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Algol (β Per) is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star" most of the time it shines at magnitude +2.1 but every two days, 20 hours and 49 minutes the star suddenly dips in brightness to magnitude +3.4, remaining dim for about 10 hours before returning to its original state.

Why the change in brightness? The Algol system consists of at least three-stars (β Per A, β Per B and β Per C) and from our perspective the orbital plane of Algol A and Algol B is in the line with the Earth. The regular dips in brightness occur when the dimmer B star moves in front of and eclipses the brighter A star. There is also an extra dimension in that a secondary eclipse occurs when the brighter star occults the fainter secondary but this results in a very small dip in brightness that can only be detected photoelectrically.

Algol System (Dept. of Physics and Astronomy - Univ. of Tennessee at Knoxville)

Algol is located in Perseus amongst the stars of the northern Milky Way. It's positioned west of magnitude +0.1 star Capella (α Aur) and southeast of the "W" of Cassiopeia. The finder chart below shows the position of Algol along with magnitude data of some of the surrounding stars for comparison purposes.

Finder Chart for Algol (Beta Persei)

Finder Chart for Algol (Beta Persei) - pdf format

For those new to variable star observing, Algol is an excellent choice to focus on. It dims quite dramatically and the brightness changes are obvious to the naked eye. The table below contains date and time information of Algol's minima for September 2015 for various World time zones that cover most regions of Europe, America, China and Australia. For time zones not listed, local eclipse times can be calculated by adjusting the table to fit to the observers local time.

Algol Minima for September 2015

DateTime (UT)Time (BST - Europe)Time (CEST - Europe)Time (EDT - N America)Time (PDT - N America)Time (CST - China)Time (AEST - Australia)
Sep 0301:4602:4603:4621:46 (Sep 2)18:46 (Sep 2)09:4611:46
Sep 0522:3523:3500:35 (Sep 6)18:3515:3506:35 (Sep 6)08:35 (Sep 6)
Sep 0819:2420:2421:2415:2412:2403:24 (Sep 9)05:24 (Sep 9)
Sep 1116:1217:1218:1212:1209:1200:12 (Sep 12)02:12 (Sep 12)
Sep 1413:0114:0115:0109:0106:0121:0123:01
Sep 1709:5010:5011:5005:5002:5017:5019:50
Sep 2006:3807:3808:3802:3823:38(Sep 19)14:3816:38
Sep 2303:2704:2705:2723:27 (Sep 22)20:27 (Sep 22)11:2713:27
Sep 2600:1601:1602:1620:16 (Sep 25)17:16 (Sep 25)08:1610:16
Sep 2821:0422:0423:0417:0414:0405:04 (Sep 29)07:04 (Sep 29)