Variable stars are any star whose observed light varies notably in intensity. The changes in brightness may be periodic, semiregular, or completely irregular. Variable stars may be classified into three broad types according to the origin and nature of their variability: (1) eclipsing, (2) pulsating, and (3) explosive.

In an eclipsing variable, one member of a double, or binary, star system partially blocks the light of its companion as it passes in front of the latter, as observed from Earth. Each time this happens, the brightness of the entire system fluctuates. Such an eclipsing variable is perhaps best exemplified by the binary star Algol, whose name means "blinking demon."

Unlike eclipsing binaries, the other two types of variable stars are intrinsically variable--that is to say, their own output of radiant energy fluctuates with time, and are used for distance scale measurements. The pulsating variables expand and contract cyclically, causing them to pulsate rhythmically in brightness and size. The Cepheids and RR Lyrae stars are typical examples of such variables. The explosive (or eruptive) variables include novas, supernovas, and similar stars that undergo sudden outbursts of radiant energy, which results in rapid brightening. This increase in brightness lasts only for a short period of time, followed by relatively slow dimming.

Excerpt from the Encyclopedia Britannica without permission.