Variable stars are stars that have regular or semi-regular, periodically varying brightnesses, and they have been known since medieval times.

One of the most famous is known as Algol, which is Arabic for "Demon Star." Its brightness varies by more than one full magnitude every 2.86739 days. This star is not a true variable; the change in brightness is due to its being an eclipsing binary star. During each orbit, the two stars eclipse one another, causing the visual brightness to drop. Variable stars of this type only occur when the binary star system is properly aligned for the observer to see one member being eclipsed by the other.

The Danish astronomer David Fabricius (1564-1617) is famous for his naked-eye discovery of the first long-term variable star in 1596. Named Omicron Ceti, this star varies from third magnitude to ninth, during an average period of 331 days. (Any star...