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The 'Demon Star' After Dark


Last updated 2/15/2024 at 10:20am

The Demon Star is actually a binary star system.

We call it the Demon Star because it winks ominously at us from the sky each evening. Well, OK, it doesn't "wink," but it does vary its brightness over time, and for much of history that was quite a portentous sign.

I'm talking about Algol, a star in the constellation Perseus that now appears high in our northeastern sky after dark. To ancient sky watchers, this star represented the hideous monster Medusa held by Perseus, and they believed it to be the most dangerous in all the heavens.

The name "Al Ghul" means a mischief-maker. The Hebrews knew it as Rosh ha Satan, Satan's Head, and the Chinese gave it the gruesome title Tseih She, the Piled-Up Corpses.

Just lovely!

Modern astronomers are also interested in Algol – aka Beta Persei – but for very different reasons. What we find interesting is that the star doesn't shine with a steady light but changes its brightness over time. In fact, we can watch the star dim noticeably for about five hours every 2.86 days, and then brighten once again.

Why this should occur is what makes the star so fascinating. Algol is one of a class of what astronomers call eclipsing binaries. Here, two stars orbit a common center of gravity and, since their orbits lie within the plane of sightline, they appear to eclipse each other every few days.

Because Algol lies some 93 light-years from us, even large telescopes can't see both stars. Instead, astronomers watch its variable brightness over time (its "light curve") to deduce the system's true nature.

This is how we've learned that one star is about three times larger than the sun, and the other is about 20% larger than the sun. Very precise observations have shown that there's a third star nearby and orbiting every 1.86 years.

This week you may want to check Algol out with your own eyes. Use the sky image I've provided to find it high in the northeastern sky after dark this week.

Begin by finding the pattern of stars that form the letter "W" or "M" on its side. This is part of the constellation Cassiopeia, the queen. Below and to its right you'll find the constellation Perseus.

During the coming week, Algol will reach its dimmest several times when stargazers can be watching. For example, it will reach its minimum on Sunday, Nov. 19 at around 2:01 EST (11:01 p.m. PST on Nov. 18) and again on Tuesday, Nov. 21 at around 10:50 p.m. EST (7:50 p.m. PST).

I suggest you check Algol for a few hours before and after these times and compare its brightness to that of nearby stars. From its brightness changes, you should be able to tell when it's entering or emerging from an eclipse.

Keep in mind, however, that this week the moon will be fairly bright, and Algol may be tough to spot without binoculars. Instead, you may want to wait until the moon is out of the sky. You can learn when future minima will occur by visiting From here, click on "Tools" (at the top) and on "Minima of Algol" in the drop-down menu that appears.