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ALGOL (Beta Persei). Surely one of the most remarkable stars ofthe sky and appropriately one of the most famed, Algol is thesecond magnitude Beta star ofPerseus, thegreat mythological hero who rescuedAndromeda fromCetus the SeaMonster. The Arabic name, "al Ghul" (related to our word "ghoul"),means "the demon," from a longer phrase that refers to the demon'shead. In Greek mythology, Algol represents the Medusa's head withwhich Perseus turned Cetus to stone, the star considered an"unlucky" one for centuries. To the eye, this class B (B8) starappears rather normal, its slightly bluish white light radiatingfrom a surface with a temperature of 12,500 K. Like theSun, it is a main sequence dwarf star fusinghydrogen in its core, though it is 3.5 times more massive. Fromits distance of 90 light years (give or take 4; second Hipparcos reduction), we calculate a visual luminosityabout 95 times that of the Sun, raised to 170 times if we factorin the invisible ultraviolet light radiated by the hot surface. Steady observation, however, reveals a surprise. As regular asclockwork, every 2.867... days, the brightness of the star plummetsfrom mid second magnitude (2.1) to the dim end of third (3.4, just30 percent of normal), the whole event (including recovery) takingonly a few hours.

The brightest star on the left-hand side of each picture isMirfak, Alpha Persei, while the brightest onthe right-hand side is Algol, Beta Per. In the reference picture at left,Algol is in its uneclipsed state and nearly as bright at Mirfak. In picture at right, taken the night of October 9, 2004, Algol is in mid-eclipse, and notably dimmed. Compare with other stars in the field as well. The starimmediately to the right of Algol,RhoPersei is also variable. See fullfull resolution.Copyright J. B. Kaler.

Though the variation was discovered in 1667, it was probably knownlong before that and is probably the reason for the star's badreputation. The cause of the sudden drop is a stellar eclipse. Algol is a close double star whose components orbit each otherevery 2.867... days. The companion to the visually observed staris a much dimmer yellow-orange class K giant star with atemperature of 4500 Kelvin and a luminosity 4.5 solar, just 2.5percent that of the class B star. (The uncertain class of thefaint star ranges from G5 to K2, from subgiant to giant. Forsimplicity, let's call it the "K giant.") The B star, at 2.9 solarradii, is smaller than the K giant (3.5 solar). Each orbit, whenthe dimmer, larger K star passes in front of the brighter B star,we see a deep eclipse. The eclipse is only partial, some of thelight of the principal component still shining brightly through. Between the deep "primary" eclipses is a smaller dip when thebright star passes partially in front of the dim one.

The eclipses of Algol were first accurately recordedphotoelectrically around 1910 with the historic 12-inch refractor at the University of Illinois Observatory. The magnitude scaledoes not reflect actual visual magnitudes. The primary eclipse atleft occurs when the bright class B star partially hides behind thecool K star. The brightness of the system then changes slightlyout of eclipse because of the tidal distortion of the stars and thereflection of the B star's light from the K star. Thirty hoursafter the primary eclipse, the smaller B star passes in front ofthe K star and cuts out a portion of its light to create thesecondary eclipse at center. Light curve by Joel Stebbins in theAstrophysical Journal, vol. 32, p. 185, 1910.

Algol is famed first as a prototype of the class ofeclipsing double stars, ofwhich thousands are known. They are among the most important kindsof stars, as they provide us with information on stellar masses anddimensions. But Algol is equally famed for the "Algol paradox." The higher the mass of a star, the shorter its lifetime, as itsfuel is used so much faster. The companion to Algol is the dyinggiant star. Yet carrying but 0.81 solar masses, it is the LESSmassive of the two (the B star weighing in at 3.7 solar). The onlyexplanation is that the dim companion has lost a great deal ofmass. The two stars are so close together, separated by only fivepercent the distance between the Earth and the Sun, that thebrighter smaller star produces tides in the larger one. Matterthen flows in from the large one (at a rate of around two hundred-millionths of a solar mass per year) to the small bright one, theeffect directly observed through the stellar spectrum as the Kgiant is being stripped nearly to its core. A third member of thesystem, Algol C, a class A or F star of 1.8 solar masses, orbitsabout 3 Astronomical Units away with a period around the inner pairof 1.86 years. The system is a source of X-rays, though whetherthey come from a corona around one of the stars or from the flow ofmatter hitting the B star is uncertain. Algol is no demon at all,but a true friend, teaching us how stars interact and die, theeffects of which you can see from your own backyard with notelescope at all. (Algol is featured in Jim Kaler'sThe HundredGreatest Stars.)

Written byJim Kaler 11/6/98; revised 11/12/04. Return toSTARS.

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