What term describes the variations in brightness of celestial objects over time?

The correct term that describes the variations in brightness of celestial objects over time is "light curves." Light curves are graphical representations that plot the brightness of an astronomical object as a function of time. These curves can reveal important information about the object's physical properties, such as its size, distance, and the presence of features like eclipses or variable states caused by factors such as pulsation or orbital motion.

Light curves are especially significant in the study of variable stars, transiting exoplanets, and supernovae, where the changes in brightness can indicate underlying physical processes. The other options, while related to light and brightness, do not directly convey the concept of temporal variation: the light spectrum relates to the distribution of different wavelengths of light, a light interval focuses on time between observed brightness changes, and brightness flux refers to the amount of light energy received per unit area, rather than describing its variation over time.