In order to know the life history of a star, astronomers must first know the mass of the star. The mass can only be determined directly when the star is a member of a binary star system: two stars orbiting around a common center of gravity. One method of detecting such a system is when the orbits of the two stars are aligned such that the two stars alternately pass in front of (occult = eclipse) one another, thus appearing brighter, then dimmer, then brighter again, and so on. The animation illustrates how the brightness of the pair of stars varies as two stars in a binary pair alternately eclipse one another.