The star Algol is one of the most famous variable stars in the sky.  Its variability earned it the nickname of the "Demon Star."

The image at left was taken on March 9, 2015, when the star was at its normal brightness.  The image at right was taken on March 11, 2015, when it had faded to about 1/3 of its maximum brightness.  The difference is hard to see in the photograph, but it's easier to see when the two exposures alternate in an animation.

You can see this effect for yourself, without the aid of a telescope.  Algol is easily visible to the naked eye, in the constellation Perseus.  It can be found using a sky map like this one.  Perseus is high in the evening sky for the northern hemisphere in fall and winter.  Once you find Algol, study its brightness relative to the other stars in the constellation.  Then, look up a time when it will be dark out and Algol will be at minimum brightness.  One source is at Sky and Telescope (free login required).

Algol fades once every 2.87 days.  It fades gradually over the course of several hours, and remains at minimum brightness for about two hours.  Once you are familiar with how the constellation should appear, Algol's near-disappearance is a remarkable sight.

What is causing this effect?  Like most stars in the sky, Algol is actually a multiple star system.  It is a pair of stars locked in a tight orbit, plus a faint third star orbiting at a greater distance.  The pair is made up of a bright, hot, massive star, and a bloated but cooler and fainter giant star.  The pair orbit each other every 2.87 days.  When the giant star passes in front of the bright star, it partially blocks it from our view, causing the whole system to appear fainter to us.  Algol is an eclipsing binary star.

Algol's name comes from the Arabic word for "the demon's head."  Early skywatchers may have noticed Algol's periodic dimming.  This variation from the norm of the steady, constant heavens earned it an evil reputation.  Now we know, however, that Algol is entirely typical, as most stars in the sky are part of a multiple star system.  Algol is special because it has more obvious variability than any other star in the sky.

Telescope: Tele Vue-85

Camera: Lumenera SkyNYX 2-2M