Time-domain astronomy is the study of how astronomical objects change with time. Although the study may be said to begin with Galileo's Letters on Sunspots, the term now refers especially to variable objects beyond the Solar System. Changes over time may be due to motion or intrinsic changes in the object itself. Common targets include supernovae, pulsating stars, novae, flare stars, blazars, and active galactic nuclei. Visible-light time-domain surveys include OGLE, HAT-South, Pan-STARRS, SkyMapper, ASAS, WASP, CRTS, and, in the near future, the LSST at the Vera C. Rubin Observatory. Time-domain astronomy studies transient astronomical events (often shortened to "transients") as well as various types of variable stars, including periodic, quasi-periodic, and those that change behavior or type. Other causes of time variability are asteroids, high proper-motion stars, planetary transits, and comets. Transients are astronomical events whose observable durations range from milliseconds to days, weeks, or even several years, in contrast to the millions or billions of years over which galaxies and their component stars evolve. More specifically, the term often refers to violent deep-sky events such as supernovae, novae, dwarf-nova outbursts, gamma-ray bursts, tidal disruption events, and gravitational microlensing. Time-domain astronomy involves long-term studies of variable stars and their changes on timescales from minutes to decades. Variability can be intrinsic—such as periodic or semi-regular pulsations, young stellar objects, stars with outbursts, and asteroseismology—or extrinsic, resulting from eclipses (in binary stars or planetary transits), stellar rotation (in pulsars and spotted stars), or gravitational microlensing. Modern time-domain surveys often use robotic telescopes, automated classification of transient events, and rapid notifications for interested researchers. Blink comparators have long been used to detect differences between photographic plates, and image subtraction became more widely used once digital imaging made it easier to normalize image pairs. Because of the large fields of view required, time-domain work involves storing and transferring vast amounts of data, requiring data mining, automated classification, and the handling of heterogeneous datasets. The importance of time-domain astronomy has been recognized by professional societies. Andrzej Udalski was recognized for his "pioneering contribution to the growth of a new field of astrophysics research, time-domain astronomy, which studies the variability of brightness and other parameters of objects in the universe on different timescales." The 2017 Dan David Prize was awarded to three leading researchers in the field of time-domain astronomy: Neil Gehrels (Swift Gamma-Ray Burst Mission), Shrinivas Kulkarni (Palomar Transient Factory), and Andrzej Udalski (Optical Gravitational Lensing Experiment).

History: Before the invention of telescopes, transient events visible to the naked eye within or near the Milky Way were very rare and sometimes separated by hundreds of years. Such events were recorded in antiquity, for example the supernova of 1054 observed by Chinese, Japanese, and Arab astronomers, and the 1572 event known as "Tycho's Supernova," after Tycho Brahe, who studied it until it faded two years later. Although telescopes made it possible to see more distant events, their small fields of view—typically less than 1 square degree—meant the chances of looking in the right place at the right time were low. Schmidt cameras and other wide-field astrographs were invented in the 20th century, but were mostly used to survey the unchanging heavens. Historically, time-domain astronomy has included the appearance of comets and the variable brightness of Cepheid-type variable stars. Old astronomical plates exposed from the 1880s through the early 1990s at the Harvard College Observatory are being digitized by the DASCH project. Interest in transients increased with the availability of large CCD detectors to the astronomical community. In the 1990s, as telescopes with wider fields of view and larger detectors came into use, the first large, regular survey observations were initiated—pioneered by gravitational microlensing surveys such as the Optical Gravitational Lensing Experiment (OGLE) and the MACHO Project. Besides discovering microlensing events, these efforts increased by orders of magnitude the number of known variable stars. Subsequent dedicated sky surveys, including the Palomar Transient Factory, the Gaia spacecraft, and the Large Synoptic Survey Telescope (LSST), focused on extending sky monitoring to fainter objects, adding optical filters, and improving astrometry and proper-motion measurements. In 2022, the Gravitational-wave Optical Transient Observer (GOTO) began searching for optical counterparts to neutron star collisions. Modern instruments can observe wavelengths invisible to the human eye—radio, infrared, ultraviolet, and X-ray—greatly increasing the information available when studying transients. In radio astronomy, the Low-Frequency Array (LOFAR) is used to search for radio transients; radio time-domain studies have long included pulsars and scintillation. Projects that look for transients in X-rays and gamma rays include the Cherenkov Telescope Array, eROSITA, AGILE, Fermi, HAWC, INTEGRAL, MAXI, Swift (the Gamma-Ray Burst Mission), and the Space Variable Objects Monitor. Gamma-ray bursts are well-known, high-energy electromagnetic transients. The proposed ULTRASAT satellite will observe a field of more than 200 square degrees continuously in the ultraviolet, a band particularly important for detecting supernovae within minutes of their occurrence.

See also: List of gamma-ray bursts; gravitational microlensing; list of gravitational wave observations; list of exoplanets detected by microlensing; X-ray transient; cataclysmic variable star; stellar pulsation; SIMBAD Astronomical Database; observational astronomy; astronomical events.