Algol
Algol /ˈælɡɒl/, 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 and one of the first non-nova variable stars to be discovered.
| 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: F1V (kA4hA9.5mF0:) |
| 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 ± 1.40 mas |
| 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 |
Algol is a three-star system, consisting of Beta Persei Aa1, Aa2, and Ab – in which the hot luminous primary β Persei Aa1 and the larger, but cooler and fainter, β Persei Aa2 regularly pass in front of each other, causing eclipses. Thus Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2.86 days during the roughly 10-hour-long partial eclipses. The secondary eclipse when the brighter primary star occults the fainter secondary is very shallow and can only be detected photoelectrically.
Algol gives its name to its class of eclipsing variable, known as Algol variables.