A star whose brightness fluctuates over time. This may be hours, days weeks or many years. The reasons for the fluctuations are varied, such as another star eclipsing it or shells of gas and dust being ejected.
An optical effect whereby the corners or edges of a telescope’s field of view are darkened. There are several ways vignetting can appear, but it can be reduced by taking flat fields (using software to remove its appearance) and in some cases by stopping down an aperture.
In planetary science, volatiles refer to chemical compounds (two or more elements bonded together) that have low boiling points and evaporate very quickly. They are commonly called ices.