Beta Persei (Algol, the "Demon Star")
November 2009 :
Are you ready for the eclipse of November 13th? I’m not talking about the sun or moon. I’m referring instead to an eclipse of the fascinating star ß Persei (Algol).
Algol is arguably the best-known example of an eclipsing binary. Every 2.867 days like clockwork, Algol dims from magnitude 2.1 to 3.4. The entire fade-away and return to normal brightness takes about 10 hours. Algol’s variability was first described by Italian astronomer Geminiano Montanari in 1667. However, its Arabic name (from Al Ra’s al Ghul “The Demon’s Head”) suggests that Algol’s odd behavior was noted centuries earlier.
Algol is comprised of a bright B8 main-sequence star orbited so closely by a fainter K-type subgiant that the two appear as a single star. Because their orbital plane is nearly edge-on to our line-of-sight, the faint member periodically passes in front of the primary, the eclipse causing a temporary dimming of the system’s light.
There are two windows of opportunity for viewing an Algol eclipse. First, you’ll need an evening from mid autumn to late winter when Perseus is well-placed in the sky. Next (unless you’re a night owl who doesn’t mind being out during the wee hours of evening) you’ll want an eclipse that begins after sunset and winds down around midnight.
According to the RASC Observer’s Handbook 2009, a favorable Algol eclipse will occur on Friday, November 13th, with mid-eclipse predicted for 8:21 pm, EST. Although the complete event takes about ten hours, most of the action can be seen within a 6-hour span. Starting about 3 hours before mid-eclipse (around 5:20 pm, or as soon as darkness permits), record your initial magnitude estimate. Use the accompanying chart, which shows the magnitudes of nearby comparison stars (to the nearest tenth, with decimals omitted). Continue at 15-minute intervals until Algol has returned to its original brightness. Special equipment won’t be necessary – Algol is readily visible to the unaided eye. One hint: go outside an evening or two before the eclipse to identify Algol and its comparison stars. You’ll avoid a lot of confusion and wasted time on eclipse night. Observing an eclipse of Algol is a great group project for an astronomy club. I took part in one a few years ago with members of the Boston ATMs. Between estimates we had time to conduct regular skygazing through our telescopes – a combination which made for a fun and fast-paced evening. Should clouds prevail on the 13th, you can scout out future Algol eclipses by consulting the Observer’s Handbook or a current issue of Sky and Telescope. Observing and recording an eclipse of Algol should be on every backyard astronomer’s “to-do” list.
Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.
