- Variable star whose changes in magnitude are caused by the partners in a binary star periodically blocking one another’s light from the observer. The best-known eclipsing binary is Algol in Perseus. Algol has eclipses every 69 hours, when its magnitude falls from 2.3 to 3.5. There is also a third member of the Algol group which does not get involved in the eclipses as seen from Earth. Observing eclipsing binaries allows us to discover the orbital characteristics of the stars involved and so to measure their mass.