| Right Ascension | 03 : 08 : 10.1 (h:m:s) |
|---|---|
| Declination | +40 : 57 : 21 (deg:m:s) |
| Distance | 100 (ly) |
| Visual brightness | 2.12 .. 3.40 (mag) |
| Period | 2.867 (days) |
| Spectral type | B8V+G5IV+Am |
Algol was discovered to be a variable star by Geminiano Montanari (1632-87) in 1667. John Goodricke (1764-86), in 1782, found that this star varied at an exactly regular period of 2 days, 20 hours, 48 minutes and 56 seconds. He was the first to explain this correctly by the assumption that this variability might be caused by a darker companion which eclipses the brighter star of a binary system. The binary nature of Algol was confirmed in 1889 by Hermann Carl Vogel (1841-1907) who found periodic Doppler shifts in the spectrum of Algol A (spectral class B8V), and the overlaying spectrum of the companion, Algol B, of type Am.
Algol A and B form a close binary system, the actual eclipsing binary, separated by only 10.4 million km. Algol A is a white main-sequence star, while Algol B is believed to be a subgiant. Spectroscopy revealed a third companion, Algol C, in orbit of about 80 million km distance to the pair AB. A fourth component, D, is suspected from spectroscopic investigations also.
Two visible companions of 12th magnitude were found by Sherburne Wesley Burnham (1838-1921) in 1878-9, who cataloged the system as Burnham 526; it is also ADS 2362.
The Algol system is about 100 ly distant, and is receding from us at about 4 km/s.