Demon Star Has Its Eye On You Tonight

The sky facing east around 6:30 p.m. local time. Use the moon and bright, white Capella to draw a triangle to Algol. You can also use the Pleiades (Seven Sisters) cluster and Capella. You can watch Algol change in brightness by comparing it to Mirfak and its next-door-neighbor Rho Persei. At maximum, Algol shines a tad fainter than Mirfak; at minimum it nearly matches Rho. Source: Stellarium
The sky facing east around 6:30 p.m. local time. Use the moon and the bright, white star Capella to draw a triangle to Algol. You can also use the Pleiades (Seven Sisters) cluster and Capella. Algol’s change in brightness is easy to see by comparing it to Mirfak and nearby Rho Persei. At maximum, Algol’s a tad fainter than Mirfak; at minimum it nearly matches Rho. Source: Stellarium

Better look over your shoulder. Algol, the Demon Star, will be giving you a clever wink tonight. What’s it up to, you wonder? This evening (Nov. 24th) across North American time zones, the star will perform one its famous fades and recoveries.

Algol is really a very close double star with a hot blue-white primary star orbited by an orange giant. During an eclipse, the giant nearly covers the brighter star. Illustration: Bob King
Algol is a very close binary star with a hot blue-white primary star orbited by an fainter orange giant. During an eclipse, the giant nearly covers the brighter star. Illustration: Bob King

Algol’s an eclipsing binary star 93 light years from Earth in the constellation of Perseus just below the W of Cassiopeia. To the eye and through most telescopes it looks like a single star, but it’s really two stars in close orbit about each other. The demon reference comes from its Arabic name, al-ghul, translated as “head of the ogre”, which may refer to its unusual if not frightening behavior in the eyes of the superstitious. Few stars so dramatically brighten and fade over a few hours time.

This animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase. Because some phases are poorly covered, Aa2 jumps at some points along its path. Credit: Dr. Fabien Baron
This animation of Algol B going around Algol A was made from 55 images taken by the high-resolution CHARA interferometer in near-infrared light. CHARA is one of the few instruments that can see Algol as two separate stars. Credit: Dr. Fabien Baron

Every 2.9 days, the larger, dimmer star glides in front of the smaller, brighter one, blocking much of its light and causing the system to dim noticeably. The entire partial eclipse takes some 10 hours, but you needn’t invest that kind of time. For the eastern half of the U.S., Algol begins the evening at magnitude +2.1, its maximum brightness. Even with the nearby nearly full moon, you should have no problem seeing it at nightfall.

Over the next few hours, the star will gradually fade and bottom out at minimum at 9:14 p.m. CST (10:14 p.m. EST;  7:14 p.m. MST and 6:14 p.m. PST). That’s the time of mid-eclipse when it’s faintest at magnitude +3.4 or about five times fainter than it began. Eclipses last about two hours before the dimmer star begins to uncover the brighter and Algol returns to its normal brightness. During the entire time, Algol will look nearly as faint as during mid-eclipse.

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 - See more at: http://www.skyandtelescope.com/astronomy-blogs/behold-algol-star-secret/#sthash.liN7fwn2.dpuf
Here’s a more detailed look at Algol’s eclipses.  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. The light curve (left) shows a gradual then steep drop from max to min.
Mike Guidry / Univ. of Tennessee

The easiest way to see the eclipse is to catch Algol early on before the fade, then go out for another look anytime between 8:15 and 10:15 p.m. CST to revel in its remarkable transformation. If you stay up really late — or live on the West Coast where the eclipse happens much earlier in the evening — you can even watch the demon re-gather its strength and brighten back up.

Night sky photo shows Algol in relation to Capella and the Pleiades in November when facing east. Credit: Bob King
Night sky photo shows Algol in relation to Capella and the Pleiades in November when facing east. Credit: Bob King

If clouds put the kibosh on your ogre oogling, you’re in luck! We’re due for another eclipse on Friday Nov. 27th with minimum  occurring at 6:03 p.m. That evening, Algol will start out at minimum for most of North American and brighten up to normal several hours later. The next easily visible evening eclipse of Algol for the Americas won’t occur until Dec. 14th.

Here’s an interactive list of Algol eclipses, so you can be ready whenever the sky clears. For fun in the meantime, be sure to check out this eclipsing binary simulator where you can play to your heart’s content with all manner of Algol-type stars.