Minima of Algol
Description
Algol, designated Beta Persei (β Persei), is also known as the 'Demon Star' or 'Gorgona'. It is a triple star system some 90 light-years away in the constellation of Perseus visually shining in beautiful blueish and orange. The young English observer, John Goodricke (1764 - 1786) discovered Algol's steady variability between 1782 and 1783 and speculated that the cause of Algol's regular dimming are eclipses by a 'dark body', perhaps a planet. He also discovered the variation of δCephei and further stars.
Algol Aa1 (or A) and Algol Aa2 (or B) form an eclipsing binary 0.062AU (15% the distance Mercury to Sun) apart with a period of 2.87 days, while the third star, Algol Ab (or C), shines in white at an average distance of 2.69 AU from the pair orbiting with a period of 680.2 days or 1.86 years. The star system is visible with the naked eye at a maximum magnitude of 2.12 lowering to 3.39 during a central eclipse by Aa2. The event occurs every 2.867328 days (the orbital period of β Per Aa1 an β Per Aa2), lasting about 9 hours and 40 minutes, minimum light for about 20 minutes. The Algol system shines brightest (2.12) when no star obstructs another.
A primary eclipse occurs when the larger, but cooler and dimmer Algol Aa2 (or B) partially hides the smaller, hotter, brighter and more massive main sequence star Algol Aa1 (or A). A secondary eclipse occurs when Algol Aa1 (A) eclipses Algol Aa2 (B), but this event can only be detected photoelectrically.
The Algol system is strong source of x-ray and radio wave flares as a result of mass transfer and magnetic fields (10x solar), respectively.
About 7.3 million years ago, Algol passed within 9.8 light-years of our Sun at an apparent magnitude of −2.5, outshining today's Sirius. Currently, the system is moving away at a radial velocity of 3.7 km per second. If placed at a distance of 10 parsecs (32.6ly) the absolute magnitude of the primary star would be −0.07, comparable with Arcturus (-0.04) in Boötes.
Location on a star chart.
DSS-IR image, 1.5° wide (click for a telescope view).
Physical Comparison
| Mass | Radius | Luminosity | Temperature | Spectral | |
|---|---|---|---|---|---|
| Algol (A) Aa1 | 3.17 | 2.73 | 182 | 13,000°K | B8 |
| Algol (B) Aa2 | 0.70 | 3.48 | 6.92 | 4,500°K | K0 |
| Algol (C) Ab | 1.76 | 1.73 | 10 | 7,500°K | A7 |
Light Curve
Prediction of Minima
A selection of 'easy' binaries (visually bright, short period, high Δmagnitude)
* elements not yet confirmed Calculator based on Sky & Telescope, February 24, 2004
| Name | Type | Epoch | Period [d] | RA (2000) | DE (2000) | Dist [ly] | Vmax | Vmin |
|---|---|---|---|---|---|---|---|---|
| Algol (Beta Persei) | eclipsing binary (prototype star Algol) | 2445641.554 | 2.867324 | 03 08 10.13 | +40 57 20.3 | 93 | 2.12 | 3.39 |
| Beta Lyrae | eclipsing binary (Algol-type) | 2455435.1 | 12.93763 | 18 50 04.79 | +33 21 45.6 | 962 | 3.25 | 4.36 |
| Chi² Hydrae | eclipsing binary (Algol-type) | 2448501.9951 | 2.2677 | 11 05 57.55 | -27 17 16.1 | 690 | 5.64 | 5.92 |
| Delta Cephei | pulsating variable prototype star δ Cep | 2436075.445 | 5.366341 | 22 29 10.25 | +58 24 54.7 | 887 | 3.48 | 4.37 |
| Delta Librae | eclipsing binary (Algol-type) | 2434456.5426 | 2.32734906 | 15 00 58.39 | -08 31 08.2 | 294 | 4.91 | 5.4 |
| Eta Aquilae | pulsating variable (δ Cephei type) | 2436084.6563 | 7.176641 | 19 52 28.36 | +01 00 20.4 | 1400 | 3.48 | 4.39 |
| i(44) Boötis | W Ursae Majoris variable | 2455352.46344 | 0.2678185 | 15 03 49.06 | +47 38 18.1 | 42 | 5.8 | 6.4 |
| Lambda Tauri | eclipsing binary (Algol-type) | 2455178.081 | 3.9529478 | 04 00 40.82 | +12 29 25.3 | 483 | 3.37 | 3.91 |
| R Arae | eclipsing binary (Algol-type) | 2455347.1559 | 4.425135 | 16 39 44.73 | -56 59 39.9 | 400 | 6.17 | 7.32 |
| R Canis Majoris | eclipsing binary (Algol-type) | 2445391.234 | 1.13595 | 07 19 28.08 | -16 23 41.7 | 144 | 5.7 | 6.34 |
| RR Lyncis | eclipsing binary (Algol-type) | 2433153.8623 | 9.945079 | 06 26 25.84 | +56 17 06.4 | 270 | 5.52 | 6.03 |
| RZ Cassiopeiae | eclipsing binary (Algol-type) | 2439025.3025 | 1.1952499 | 02 48 55.51 | +69 38 03.1 | 204 | 6.18 | 7.72 |
| Sigma Aquilae | eclipsing binary (Algol-type) | 2422486.7975 | 1.95027 | 19 39 11.64 | +05 23 52.0 | 204 | 5.14 | 5.32 |
| U Cephei | eclipsing binary (Algol-type) | 2444541.6031 | 2.4930475 | 01 02 18.34 | +81 52 32.1 | 826 | 6.75 | 9.24 |
| U Sagittae | eclipsing binary (Algol-type) | 2440774.4638 | 3.380626 | 19 18 48.41 | +19 36 37.7 | 1013 | 6.45 | 9.28 |
| V1010 Ophiuchus | eclipsing binary (Algol-type) | 2455758.3547 | 0.66143 | 16 49 27.67 | -15 40 04.7 | 1240 | 6.1 | 7 |
| VZ Cassiopeiae | eclipsing binary (Algol-type) | 2451565.405 | 4.467224 | 00 45 39.11 | +74 59 17.3 | 340 | 5.78 | 6.12 |
| W Sagittarii | pulsating variable (δ Cephei type) | 2437678.578 | 7.594 | 18 05 01.22 | -29 34 48.3 | 1335 | 4.29 | 5.14 |
| Zeta Geminorum | pulsating variable (δ Cephei type) | 2436791.922 | 10.15082 | 07 04 06.54 | +20 34 13.1 | 1183 | 3.62 | 4.18 |
| μ Herculis | eclipsing binary (Algol-type) | 2448852.3679 | 2.051027 | 17 17 19.57 | +33 06 00.4 | 27 | 4.69 | 5.37 |
| μ¹ Scorpii | eclipsing binary (Algol-type) | 2449534.178 | 1.44627 | 16 51 52.23 | -38 02 50.6 | 294 | 2.89 | 3.15 |
