Eclipsing Binary Stars ‒ Algol

The bright stars of Algol can be found in the sky in the Perseus constellation. Though it is perceived with a naked eye as a single star, astronomical observations have shown that it actually consists of three stars - Algol is therefore a three-star system. It can be imagined as follows: the base consists of two stars (known as Algol A and Algol B) that orbit close to each other around a common center of gravity. Around this duo a third star called Algol C circulates.



Coincidentally the Algol system is positioned in such a way that when viewed from Earth, the stellar components A and B periodically cover each other in their circulation. More specifically, the plane in which the orbit of both components is located almost coincides with the plane of our sight. One orbit around the center takes 2 days 20 hours and 49 minutes for both Algol A and B.



The result of these repeated occultations (when the first star's light is shielded by the second star) is a periodic variation in brightness of Algol visible from Earth. These brightness variations are so distinct that even a naked eye can notice them. Hence the Algol is also called an eclipsing binary star. At the same time it is one of the first recognized variable stars - the stars whose brightness change.



Animation shows both the Algol A and Algol B stars orbiting. Since the stars have a different mass, center of gravity is greatly shifted towards the heavier one. Part "Line of Sight from Earth" also shows the inclination of the entire system to an observer on the Earth's surface.