of comets or the sudden appearance of new “stars”, which we know today as variable stars, supernovas, and other transient phenomena.

Similarly, modern time domain astronomy involves both predictable and unpredictable changes in the sky:

  • Monitoring stars for exoplanet transits: the small amount of light blocked as a planet crosses between its host star and us. The duration of the transit and time between transits tells us how long the planet’s year is. To discover transits, astronomers point observatories at a patch of the sky for a long period to see fluctuations — an ideal example of time domain research.

  • Similarly, studying variations in stars themselves. Fluctuations in their magnetic fields or vibrations in their interiors produce changes that can only be noticed by observing the star over a period of time. That includes pulsations, flares, and other transient events.

  • Tracking the infall of matter onto black holes and other compact objects produces jets and flares, which fluctuate measurably over time. The Event Horizon Telescope (EHT) is an array of many observatories dedicated to creating images of black holes at the center of the Milky Way and the nearby galaxy M87. The EHT also measures variations in light from matter orbiting the black hole, which provides a wealth of information about this extreme environment.

  • Looking for new asteroids and comets within the Solar System.

  • Detecting the most energetic events in the Universe, which burn brightly then fade away. This includes supernova explosions, gamma-ray bursts, and the bursts of light that happen when stars pass too close to a supermassive black hole and are torn apart by the gravitational tidal force.

  • Observing the collisions of black holes and neutron stars, which are the source of gravitational waves. In addition, neutron star collisions — known as “kilonovas” — are explosive events producing light, which allows for multiple ways to study the event.

Some transient phenomena last fractions of a second, while others take decades or longer to change noticeably. Time domain astronomy encompasses all of these. Next-generation observatories such as the Large Synoptic Survey Telescope (LSST) are designed to spot many transient events by watching large swaths of the sky again and again. These observations require new methods of data analysis, including machine learning and advanced statistical techniques.


Historical Data as Time Domain Astronomy

Researchers in the Digital Access to a Sky Century at Harvard (DASCH) project are engaged in digitizing photographic plates taken by astronomers from 1885 through 1992. These cover many different observatories mapping much of the sky. Comparing these historical images with each other and with more recent photographs allows astronomers to spot transient events from more than a century of observations.