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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 brightness change. 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 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 June 2019 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 eclipse dates and times for June 2019

DateTime (UT-GMT)Time (BST-UK)Time (CEST-Europe)Time (EDT-N America)Time (PDT-N America)Time (CST-China)Time (AEST-Australia)
Jun 118:3619:3620:3614:3611:3602:36 (Jun 2)04:36 (Jun 2)
Jun 415:2516:2517:2511:2508:2523:2501:25 (Jun 5)
Jun 712:1413:1414:1408:1405:1420:1422:14
Jun 1009:0310:0311:0305:0302:0317:0319:03
Jun 1305:5206:5207:5201:5222:52 (Jun 12)13:5215:52
Jun 1602:4103:4104:4122:41 (Jun 15)19:41 (Jun 15)10:4112:41
Jun 1823:3000:30 (Jun 19)01:30 (Jun 19)19:3016:3007:30 (Jun 19)09:30 (Jun 19)
Jun 2120:1921:1922:1916:1913:1904:19 (Jun 22)06:19 (Jun 22)
Jun 2417:0818:0819:0813:0810:0801:08 (Jun 25)03:08 (Jun 25)
Jun 2713:5614:5615:5609:5606:5621:5623:56
Jun 3010:4511:4512:4506:4503:4518:4520:45