From the November 2006 issue

Glenn Chaple’s observing basics: A view to an eclipse

November 2006: Examine Algol, Perseus' winking mystery star.
By | Published: November 1, 2006 | Last updated on May 18, 2023
Here’s a little astronomical riddle for you: Which star in the late-autumn sky winks at us once every 2 days, 20 hours, and 49 minutes? Each wink lasts about 10 hours. If you identified the mystery star as the eclipsing binary Algol (Beta [β] Persei), you’re right!

In its parent constellation, Algol represents the head of the mythical Medusa, a hideous snake-haired creature slain by Perseus. “Algol” comes from the Arabic Ra’s al Ghul, the “demon’s head.” Such sinister connotations suggest the ancients were aware of this star’s odd behavior and regarded it with suspicion. However, no evidence exists anyone officially described Algol’s variability until the late 17th century, when Italian astronomer Geminiano Montanari reported its brightness changes.

Perseus map
Use this finder chart to locate Algol in the night sky.
Astronomy: Roen Kelly
From 1782 to 1783, young English astronomer John Goodricke observed this star and noted the fading occurred at regular intervals. He surmised a dark, orbiting companion partially eclipsed Algol. A planet, he wondered? Not quite.

A century later, spectroscopic studies showed that Algol is a close, unequally bright binary system whose orbital plane is nearly edge-on to our line of sight. As a result, the fainter star periodically passes in front of the brighter one and produces an eclipse that dims the system’s brightness.

Even though we can observe an eclipse of Algol with the unaided eye (the star fades from a maximum magnitude of 2.1 to a minimum of 3.4), few observers choose to view the event after they learn it lasts 10 hours. At a meeting of the American Association of Variable Star Observers (AAVSO) a few years ago, I chatted with Marvin Baldwin and Gerard Samolyk — two renowned eclipsing-binary observers. I sheepishly admitted that, although I had studied a fair number of eclipsing variable stars, I had yet to observe an eclipse of Algol. Marv and Gerry pointed out that I didn’t need to observe Algol for the full 10 hours because most of the “action” happens during a 6-hour span centered around mid-eclipse. Encouraged by their suggestions, I made plans to view Algol’s next favorable eclipse.

Beginning 3 hours before the predicted time of mid-eclipse, I compared Algol to nearby stars of known magnitude to get a brightness estimate. I recorded the date, time, and magnitude in my observing log. Every 15 minutes, I made a follow-up observation. At first, the changes were subtle, but as mid-eclipse approached, Algol faded precipitously. The return was just as rapid, and by the end of the 6-hour run, Algol had regained its former brightness.

If you’re not inclined to commit to a 6-hour observing run, try giving Algol a “quick look.” The evening before a scheduled eclipse, estimate its brightness. At the time of mid-eclipse the next night, give Algol another look. You’ll be surprised how much fainter it appears.

Questions, comments, or suggestions? E-mail me here.