Viewed from a reference frame where one of the stars is at rest, the secondary star moves in an elliptical orbit. The secondary star is at periastron when the two stars have their smallest separation.

In a binary star system, each star moves around the other in an elliptical orbit. The point of closest approach between the two stars is called the periastron. This position is the same as the periapsis of the orbit, but specifically refers to orbits around other stars.

The point of maximum separation between the two stars is the apastron.