ALGOL (Beta Persei). Surely one of the most remarkable stars of the sky and appropriately one of the most famed, Algol is the second magnitude Beta star of Perseus, the great mythological hero who rescued Andromeda from Cetus the Sea Monster. The Arabic name, "al Ghul" (related to our word "ghoul"), means "the demon," from a longer phrase that refers to the demon's head. In Greek mythology, Algol represents the Medusa's head with which Perseus turned Cetus to stone, the star considered an "unlucky" one for centuries. To the eye, this class B (B8) star appears rather normal, its slightly bluish white light radiating from a surface with a temperature of 12,500 K. Like the Sun, it is a main sequence dwarf star fusing hydrogen in its core, though it is 3.5 times more massive. From its distance of 90 light years (give or take 4; second Hipparcos reduction), we calculate a visual luminosity about 95 times that of the Sun, raised to 170 times if we factor in the invisible ultraviolet light radiated by the hot surface. Steady observation, however, reveals a surprise. As regular as clockwork, every 2.867... days, the brightness of the star plummets from mid second magnitude (2.1) to the dim end of third (3.4, just 30 percent of normal), the whole event (including recovery) taking only a few hours.
The brightest star on the left-hand side of each picture is Mirfak, Alpha Persei, while the brightest on the 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 star immediately to the right of Algol, Rho Persei is also variable. See full full resolution. Copyright J. B. Kaler.
Though the variation was discovered in 1667, it was probably known long before that and is probably the reason for the star's bad reputation. The cause of the sudden drop is a stellar eclipse. Algol is a close double star whose components orbit each other every 2.867... days. The companion to the visually observed star is a much dimmer yellow-orange class K giant star with a temperature of 4500 Kelvin and a luminosity 4.5 solar, just 2.5 percent that of the class B star. (The uncertain class of the faint star ranges from G5 to K2, from subgiant to giant. For simplicity, let's call it the "K giant.") The B star, at 2.9 solar radii, is smaller than the K giant (3.5 solar). Each orbit, when the dimmer, larger K star passes in front of the brighter B star, we see a deep eclipse. The eclipse is only partial, some of the light of the principal component still shining brightly through. Between the deep "primary" eclipses is a smaller dip when the bright star passes partially in front of the dim one.
The eclipses of Algol were first accurately recorded photoelectrically around 1910 with the historic 12-inch refractor at the University of Illinois Observatory. The magnitude scale does not reflect actual visual magnitudes. The primary eclipse at left occurs when the bright class B star partially hides behind the cool K star. The brightness of the system then changes slightly out of eclipse because of the tidal distortion of the stars and the reflection of the B star's light from the K star. Thirty hours after the primary eclipse, the smaller B star passes in front of the K star and cuts out a portion of its light to create the secondary eclipse at center. Light curve by Joel Stebbins in the Astrophysical Journal, vol. 32, p. 185, 1910.
Algol is famed first as a prototype of the class of eclipsing double stars, of which thousands are known. They are among the most important kinds of stars, as they provide us with information on stellar masses and dimensions. But Algol is equally famed for the "Algol paradox." The higher the mass of a star, the shorter its lifetime, as its fuel is used so much faster. The companion to Algol is the dying giant star. Yet carrying but 0.81 solar masses, it is the LESS massive of the two (the B star weighing in at 3.7 solar). The only explanation is that the dim companion has lost a great deal of mass. The two stars are so close together, separated by only five percent the distance between the Earth and the Sun, that the brighter smaller star produces tides in the larger one. Matter then 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, the effect directly observed through the stellar spectrum as the K giant is being stripped nearly to its core. A third member of the system, Algol C, a class A or F star of 1.8 solar masses, orbits about 3 Astronomical Units away with a period around the inner pair of 1.86 years. The system is a source of X-rays, though whether they come from a corona around one of the stars or from the flow of matter hitting the B star is uncertain. Algol is no demon at all, but a true friend, teaching us how stars interact and die, the effects of which you can see from your own backyard with no telescope at all. (Algol is featured in Jim Kaler's The Hundred Greatest Stars.)
Written by Jim Kaler 11/6/98; revised 11/12/04. Return to STARS.