November 4, 2008

In Today's One Minute Astronomer...

Algol, The "Demon Star"

Algol, the second brightest star in the northern constellation Perseus, is a beautiful example of an eclipsing variable star. While the ancients were frightened by Algol… its name comes from the Arabic “al Ghul” meaning “The Demon”… it's a beautiful example of two stars revolving about each other in a rhythmic gravitational dance. And you can see it from your backyard. No telescope required.

The Basics

• Algol lies some 90 light years from Earth. Like many stars, it's a binary: a hot blue-white star and a cooler yellow-orange star that revolve around each other. The stars are only 1/20 of the separation of the Earth and Sun so they revolve around each other quickly… once every 2.867 days.

• But unlike many binary stars, from our point of view on Earth the fainter cool star partly blocks the brighter hot star once per revolution. So the total brightness of the two stars appears to drop suddenly by roughly 70% every 2.867 days. And when the brighter star blocks the cool star, the total brightness drops again by a much smaller amount. The period of brightness variability is like clockwork.

• You can see this variability for yourself. Algol is easily visible in Perseus just east of Mirphak, the brightest star in the constellation. Compare the brightness against Mirphak or nearby gamma Andromedae.

Algol, the "Demon Star" in Perseus

A Deeper Look

• Eclipsing binary stars like Algol are more than a passing curiosity. They enable determination of stellar brightness and mass that help astronomers refine theories and mathematical models of how stars work.

• The measure of the brightness (or magnitude) of a variable star like Algol gives what's called a light curve. An eclipsing variable gives a distinctive light curve shown below. The deep dip in brightness is the time when the faint star blocks the bright star. When the bright star blocks the faint star, you see a shallower dip in brightness.

A sample light curve of the eclipsing binary star Algol.

A Bit Of History

In the Greek myths, Algol represents the head of Medusa, which Perseus used to turn Cetus the sea monster to stone. For this reason, the ancients considered Algol to represent misfortune. It's likely they were unnerved by its regular change in brightness.

Personal View

This time of year, as the night air turns crisp, I like to pan through Perseus looking for star clusters, especially the wonderful Double Cluster. Of course I always check out Algol to see if it's in eclipse. This star is like a cosmic wristwatch, revolving constantly to the rhythms of gravity and winking to watchful observers on Earth.

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