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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 mag. +2.1 but every two days, 20 hours and 49 minutes it suddenly dips in brightness to mag. +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) with the orbital plane of Algol A and B directly in 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, resulting in a very small dip in brightness that can be detected with photo-electrical equipment.

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

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

Finder Chart for Algol (credit:- freestarcharts)

Finder Chart for Algol - pdf format (credit:- freestarcharts)

For those new to variable star observing, Algol is an excellent first choice target. It dims quite dramatically and the regular brightness changes are obvious to the naked eye. The table below contains date and time information of Algol's minima for July 2017 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 observer's local time.

Algol Minima for July 2017

DateTime (UT-GMT)Time (BST-UK)Time (CEST-Europe)Time (EDT-N America)Time (PDT-N America)Time (CST-China)Time (AEST-Australia)
Jul 0203:4004:4005:4023:40 (Jul 1)20:40 (Jul 1)11:4013:40
Jul 0500:2801:2802:2820:28 (Jul 4)17:28 (Jul 4)08:2810:28
Jul 0721:1722:1723:1717:1714:1705:17 (Jul 8)07:17 (Jul 8)
Jul 1018:0619:0620:0614:0611:0602:06 (Jul 11)04:06 (Jul 11)
Jul 1314:5515:5516:5510:5507:5522:5500:55 (Jul 14)
Jul 1611:4312:4313:4307:4304:4319:4321:43
Jul 1908:3209:3210:3204:3201:3216:3218:32
Jul 2205:2106:2107:2101:2122:21 (Jul 21)13:2115:21
Jul 2502:1003:1004:1022:10 (Jul 24)19:10 (Jul 24)10:1012:10
Jul 2722:5823:5800:58 (Jul 28)18:5815:5806:58 (Jul 28)08:58 (Jul 28)
Jul 3019:4720:4721:4715:4712:4703:47 (Jul 31)05:47 (Jul 31)