Algol

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
Radial velocity (Rv)	3.7 km/s
Proper motion (μ)	RA: 2.99[1] mas/yr Dec.: −1.66[1] mas/yr
Parallax (π)	34.7±0.6 mas[7]
Distance	94 ± 2 ly (28.8 ± 0.5 pc)[7]
β Per Aa1
Absolute magnitude (MV)	−0.07[8]
β Per Aa2
Absolute magnitude (MV)	2.9[8]
β Per Ab
Absolute magnitude (MV)	2.3[8]
Orbit[9]
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[9]
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[9] M☉
Radius	2.73 ± 0.20[9] R☉
Luminosity	182[8] L☉
Surface gravity (log g)	4.0[10] cgs
Temperature	13,000[10] K
Rotational velocity (v sin i)	49[11] km/s
Age	570[8] Myr
β Per Aa2
Mass	0.70±0.08[9] M☉
Radius	3.48±0.28[9] R☉
Luminosity	6.92[8] L☉
Surface gravity (log g)	3.5[10] cgs
Temperature	4,500[10] K
β Per Ab
Mass	1.76±0.15[9] M☉
Radius	1.73±0.33[9] R☉
Luminosity	10.0[8] L☉
Surface gravity (log g)	4.5[10] cgs
Temperature	7,500[10] 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/,^[12] 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.^[13]

Algol gives its name to its class of eclipsing variable, known as Algol variables.