A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star or secondary.
Binary stars are often detected optically, in which case they are called visual binaries. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as spectroscopy (spectroscopic binaries) or astrometry (astrometric binaries). If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other; these pairs are called eclipsing binaries, or, as they are detected by their changes in brightness during eclipses and transits, photometric binaries.
The first GIF shows an artist’s impression of an eclipsing binary star system. As the two stars orbit each other they pass in front of one another and their combined brightness, seen from a distance, decreases.
Algol, known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, (2nd GIF) although Algol is actually a three-star system (Beta Persei A, B, and C) in which the large and bright primary Beta Persei A is regularly eclipsed by the dimmer Beta Persei B.
The second animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase.