When two or more stars orbit each other, they are called star systems. A binary star is a star system which is made up of two stars that orbit a common center of mass because of gravitational attraction. The brighter and larger star is usually called the primary and the other one the companion star.

There are four subcategories of binary stars, which are the visual binaries, the eclipsing binaries, the astro-metric binaries and the spectroscopic binaries.

A visual star is a star system where the two stars can be seen through a telescope with the right resolving power. However, if the primary star is much brighter than its companion star, it can be difficult to detect a visual star.
An eclipsing binary star, meanwhile, is a star system with the two stars’ orbit plane lying nearly at the observer’s line of sight. It gives the illusion that the orbits form a horizontal line, making the stars undergo mutual eclipses.

If there is a star in space that seems to wobble without a companion star, then it may be an astro-metric binary star. Even though only one star can be seen, the wobble is meant to indicate that a smaller star is attracting the larger star.
Finally, the spectroscopic binary star is a system which has a very high orbital velocity, meaning the stars orbit each other very quickly. The stars themselves are very close and can be difficult to see through a telescope.

Colliding Binary Star