The Algol Eclipsing Binary System

The most famous eclipsing binary system is Algol. The Algol system is illustrated in the following figure:

The Algol System
John Goodricke proposed in 1783 that Algol might be an eclipsing system, but had no way to prove that it was a binary. It was 1889 before Hermann Vogel showed that Algol is a (single-line) spectroscopic binary. Modern analysis indicates that there are three stars in the system, with two of them responsible for the eclipses observed from Earth.
The alternative hypothesis that the variation in Algol's light output is because of a pulsating variable can be ruled out by a careful analysis of the details.

Variability of Algol

Normally Algol is a magnitude 2 star, but every 68.75 hours it fades for several hours to about a third of its normal brightness before returning to magnitude 2. Half-way in between these major dips in brightness there is another very small decrease in brightness for several hours. As the preceding figure of the the Algol system illustrates, the variation in light output can be attributed to Algol being an eclipsing binary, with one star brighter than the other.

Eclipses in the Algol System

The primary star, called Algol A, is a spectral class B8 main sequence star of about 3 solar diameters, while the companion Algol B is a cooler spectral class K2 star with a diameter of about 3.5 solar diameters. The center of mass for the system (denoted by an X) is just inside the primary star. It is believed that the shape of the secondary is distorted by the gravitational influence of the primary. The partial eclipses, which cause light variation that is visible to the naked eye, are deepest when part of the hotter star is hidden and more shallow when part of the cooler star is hidden in eclipse. These eclipses occur with a period of 68.75 hours.