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Algol (beta Persei), the Eclipsing Binary

Algol (beta Persei) is the brightest eclipsing binary in the sky, and the most famous of the eclipsing variable stars. Algol means "Demon Star" in Arabic, and this suggests that its strange variability might have been known in antiquity, although there is no concrete evidence to support this conjecture. The name is from the Arabic Al Ra's al Ghul, which translates to "The Demon's Head." The Hebrews called the star "Satan's Head." In some other traditions, it is identified with the mysterious and sinister Lilith, the legendary first wife of Adam. Medieval astrologers considered Algol the most dangerous and unlucky star in the heavens.

Although Algol's name suggests that its light changes were known to the medieval Arabs, the first written account was made by the Italian astronomer Geminiano Montanari of Bologna in 1667. The English astronomer John Goodricke is credited with establishing the period of Algol in 1782. Goodricke proposed that the variation in Algol's brightness was due to its being eclipsed by an unseen companion, possibly a planet. In 1881, Edward Pickering, the Director of Harvard College Observatory and one of the founders of AAVSO, presented evidence which showed that Algol was an eclipsing binary star. One peculiar feature of the Algol system, shared by other binaries of the same type, is that the fainter and less massive star has evolved to the subgiant stage, while the primary star remains a main sequence object. This is a stellar evolutionary paradox, for if the stars are of the same age, the brighter and more massive star should evolve more rapidly. Binary stars form together from the same condensing cloud of gas and dust, and therefore have to be the same age. Astronomer Fred Hoyle suggested the following solution to the dilemma. The fainter star was originally the more massive and luminous of the pair. As it began its evolutionary expansion it lost great quantities of matter to the close companion. It thus became fainter as it evolved to the subgiant stage. At the same time the companion grew more brilliant as the result of its increased mass. This is now considered to be the case. Although Algol is the most studied eclipsing binary, high-resolution spectroscopy has only recently begun to reveal the details of its behavior.

Algol is actually a three-star system 92 light-years away. The primary star is a bright B8 main-sequence star. The primary is eclipsed every 2.87 days by the secondary star, a larger, dimmer, less massive K2 subgiant with a very active surface covered with starspots. The K2 subgiant and the B8 primary are in a very close orbit. In the distance a tertiary F 1 main-sequence star orbits the binary pair every 1.86 years. Algol varies in magnitude from 2.1 at maximum to 3.4 at primary minimum, with a period of 2.87 days. The period is slowly lengthening due to the mass transfer of material between the two stars. The primary eclipse occurs when the fainter K2 secondary passes in front of the brighter B8 star, and lasts for -10 hours. To us, the eclipse is a partial one, due to the angle from which we observe it. There is also a shallow secondary eclipse when the B8 star passes in front of the K2 star. This can only be detected photoelectrically. The primary eclipse, however, can easily be detected with the unaided eye.

The K2 subgiant has expanded to fill its Roche lobe, a teardrop-shaped volume of space in which its gravity is strong enough to hold onto its loose atmospheric material. The tip of the teardrop shape points in the direction of the primary. As the K2 subgiant tries to expand further, a thin but powerful stream of gas spills from the point of the Roche lobe and crashes down onto the 138 primary star. A binary system such as this, in which one component has filled its Roche lobe and is transferring material to its companion, is called a semidetached binary. The speed of the stream of gas is 520 km/s when it slams into the 138 star. The stream of gas, now heated to 100,000K, strikes the 138 star's surface at a low angle and kicks a spray of gas forward and upward. This spray forms a variable, asymmetric accretion disk that circles the primary before settling onto the surface. The disk varies in size and shape, indicating that the gas stream varies also. The K2 star must overflow intermittently. If the 138 star were smaller, or if the stars' separation were wider, there would be room for the formation of a permanent, stable accretion disk. Instead, the surface of the B8 star gets in the way. Algol-type binaries with orbital periods greater than 5 or 6 days do have room to acquire permanent accretion disks, but Algol itself revolves in only 2.87 days.

Algol is a strong radio source. The radio emissions come from the hot corona, the layer of atmosphere directly above the photosphere surrounding the K2 star. The star probably rotates in step with its orbital period, generating a strong magnetic dynamo effect within the star, intense surface activity, and a strong radio-emitting corona. This was confirmed by very long baseline interferometry (VLBI), a method of simultaneously pointing several radio telescopes (widely separated by long distances) at an object. Radio astronomers also announced that the orbital plane of the close binary pair is oriented at a right angle to the orbital plane of the distant F 1 star, contrary to theories relating to the formation of multiple star systems. Another study has reported the opposite-that all three stars lie in the same orbital plane.

In September 1990, the second-brightest eclipsing binary was discovered, and it happens to be located in the same constellation. The star is 3rd magnitude gamma Persei. The eclipses of gamma Persei occur rarely-approximately every 14.67 years. The next eclipse is expected in April of 2005. However, the star will then be in superior conjunction with the Sun, and so will not be visible from Earth. (Objects are in superior conjunction when they are on the opposite side of the Sun from the Earth.)

Gamma Persei consists of a cool, giant G8 primary star in orbit with a hot, main-sequence A2 secondary. It is a composite-spectrum binary (also called a spectroscopic binary): spectroscopic analysis shows the presence of features from two different stars. The composite nature of the spectrum was recognized in 1897 by Antonia Maury at Harvard. Gamma Persei was resolved into its two components for the first time in 1973, and it was extensively analyzed in 1987. At this time it was predicted that the A2 star would pass behind the G8 star in the fall of 1990. The eclipse occurred on the evening of September 12th, and was recorded at several observatories. The secondary star "set" more or less vertically behind the giant star's limb, so the eclipse was central, or behind the middle of the G8 star, and lasted for an entire week. The eclipse was 0.3 magnitude deep visually, so it was detectable-though certainly not conspicuous-with the unaided eye. Gamma Persei will not eclipse again for unaided-eye observers until November 2019.