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Position Beta.png
Observational Data
Astronomical designation Beta Persei
Right ascension 03h 08m 10.1315s
Declination +40o 57′ 20.332″
Constellation Perseus
Type of object Binary star
Magnitude Absolute Mag: -0.15
Apparent Mag: 2.12
Distance from Earth 93 ±2 ly
Radial Velocity 3.7 km/s
Proper Motion RA: 2.39 mas/yr
Dec.: -1.44 mas/yr
Parallax 35.14 ±0.90 mas

Algol, also known as Beta Persei, is a triple star system located 92.8 light years away in the constellation of Perseus. A star system that has been well known throughout human history, it is also the first and most well known eclipsing binary. Normally Algol's apparent magnitude is 2.1, but every 2 days, 20 hours, and 49 minutes, the magnitude drops to 3.4 for some 10 hours. This is due to Algol A (Beta Persei A) being regularly eclipsed from our point of view, by the dimmer Algol B (Beta Persei B).[1]


Algol in History

The Algol star system was known at least since antiquity. The Ancient Greeks called Algol the "Evil eye" of Medusa. This was most likely due to the regular occurrences of the change in brightness and color as Beta Persei B eclipsed Beta Persei A. The name Algol itself is derived from the Arabic term أس الغول ra's al-ghūl, meaning the Demon's Head or the Head of the Ogre. The name likely stems from the Greek view of the star system, specifically Ptolemy, who named it as τῶν ἐν γοργονίῳº ὁ λαμπρός, meaning "the bright one of those in the Gorgon's head". The names "Blinking Demon" and "Demon Star" are English translations of the Arabic name.[2]

In Hebrew legend, the star was seen as Rōsh ha Sāṭān or 'Satan's head'. It was also linked with the mythical Lilith. In Latin it was called Caput Larvae, or 'Spectre's Head'.[3] Medieval Europeans saw Algol as one of the 15 Behenian stars used for astrology.[4] The Chinese named the star 大陵五, meaning the Fifth Star of the Mausoleum, although they also named it 叠尸 or Tseih She, meaning 'Piled up corpses'.[3]

The first known recording of Algol's regular variation in its magnitude was in 1667 by Geminiano Montanari. In May 1783, John Goodricke presented his findings on the proposed cause of Algol's variability to the Royal Society. He believed a dark body regularly passed in front of the star, or that the star itself had some dark spot.[5] It wasn't until 1881, that Edward Pickering, an astronomer from Harvard, brought forth evidence that Algol was an eclipsing binary star system.[6] This was confirmed in 1889 by Hermann Carl Vogel, who discovered periodic Doppler shifts in the spectrum of Algol A and the eclipsing Algol B.[7]

Star System

Algol is a triple star system. The first two stars make up the famous eclipsing binary and are separated from each other by a distance of only 0.062 AU. The orbit of the two stars is highly circular and take only 2.87 days to complete. From Earth, the orbit is inclined at 97.69°.[8] A third star orbits the first two at the distance of 2.69 AU, taking some 1.86 years to complete with an inclination of 83.98°.[9]

Beta Persei A

Algol A (Beta Persei A) is a blue white main sequence dwarf star of spectral type B5-8 V, and the most massive of the triple star system. The star has a diameter 2.88 times that of our Sun with a mass 3.59 times as great.[10] The star is considerably brighter than the Sun. Its visual luminosity is 98 times of the Sun, and the total luminosity though is some 200 times as great, accounting for ultraviolet radiation. Algol A rotates at a quick 65 km/sec and could possibly be less then 300 million years old.

Beta Persei B

Algol B (Beta Persei B) is a orange-red subgiant star of spectral class K0-2 IV. Once the most massive of the three stars of the system, it has become a cool, low-mass star that has distorted into a teardrop shape from the tidal forces caused by its companion, Algol A. The gas from Algol B itself is being drained slowly away in an accretion stream towards its close companion, now the more massive of the two stars. Algol B itself has around 79 percent of our Sun's mass and 3.54 times its diameter. As a subgiant, the star is also 3.4 times as luminous as our Sun.[10]

Beta Persei C

Algol C (Beta Persei C) is a blue white main sequence dwarf of spectral type A5 V. The star has 1.7 times our Sun's diameter and 1.67 times its mass. Its luminosity is around 4.1 times that of our Sun. Algol C orbits the other two stars with an average orbit of 2.69 AU.[10]

For an Earth-like world to have liquid water on its surface, it would have to be around 14 AU from the three stars, or the equivalent of an orbit between Saturn and Uranus. The hypothetical world would have an orbital period of over 27 years. Because the system itself is very young, it is unlikely such a planet would have time to cool to the point of holding water.


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