Transient astronomy is the study of phenomena that lasts for a limited amount of time e.g., compared to a human lifetime, such a change in the appearance of an object, perhaps temporary. Some are over in seconds, some produce results lasting weeks, months, or years, and some repeat. The term transient astronomical event (or just transient), is used for such phenomena. Examples:
I've heard there might exist many short visible light bursts bright enough to observe, but dim and short enough that you'd have had to know when and where to observe; the same could be true of any spectral band. The challenge in all transient astronomy is to know when and where to look, or how to catch them early to observe as much of the phenomena as possible. Surveys optimized toward spotting them are now carried out that automatically recheck the same region of the sky regularly. Examples:
When a change is discovered, other telescopes can be used to follow up, observing the locations of the celestial sphere where the changes have been found. Systems to quickly notify astronomers to do this have been of interest as well as automated follow-ups.