Variable Star

A Variable Star is a star whose brightness fluctuates. This may happen either because of of external interference between the star and the observer (e.g. clouds of dust, eclipsing companion, etc.) in which case the star is an “Extrinsic Variable”, or because the star’s luminosity does actually change for some reason (e.g. the star periodically grows and shrinks) in which case the star is an “Intristic Variable”.

Intrinsic Variables are further subdivided into three categories: There are many, many types of variable stars, all of which are covered in great detail here.
 * 1) Pulsating Variables, stars whose radius expands and contracts periodically as part of their ageing. These stars are generally found in a part of the HR diagram known as the instability strip.
 * 2) Eruptive Variables, stars which briefly get brighter due to eruptions on the star’s surface such as a flare or mass ejection.
 * 3) Cataclysmic Variables, stars which get brighter due to cataclysmic events such as supernovae.

Worldbuilding in Practice
"Eta Rodriguez A is a particularly fascinating star as it is a variable star (a Delta Cepheid variable, to be precise) which regularly pulsates in a four day cycle. At the peak of this cycle, it is about 11 times brighter than it was two days previously. Even from its distant companion Eta Rodriguez E, this cycle is very much apparent."