Beta Persei - Algol

Algol or Beta Persei, at magnitude 2.1, is usually the second-brightest star in Perseus. It is one of the best known eclipsing binaries, and the first such star to be discovered.

Mythology and Historical Observation

Perseus was the mythical Greek hero who used the Gorgon Medusa's severed head to turn the sea monster Cetus into stone. In the constellation Perseus, Algol represented the "evil eye" of Medusa to the ancient Greeks, probably because of its regularly changing brightness and color.

Known in English as the Demon Star, the name Algol derives from Arabic "ra's al-ghul," meaning "head of the ghoul", also probably given due to the star's peculiar behavior. In Hebrew folklore, Algol was known as "Rosh ha Satan", or "Satan's Head". A Latin term from the 16th century was Caput Larvae, "Spectre's Head". Algol is known as the Fifth Star of the Mausoleum in Chinese astronomy, and also bore the grim name "Tseih She", meaning "Piled up corpses".

The variability of Algol was first recorded in 1667 by Geminiano Montanari, but was noticed long before this time. The first person to propose a mechanism for Algol's variability was the British amateur astronomer John Goodricke. In May, 1783 he presented his findings to the Royal Society, suggesting that the periodic variability was caused by a dark body passing in front of the star (or that the star had a darker region that periodically turned toward the Earth.)

In 1881, the Harvard astronomer Edward Pickering presented evidence that Algol was actually an eclipsing binary. This was confirmed in 1889, when the Potsdam astronomer Hermann Carl Vogel found periodic Doppler shifts in Algol's spectrum, inferring variations in the system's radial velocity. Thus Algol became one of the first known spectroscopic binaries.

Variability and Orbit

Algol is actually a three-star system (designated β Per A, B and C) about 93 light years away. The large, bright primary (β Per A) is regularly eclipsed by the dimmer β Per B every 2 days, 20 hours and 49 minutes (2.86739 days). Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 during the roughly 10-hour long partial eclipses. There is also a secondary eclipse when the brighter star occults the fainter secondary, but this secondary eclipse can only be detected photoelectrically.

The eclipsing binary pair is separated by only 0.062 AU, with a near-circular orbit. The third star (β Per C) is at an average distance of 2.69 AU from the AB pair, with an orbital period of 681 days (1.86 years), and a moderately eccentric orbit (e=0.225). The total mass of the system is about 5.8 solar masses, and the mass ratios of A, B and C are about 4.5:1:2. Algol also has four optical line-of-sight companions.

Properties and Evolution

Algol A is a blue-white main sequence star of spectral class B5 V, with a surface temperature of 12,500 K. The star has 98 times the Sun's visual luminosity, or around 200 times including ultraviolet. It is 2.88 times the Sun's diameter, and contains 3.59 times as much mass. It has a fast rotational velocity of 65 km/sec, and is less than 300 million years old. The companion, β Per B, is an orange subgiant of spectral class K0 IV and a temperature of 4500 K. It has around 3.4 times the Sun's luminosity, 3.54 times its diameter, and around 79% of its mass. Resolved by speckle interferometry, β Per C is a white main-sequence star of spectral class A5 V. It has around 4.1 times the Sun's luminosity, 1.7 times its diameter, and 1.67 times its mass.

Tidal forces from β Per A have distorted the outer gas envelope of β Per B into a teardrop shape within its "Roche lobe" or gravitational limit, pulling out a stream that falls onto the more massive β Per A. The system also exhibits x-ray and radio flares, caused by the AB components' magnetic fields interacting with the mass transfer. The radio emissions may be created by magnetic cycles similar to sunspots, but more powerful and longer lasting, as the magnetic fields around these stars are up to ten times stronger than the Sun's.

Studies of Algol led to the "Algol paradox" in stellar evolution: both components of a binary star form at the same time, and massive stars evolve much faster than less massive ones. Yet the more massive A component is still in the main sequence, while the less massive B component is a subgiant at a later evolutionary stage. To solve this paradox, β Per B must have once been more massive than β Per A, and has transferred much of its original mass to A. β Per B is now losing mass to its brighter companion at the rate of up to 2.0 x 10-11 solar mass/year.

If β Per A swells up to become a red giant after B becomes a white dwarf, then A will overfill its own Roche lobe, and transfer mass to the white dwarf companion B. This could result in a cataclysmic nova explosion, or even a type-Ia supernova, if the white dwarf is loaded beyond 1.4 solar masses.

Motion

About 7.3 million years ago, Algol passed within 9.8 light years of Earth, and its apparent magnitude was approximately -2.5 - considerably brighter than Sirius is today. The total mass of the system is 5.8 solar masses, possibly enough to perturb our solar system's Oort cloud slightly, despite the fairly large distance at closest approach, and to increase the number of comets entering the inner solar system. However, the actual increase in the net cratering rate was probably quite small.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]