Algol
| 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) | 34.7±0.6 mas |
| Distance | 94 ± 2 ly (28.8 ± 0.5 pc) |
| A or Aa1 | |
| Absolute magnitude (MV) | −0.07 |
| B or Aa2 | |
| Absolute magnitude (MV) | 2.9 |
| C or Ab | |
| Absolute magnitude (MV) | 2.3 |
| Orbit | |
| Primary | A or Aa1 |
| Companion | B or 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 | Aa |
| Companion | Ab |
| 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 | |
| A or 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 |
| B or 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 |
| C or 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 | |
| Algol, Gorgona, Gorgonea Prima, Demon Star, El Ghoul, β Persei, β Per, 26 Persei, BD+40°673, FK5 111, GC 3733, HD 19356, HIP 14576, HR 936, PPM 45864, SAO 38592. | |
| Database references | |
| SIMBAD | data |
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.
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.