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 a secondary eclipse 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 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 magnitudes of some of the surrounding stars for comparison purposes.
Finder Chart for Algol (Beta Persei) - pdf format
For those new to variable star observing, Algol is an excellent first choice. It dims quite dramatically and the brightness changes are obvious to the naked eye. The table below contains the dates and times of Algol's minima for August 2015 for various time zones.
Algol Minimums for August 2015
| Date | Time (UT) | Time (BST - Europe) | Time (CET - Europe) | Time (CEST - Europe) | Time (EDT - N America) | Time (EST - N America) | Time (PDT - N America) | Time (PST - N America) | Time (CST - China) | Time (AEST - Australia) |
|---|---|---|---|---|---|---|---|---|---|---|
| August 02 | 11:20 | 12:20 | 12:20 | 13:20 | 07:20 | 06:20 | 04:20 | 03:20 | 19:20 | 22:20 |
| August 05 | 08:09 | 09:09 | 09:09 | 10:09 | 04:09 | 03:09 | 01:09 | 00:09 | 16:09 | 19:09 |
| August 08 | 04:57 | 05:57 | 05:57 | 06:57 | 00:57 | 23:57 (Aug 7) | 21:57 (Aug 7) | 20:57 (Aug 7) | 12:57 | 15:57 |
| August 11 | 01:46 | 02:46 | 02:46 | 03:46 | 21:46 (Aug 10) | 20:46 (Aug 10) | 18:46 (Aug 10) | 17:46 (Aug 10) | 09:46 | 12:46 |
| August 13 | 22:35 | 23:35 | 23:35 | 00:35 (Aug 14) | 18:35 | 17:35 | 15:35 | 14:35 | 06:35 (Aug 14) | 09:35 (Aug 14) |
| August 16 | 19:23 | 20:23 | 20:23 | 21:23 | 15:23 | 14:23 | 12:23 | 11:23 | 03:23 (Aug 17) | 06:23 (Aug 17) |
| August 19 | 16:12 | 17:12 | 17:12 | 18:12 | 12:12 | 11:12 | 09:12 | 08:12 | 00:12 (Aug 20) | 03:12 (Aug 20) |
| August 22 | 13:00 | 14:00 | 14:00 | 15:00 | 09:00 | 08:00 | 06:00 | 05:00 | 21:00 | 00:00 (Aug 23) |
| August 25 | 09:49 | 10:49 | 10:49 | 11:49 | 05:49 | 04:49 | 02:49 | 01:49 | 17:49 | 20:49 |
| August 28 | 06:37 | 07:37 | 07:37 | 08:37 | 02:37 | 01:37 | 23:37 (Aug 27) | 22:37 (Aug 27) | 14:37 | 17:37 |
| August 31 | 03:26 | 04:26 | 04:26 | 05:26 | 23:26 (Aug 30) | 22:26 (Aug 30) | 20:26 (Aug 30) | 19:26 (Aug 30) | 11:26 | 14:26 |