Algol A
 Location of β Persei (circled) |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Perseus |
| Right ascension | 03h 08m 10.13245s |
| Declination | +40° 57′ 20.3280″ |
| Apparent magnitude (V) | 2.12 (- 3.39) |
| Characteristics | |
| Spectral type |
Aa1: B8V Aa2: K0IV Ab: A7m |
| U−B colour index | −0.37 |
| B−V colour index | −0.05 |
| Variable type | EA/SD |
| Astrometry | |
| Radial velocity (Rv) | 3.7 km/s |
| Proper motion (μ) |
RA: 2.99 mas/yr Dec.: −1.66 mas/yr |
| Parallax (π) | 36.27 ± 1.40mas |
| Distance | 90 ± 3 ly (28 ± 1 pc) |
| β Per Aa1 | |
| Absolute magnitude (MV) | −0.07 |
| β Per Aa2 | |
| Absolute magnitude (MV) | 2.9 |
| β Per Ab | |
| Absolute magnitude (MV) | 2.3 |
| Orbit | |
| Primary | β Per Aa1 |
| Companion | β Per Aa2 |
| Period (P) | 2.867328 days |
| Semi-major axis (a) | 0.00215″ |
| Eccentricity (e) | 0 |
| Inclination (i) | 98.70° |
| Longitude of the node (Ω) | 43.43° |
| Orbit | |
| Primary | β Per A |
| Companion | β Per B |
| Period (P) | 680.168 days |
| Semi-major axis (a) | 0.09343″ |
| Eccentricity (e) | 0.227 |
| Inclination (i) | 83.66° |
| Longitude of the node (Ω) | 132.66° |
| Periastron epoch (T) | 2446927.22 |
|
Argument of periastron (ω) (primary) |
310.02° |
| Details | |
| β Per Aa1 | |
| Mass | 3.17 ± 0.21 M☉ |
| Radius | 2.73 ± 0.20 R☉ |
| Luminosity | 182 L☉ |
| Surface gravity (log g) | 4.0 cgs |
| Temperature | 13,000 K |
| Rotational velocity (v sin i) | 49 km/s |
| Age | 570 Myr |
| β Per Aa2 | |
| Mass | 0.70 ± 0.08 M☉ |
| Radius | 3.48 ± 0.28 R☉ |
| Luminosity | 6.92 L☉ |
| Surface gravity (log g) | 3.5 cgs |
| Temperature | 4,500 K |
| β Per Ab | |
| Mass | 1.76 ± 0.15 M☉ |
| Radius | 1.73 ± 0.33 R☉ |
| Luminosity | 10.0 L☉ |
| Surface gravity (log g) | 4.5 cgs |
| Temperature | 7,500 K |
| Other designations | |
| Database references | |
| SIMBAD | data |
| The constellation Perseus and Algol, the Bright Star in the Gorgon's head | |
|---|---|
Johannes Hevelius, Uranographia, 1690
|
Algol, designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation of Perseus. It is the first and best known eclipsing binary, and one of the first non-nova variable stars to be discovered. It is a three-star system, consisting of Beta Persei Aa1, Aa2, and Ab - in which the large and bright primary β Persei Aa1 is regularly eclipsed by the dimmer β Persei Aa2. Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2.86 days (2 days, 20 hours and 49 minutes) during the roughly 10-hour-long partial eclipses. There is also a secondary eclipse (the "second minimum") when the brighter star occults the fainter secondary. This secondary eclipse can only be detected photoelectrically.
Algol gives its name to its class of eclipsing variable, known as Algol variables.
An Ancient Egyptian Calendar of Lucky and Unlucky Days composed some 3,200 years ago is claimed to be the oldest historical document of the discovery of Algol.
The association of Algol with a demon-like creature (Gorgon in the Greek tradition, ghoul in the Arabic tradition) suggests that its variability was known long before the 17th century, but except for the Ancient Egyptian discovery there is still no indisputable evidence for this.
The variability of Algol was noted in 1667 by Italian astronomer Geminiano Montanari, but the periodic nature of its variations in brightness was not recognized until more than a century later, when the British amateur astronomer John Goodricke also proposed a mechanism for the star's variability. In May 1783, he presented his findings to the Royal Society, suggesting that the periodic variability was caused by a dark body passing in front of the star (or else that the star itself has a darker region that is periodically turned toward the Earth). For his report he was awarded the Copley Medal.
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