Time domain astronomy is the study of how astronomical objects, especially those beyond the solar system, change with time. This may be due to movement or changes in the object itself. Common targets included are supernovae, novas, flare stars, blazars and active galactic nuclei. Visible light time domain studies include Hat-South, LSST PanSTARRS, PSST, SkyMapper, WASP3 and Catalina Real-time Transient Survey. in Radio Astronomy the LOFAR is looking for radio transients. Radio time domain studies have long included pulsars and scintillation. Cherenkov Telescope Array, eROSITA, Fermi, HAWC, INTEGRAL, MAXI, and Swift Gamma-Ray Burst Mission will look for transients in X-ray and gamma rays. Gamma ray bursts are a well known high energy electromagnetic transient.
Tools used are robotic telescopes, automatic classification of transient events, rapid notification of interested people. Blinking two photographic plates enabled a person to spot the differences. Image subtraction requires a normalization of the pairs of images. Challenges include handling a huge amount of data storage and transfer, data mining techniques, classification, and heterogeneous data.
Historically time domain astronomy has come to include appearance of comets, and cepheid variable. Old astronomical plates exposed from the late 1800s through the early 1990s held by the Harvard College Observatory are being digitized by the DASCH project.
Other causes of time variability are asteroids, eclipses, microlensing, planetary transits, variable stars.