Algol

Beta Persei A/B/C

The position of Algol.
Observation data Epoch J2000      Equinox J2000
Constellation	Perseus
Right ascension	03h 08m 10.1315s[1]
Declination	+40° 57′ 20.332″[1]
Apparent magnitude (V)	2.12[1]
Characteristics
Spectral type	B8V (A)[1] /K02IV (B)[2] /A5V (C)
U-B color index	-0.37
B-V color index	-0.05
Variable type	Eclipsing binary
Astrometry
Radial velocity (Rv)	3.7 km/s
Proper motion (μ)	RA: 2.39 mas/yr Dec.: -1.44 mas/yr
Parallax (π)	35.14 ± 0.90 mas
Distance	93 ± 2 ly (28.5 ± 0.7 pc)
Absolute magnitude (MV)	-0.15
Details
Mass	3.59/0.79/1.67 M☉
Radius	2.3/3.0/0.9 R☉
Luminosity	98/3.4/4.1 L☉
Temperature	12,000/4,500/8,500 K
Metallicity	Not available
Rotation	65 km/s.
Age	< 3 × 108 years
Other designations
Algol, Gorgona, Gorgonea Prima, Demon Star, El Ghoul, 26 Per, GJ 9110, HR 936, BD +40°673, HD 19356, GCTP 646.00, SAO 38592, FK5 111, Wo 9110, ADS 2362, WDS 03082+4057A, HIP 14576.

Algol (β Per / Beta Persei), known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, the first such star to be discovered, and also one of the first (non-nova) variable stars to be discovered. Algol is actually a three-star system (Beta Persei A, B, and C) in which the large and bright primary Beta Persei A is regularly eclipsed by the dimmer Beta Persei B. Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every two days, 20 hours and 49 minutes during the roughly 10-hour long partial eclipses. There is also a secondary eclipse when the brighter star occults the fainter secondary. This secondary eclipse can only be detected photoelectrically.^[3]

Observation history

The variability of Algol was first recorded in 1667 by Geminiano Montanari, but it is probable that this property was noticed long before this time. The first person to propose a mechanism for the variability of this star 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 else that the star itself has a darker region that is periodically turned toward the Earth.) For his report he was awarded the Copley Medal.^[4]

In 1881, the Harvard astronomer Edward Pickering presented evidence that Algol was actually an eclipsing binary.^[5] This was confirmed a few years later, in 1889, when the Potsdam astronomer Hermann Carl Vogel found periodic doppler shifts in the spectrum of Algol, inferring variations in the radial velocity of this binary system.^[6] Thus Algol became one of the first known spectroscopic binaries.

System

Algol A and Algol B are an eclipsing binary, because their orbital plane coincidentally contains the Earth's line of sight. To be more precise, however, Algol is a triple-star system: the eclipsing binary pair is separated by only 0.062 AU, while the third star in the system (Algol C) is at an average distance of 2.69 AU from the pair and the mutual orbital period is 681 days (1.86 years). 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.

Studies of Algol led to the Algol paradox in the theory of stellar evolution: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the main sequence, while the less massive Algol B is a subgiant star at a later evolutionary stage. The paradox can be solved by mass transfer: when the more massive star became a subgiant, it filled its Roche lobe, and most of the mass was transferred to the other star, which is still in the main sequence. In some binaries similar to Algol, a gas flow can be seen.^[9]

This system also exhibits variable activity in the form of x-ray and radio flares. The former is thought to be caused by the magnetic fields of the AB components interacting with the mass transfer.^[10] The radio emissions may be created by magnetic cycles similar to sunspots, but, as the magnetic fields around these stars are up to ten times stronger than that of the Sun, these radio flares are more powerful and longer lasting.^[11]

Algol is 92.8 light years from Earth; however, about 7.3 million years ago it passed within 9.8 light years^[12] and its apparent magnitude was approximately −2.5, considerably brighter than Sirius is today. Because the total mass of the system is 5.8 solar masses, and despite the fairly large distance at closest approach, this may have been enough to perturb the solar system's Oort cloud slightly and to increase the number of comets entering the inner solar system. However, the actual increase in net cratering rate is believed to have been quite small.^[13]

Etymology and cultural significance

The name Algol derives from Arabic رأس الغول ra's al-ghūl : head (ra's) of the ogre (al-ghūl) (see "the ghoul") which was given from its position in the constellation Perseus, representing the head of Gorgon Medusa. The English names of Demon Star and Blinking Demon are direct translations.^[14] In Hebrew folklore it was known as Rōsh ha Sāṭān 'Satan's Head', via Edmund Chilmead, who called it 'Divels head' or Rosch hassatan. A Latin term from the 16th century was Caput Larvae 'Spectre's Head'. It was also linked with Lilith.^[14]. Hipparchus and Pliny made this a separate, though connected, constellation.^[14]

It is known as 大陵五 (the Fifth Star of the Mausoleum) in Chinese astronomy, and also bore the grim name Tseih She (叠尸 - die2 shi1 in Modern Pinyin), meaning 'Piled up corpses'.^[14]

Astrology

Astrologically, Algol is considered the most unfortunate star in the sky.^[14] In the Middle Ages it was one of the 15 Behenian stars,^[15] associated with the diamond and hellebore, and marked with the kabbalistic sign:

Modern fiction

This section does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (October 2009)

One of the earliest films about alien invasion was a 1920 German silent film titled Algol. Renowned at the time for its sets, it featured Emil Jannings as Mephisto, an alien from Algol. All prints of the film were believed to have been lost,^[16] but an intact copy has been recovered.

Algol is also depicted as the main antagonist in the 2008 fighting videogame Soul Calibur IV.

Algol is also the name of the solar system featured in the original Phantasy Star series of role-playing video games on the Sega Master System and Sega Genesis/Mega Drive.

Progressive Viking/folk metal band Vintersorg's 2000 album, Cosmic Genesis, features a song titled "Algol".

The name of a boss in World of Warcraft's raid dungeon, Ulduar, is Algalon, whose name is based on Algol.

Algol is also referred as the only star visible in Hell in the novel "God's Demon" by artist Wayne Barlowe.

In H. P. Lovecraft's short story Beyond The Wall of Sleep Algol is the nemesis of the entity possessing Joe Slater or "Slaader."

In Star Trek: The Next Generation, "Algolians" were an alien species who appeared in the episodes "Ménage à Troi" and "Qpid."

References

External links

• "HD 19356 -- Eclipsing binary of Algol type". SIMBAD. http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=Beta+Persei&NbIdent=1&Radius=10&Radius.unit=arcmin&CooFrame=FK5&CooEpoch=2000&CooEqui=2000&output.max=all&o.catall=on&output.mesdisp=N&Bibyear1=1983&Bibyear2=2006&Frame1=FK5&Frame2=FK4&Frame3=G&Equi1=2000.0&Equi2=1950.0&Equi3=2000.0&Epoch1=2000.0&Epoch2=1950.0&Epoch3=2000.0. Retrieved 2006-07-31. 
• "Beta Persei (Algol)". AAVSO. January 1999. http://www.aavso.org/vstar/vsots/0199.shtml. Retrieved 2006-07-31. 
• "The Horror-Scope of Algol". Skyscript. July 2005. http://skyscript.co.uk/algol.html. Retrieved 2006-07-31. 
• "Algol 3". SolStation. http://www.solstation.com/stars2/algol3.htm. Retrieved 2006-07-31. 
• "4C02517". ARICNS. 1998-03-04. http://www.ari.uni-heidelberg.de/aricns/cnspages/4c02517.htm. Retrieved 2006-07-31. 
• "Algol". Alcyone ephemeris. http://www.alcyone.de/SIT/mainstars/SIT000666.htm. Retrieved 2006-06-08. 
• Kaler, Jim. "Algol (Beta Persei)". Stars. http://www.astro.uiuc.edu/~kaler/sow/algol.html. Retrieved 2006-06-08. 
• Bezza, Giuseppe [translated by Daria Dudziak]. "Al-ghûl, the ogre.". Cielo e Terra. http://www.cieloeterra.it/eng/eng.articoli.algol/eng.algol.html. Retrieved 2006-06-08. 
• Czech space metal band "Realm of Algol"

Coordinates: 03^h 08^m 10.1315^s, +40° 57′ 20.332″