The Eclipsing Variable Star
Algol (Beta Persei)

Algol (Beta Persei) is the brightest eclipsing binary with deep eclipses. It is also the brightest and closest semi-detached binary, a type of binary system in which one component has filled its Roche Lobe (the volume within which gas is gravitationally bound to the star) and is now transferring material to its companion.

Algol means "Demon Star" in Arabic. This would tend to suggest that its strange variability may have been known in antiquity, though there is no concrete evidence to support this conjecture. The English astronomer John Goodricke (1764-1786) is credited with the discovery of the periodicity of Algol in 1782-83, though it was apparently discovered independently by a German farmer named Palitzch. Goodricke and his colleague Edward Pigott (1753?-1825) also proposed that the variability of Algol might be caused by eclipses - but by a planet revolving around Algol. It was Edward Pickering, Director of Harvard College Observatory and a founder of the American Association of Variable Star Observers (AAVSO) who, in 1881, presented convincing evidence that Algol was an eclipsing binary star.

According to the recent definitive study by M.T. Richards, S.W. Mochnacki and C.T. Bolton of the University of Toronto (Astronomical Journal, 96, 326 (1988)), the properties of the components of Algol are as follows: the primary is a B8V main sequence star (Note 1) with a mass of 3.7+/-0.3 suns, and a radius of 2.90+/-0.04 suns; the secondary is a K2IV subgiant with a mass of 0.81+/-0.05 suns, and a radius of 3.5+/-0.1 suns. The inclination of the orbit of these two stars to the sky is 81.4 degrees. Near-infrared photometry shows that the K2IV star is a spotted rotating variable star of the RS CVn type (RS Canum Venaticorum: Note 2). There is also a third component in wide orbit around the close pair: an A4m (Note 3) metallic-line star with a mass of 1.6+/-0.1 suns, and a radius of 1.4+/-0.1 suns. The system also contains circumstellar material lost from the K2IV component. Because of its brightness, Algol is potentially one of the most important star systems for studying mass transfer and mass loss in an interacting binary.

Algol varies in visual magnitude from 2.1 at maximum to 3.4 at primary minimum, with a period of 2.867315 days; this period, however, is slowly lengthening. The primary eclipse occurs when the fainter K2IV star passes in front of the brighter B8V star, and lasts for some 10 hours in total. Because the eclipse is partial, the minimum (of the light curve) is not flat, but rounded. There is also a shallow secondary eclipse when the B8V star passes in front of the K2IV star. It can only be detected photoelectrically. The primary eclipse, however, can easily be detected with the unaided eye, and the magnitude and time of minimum can be measured. Algol therefore provides a convenient and interesting introduction to variable star observing.

Long-term studies of the times of minimum of Algol have shown that the orbital period varies due to mass transfer and mass loss. For this reason, careful visual observation of Algol and other eclipsing binaries is useful to astronomers. The AAVSO has an active program of observation and analysis of eclipsing binaries, coordinated by Marvin E. Baldwin, who has written several excellent articles on the subject, including "Techniques for Visual Observation of Eclipsing Binary Stars", in Journal of the AAVSO, 4, 10 (1975) and "Observing Eclipsing Binaries", in Journal of the AAVSO, 21, 127 (1992). See also the articles on the bright eclipsing binary Beta Lyrae by John Isles in Sky and Telescope, 85, 72 (June 1993) and 87, 72 (June 1994). For more information about the AAVSO program, write to:

American Association of Variable Star Observing
25 Birch Street
Cambridge, Massachusetts 02138-1205

Note 2: RS Canum Venaticorum stars are rapidly rotating stars, usually close binary systems, which undergo small amplitude changes in light that may be due to dark or bright spots on their stellar surface. Eclipses may also be present in such systems.

Note 3: A4m (m indicates the star has strong metallic lines in its spectra).