If you’re out watching the meteors this summer, especially the Perseids in August, you may want to take the time to "time" another, though more subtle, celestial spectacle – a "blinking demon star." What’s more, you don’t have to travel to a dark-sky site to see it blink. You can watch this star from your own backyard. All you need is a star wheel or planisphere (available at most museum or planetarium gift shops or "nature-type" stores) to help you find the constellations on any given night and time of the year.
Our "winker" is more commonly called Algol, and it’s the second brightest star in the constellation Perseus. The close-up chart of Perseus shown here (courtesy Jack Horkheimer, TV’s Stargazer), shows the position of Mirfak, the brightest star in Perseus, Algol (our winking star), and the point in the sky where the Perseid meteors appear to shoot from.
Algol is the "demon star" because in ancient times it was seen as the evil eye of the snake-haired gorgon Medusa, who Perseus slayed. When we look to the heavens at Perseus, we see a strong youth wearing winged sandals and a scarf tossed over his shoulder; in his left hand he holds the bloody head of Medusa, which he severed during his daring rescue of the fair maiden, Andromeda. In ancient times, many looked upon Algol and its surrounding stars as a separate constellation, the "Head of Medusa." Although it is still seen as such today, the head of Medusa is officially associated with the constellation Perseus. One rather gross note of interest: to the ancient Chinese, Algol had another special name, Tseih She, which means "Piled-up Corpses."
But why is Algol a blinking star? The reason Algol blinks is not for the same reason that stars twinkle. Stars twinkle because their light is shimmering in our constantly moving atmosphere. Algol blinks because it is a variable star – a star that dims and
beightens over a certain interval of time. The 17th-century Italian astronomer Geminiano Montanari first noticed that the star varies in brightness. Then, over the centuries, other astronomers made precise measurements of the star’s fluctuations.
Today we now know that the star is an eclipsing binary star, meaning that the single star we see with our eyes is actually a system of two stars (one dim, one bright) too close together to separate with our eyes. The stars orbit one another in such a way that they pass in front of one another from our line of sight. When the dim star passes in front of the brighter star, we see an eclipse, and a drop in brightness. When the bright star eclipses the dim one, we essentially see Algol at its normal brightness.
That’s how the star we see with the naked eye blinks!
Algol was the first eclipsing variable star ever discovered, and it’s
still the most famous one. Algol fades and rebrightens like clockwork
every 2.9 days. When in eclipse, Algol can fade more than one stellar
magnitude – that’s more than a hundred times in brightness. It takes a couple of hours for the star to fade to its minimum light and it stays dim for nearly two hours before it gradually returns to normal
Here are some key times to look for Algol this summer when it is dim (mid-eclipse). That means you will have to go out about an hour or so before these time to catch the entire show. Also note that you will have to convert the times given here (Eastern Daylight Time) to that of your own time zone, if necessary.