When Algol Winks, Will You Wink Back?

Here's what an eclipse would look like if you could see it up close. The main eclipse (at right) occurs when the larger but dimmer companion star, a K2 orange subgiant, partially eclipses Algol A, a more massive but smaller main sequence star. A small secondary eclipse (left) is observed when the B star passes around the back of the primary.
 Mike Guidry / Univ. of Tennessee

By: Bob King | November 19, 2014

The dark ways of Algol, the Demon Star, and what it can teach us about stellar evolution.

What if I told you that you could stand in your front yard and watch one star eclipse another 93 light-years away from Earth with nothing but your eyeballs? As improbable as it sounds, this sight is within easy reach of northern hemisphere skywatchers once or twice a month throughout fall and winter.

Algol, nicknamed the Demon Star, resides in the constellation Perseus directly below the “W” of Cassiopeia. The name derives from Arabic “Al-Ghul,” (ghoul) and may hint at its then-mysterious habit of fading away and returning to normal brightness in a matter of hours. We now understand that this evil-spirited behavior can be blamed on Algol B, its dimmer companion star. Every 2.87 days, Algol B partially eclipses the hotter and brighter Algol A. Astronomers refer to this pair as an eclipsing binary.

During eclipse, Algol fades from magnitude +2.1 (as bright as one of the Big Dipper stars) to +3.4, then returns to its original brightness. The change is dramatic, looking as if the star might disappear altogether. But as surely as the Moon releases its hold on the Sun during a solar eclipse, Algol A and B dance about their center of gravity until the primary star returns to its previous brightness. The entire cycle takes about ten hours.