A stellar system of two stars is known as a "binary star", "binary star system" or "physical double star". If there are no tidal effects, no disturbance from other forces, and no transfer of mass from one star to the other, such a system is stable. Both stars will orbit around the center of mass of the system indefinitely. Examples of binary systems are Sirius, Procyon and Cygnus X-1, the last of which probably consists of a star and a black hole.