Algol ,[11] 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.
Quick Facts Observation data Epoch J2000 Equinox J2000, Constellation ...
Algol
Location of β Persei (circled) |
Observation data Epoch J2000 Equinox J2000 |
Constellation |
Perseus |
Right ascension |
03h 08m 10.13245s[1] |
Declination |
+40° 57′ 20.3280″[1] |
Apparent magnitude (V) |
2.12[2] (- 3.39[3]) |
Characteristics |
Spectral type |
Aa1: B8V[4] Aa2: K0IV[4] Ab: F1V[5] (kA4hA9.5mF0:[6]) |
U−B colour index |
−0.37[2] |
B−V colour index |
−0.05[2] |
Variable type |
EA/SD[3] |
Astrometry |
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Radial velocity (Rv) | 3.7 km/s |
Proper motion (μ) | RA: 2.99[1] mas/yr Dec.: −1.66[1] mas/yr |
Parallax (π) | 36.27 ± 1.40 mas[1] |
Distance | 90 ± 3 ly (28 ± 1 pc) |
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β Per Aa1 |
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Absolute magnitude (MV) | −0.07[7] |
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β Per Aa2 |
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Absolute magnitude (MV) | 2.9[7] |
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β Per Ab |
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Absolute magnitude (MV) | 2.3[7] |
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Orbit[8] |
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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[8] |
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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 |
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β Per Aa1 |
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Mass | 3.17 ± 0.21[8] M☉ |
Radius | 2.73 ± 0.20[8] R☉ |
Luminosity | 182[7] L☉ |
Surface gravity (log g) | 4.0[9] cgs |
Temperature | 13,000[9] K |
Rotational velocity (v sin i) | 49[10] km/s |
Age | 570[7] Myr |
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β Per Aa2 |
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Mass | 0.70±0.08[8] M☉ |
Radius | 3.48±0.28[8] R☉ |
Luminosity | 6.92[7] L☉ |
Surface gravity (log g) | 3.5[9] cgs |
Temperature | 4,500[9] K |
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β Per Ab |
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Mass | 1.76±0.15[8] M☉ |
Radius | 1.73±0.33[8] R☉ |
Luminosity | 10.0[7] L☉ |
Surface gravity (log g) | 4.5[9] cgs |
Temperature | 7,500[9] K |
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Other designations |
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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 |
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SIMBAD | data |
Close
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.[12]
Algol gives its name to its class of eclipsing variable, known as Algol variables.