The red dot shows the location of Algol in Perseus.
Epoch J2000 Equinox J2000
|Right ascension||03h 08m 10.1315s|
|Declination||+40° 57′ 20.332″|
|Apparent magnitude (V)||2.12|
|Spectral type||B8V (A) /K0IV (B) /A5V (C)|
|U−B color index||−0.37|
|B−V color index||−0.05|
|Variable type||Eclipsing binary|
|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|
|Age||< 3×108 years|
Algol (Beta Per, β Persei, β Per), 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. Algol gives its name to its class of eclipsing variable, known as Algol variables.
Observation history 
The association of Algol with a demon-like creature (Gorgon in the Greek tradition, ghoul in the Arabic tradition) strongly suggests that its variability was known long before the 17th century, but there is still no conclusive evidence for this. Investigating a calendar for lucky and unlucky days composed in Egypt some 3200 years ago, a periodicity of 2.85 days has been noted and it has been conjectured to correspond to the dimming of Algol.
The variability of Algol was first unambiguously recorded in 1667 by Geminiano Montanari, but the periodic nature of its brightness variations was not recognized until more than a century later by the British amateur astronomer John Goodricke (who also proposed a mechanism for the star's variability). 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.
In 1881, the Harvard astronomer Edward Charles Pickering presented evidence that Algol was actually an eclipsing binary. 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. Thus Algol became one of the first known spectroscopic binaries. Dr. Joel Stebbins at the University of Illinois Observatory used an early selenium cell photometer to produce the first-ever photoelectric study of a variable star. The light curve revealed the second minimum and the reflection effect between the two stars. Some difficulties in explaining the observed spectroscopic features led to the conjecture that a third star may be present in the system; four decades later this conjecture was found to be correct.
From the point of view of the Earth, Algol A and Algol B form an eclipsing binary because their orbital plane contains the line of sight to the Earth. To be more precise, Algol is a triple-star system: the eclipsing binary pair is separated by only 0.062 astronomical units (AU) from each other, whereas 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 of the trio is 681 Earth days. 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 to 1 to 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.
This system also exhibits variable activities in the forms of x-ray and radio wave flares. The former is thought to be caused by the magnetic fields of the A and B components interacting with the mass transfer. The radio wave emissions might be created by magnetic cycles similar to those of sunspots, but since the magnetic fields of these stars are up to ten times stronger than the field of the Sun, these radio flares are more powerful and more persistent.
Algol is located about 92.8 light years from the Sun, but about 7.3 million years ago it passed within 9.8 light years of the Solar System and its apparent magnitude was about -2.5, which is considerably brighter than the star Sirius is today. Because the total mass of the Algol system is about 5.8 solar masses, at the closest approach this might have given enough gravity to perturb the Oort cloud of the Solar System somewhat and hence increase the number of comets entering the inner solar system. However, the actual increase in net cometary collisions is thought to have been quite small.
In Hebrew folklore, Algol was called Rōsh ha Sāṭān or "Satan's Head", as stated by Edmund Chilmead, who called it "Divels head" or Rosch hassatan. A Latin name for Algol from the 16th century was Caput Larvae or "the Spectre's Head". Algol was also linked with "Lilith". Hipparchus and Pliny made this a separate, though connected, constellation.
In Chinese, 大陵 (Dà Líng), meaning Mausoleum, refers to an asterism consisting of β Persei, 9 Persei, τ Persei, ι Persei, κ Persei, ρ Persei, 16 Persei and 12 Persei. Consequently, β Persei itself is known as 大陵五 (Dà Líng wu, English: the Fifth Star of Mausoleum.). According to R.H. Allen the star bore the grim name of Tseih She 叠尸 (Dié Shī), meaning "Piled up Corpses" but this appears to be a misidentification.
Cultural significance 
|The constellation Perseus and Algol, the Bright Star in the Gorgon's head|
Johannes Hevelius, Uranographia, 1690
In the Tetrabiblos, the 2nd century astrological text of the Alexandrian astronomer Ptolemy, Algol is referred to as "the Gorgon of Perseus" and associated with death by decapitation: a theme which mirrors the myth of the hero Perseus’ victory over the snake-headed Gorgon Medusa. Astrologically,[clarification needed] Algol is considered one of the unluckiest stars in the sky, and was listed as one of the 15 Behenian stars. Historically, the star has received a strong association with bloody violence across a wide variety[which?] of cultures.
See also 
- Database entry for Algol A, SIMBAD. Accessed online February 9, 2008.
- Database entry for Algol B, SIMBAD. Accessed online February 9, 2008.
- Rhee, Joseph H. et al. (May 2007), "Characterization of Dusty Debris Disks: The IRAS and Hipparcos Catalogs", The Astrophysical Journal 660 (2): 1556–1571, arXiv:astro-ph/0609555, Bibcode:2007ApJ...660.1556R, doi:10.1086/509912
- Beta Persei, American Association of Variable Star Observers. http://www.aavso.org/vstar/vsots/0199.shtml[dead link]
- Wilk, Stephen R. (1996). "Mythological Evidence for Ancient Observations of Variable Stars". The Journal of the American Association of Variable Star Observers 24 (2): 129–33. Bibcode:1996JAVSO..24..129W.
- G.A. Davis, "Why did the Arabs Call Beta Persei "al-Ghul"?", Sky and Telescope, 16 (1957), 177 ADS.
- Porceddu S., Jetsu L., Markkanen T. and Toivari-Viitala J.,(2008) Evidence of Periodicity in Ancient Egyptian Calendars of Lucky and Unlucky Days, Cambridge Archaeological Journal, Vol.18, Oct. 2008, Iss.3, pp 327-39, DOI: 10.1017/S0959774308000395
- L. Jetsu, S. Porceddu, J. Lyytinen, et al, "Did the ancient egyptians record the period of the eclipsing binary Algol - the Raging one?" ; see also"Cairo Calendar shows Egyptians discovered binary Algol first". 2 May 2012. Retrieved 3 May 2012.
- G. Montanari, "Sopra la sparizione d'alcune stelle et altre novità celesti", in: Prose de Signori Accademici Gelati di Bologna (Bologna: Manolessi, 1671), pp. 369-392 (Google books).
- O.J. Eggen,"An Eighteenth Century Discussion of Algol", The Observatory, 77 (1957), 191-197 ADS.
- "John Goodricke, The Discovery of the Occultating Variable Stars". 6 August 2003. Archived from the original on 22 June 2006. Retrieved 31 July 2006.
- Pickering, Edward C. (1881). "Dimensions of the Fixed Stars, with especial reference to Binaries and Variables of the Algol type". Astronomical register 50 (1-2): 253–56. Bibcode:1881AReg...19..253.
- A. H. Batten (1989). "Two Centuries of Study of Algol Systems". Space Science Reviews 50 (1/2): 1–8. Bibcode:1989SSRv...50....1B. doi:10.1007/BF00215914.
- J. Stebbins (1910). "The Measurement of the Light of Stars with a Selenium Photometer with an Application to the Variation of Algol". Astrophysical Journal 32: 65–80.
- Meltzer, Alan S., A Spectroscopic Investigation of Algol, Astrophysical Journal, vol. 125, (1957), p.359, BibCode:1957ApJ...125..359M
- L. A. Molnar, R. L. Mutel (1996). "Dynamical Evolution of the Algol Triple System". Bulletin of the American Astronomical Society 28 (1-2): 921. Bibcode:1996AAS...188.6014M.
- AAVSO observation data
- W.I. Hartkopf, B.D. Mason (30 July 2006). "Sixth Catalog of Orbits of Visual Binary Stars". U.S. Naval Observatory. Archived from the original on 6 August 2006. Retrieved 31 July 2006.
- Pustylnik, Izold (1995). "On Accretion Component of the Flare Activity in Algol". Baltic Astronomy 4 (1-2): 64–78. Bibcode:1995BaltA...4...64P.
- M.J. Sarna, S.K. Yerli, A.G. Muslimov (1998). "Magnetic Activity and Evolution of Algol-type Stars - II". Monthly Notices of the Royal Astronomical Society 297 (3): 760–68. Bibcode:1998MNRAS.297..760S. doi:10.1046/j.1365-8711.1998.01539.x.
- Blue, Charles E. (3 June 2002). "Binary Stars "Flare" With Predictable Cycles, Analysis of Radio Observations Reveals". National Radio Astronomy Observatory. Archived from the original on 2 July 2006. Retrieved 31 July 2006.
- Garcia-Sanchez, J.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Weissman, P. R.; Latham, D. W. (25 August 1997). "A Search for Stars Passing Close to the Sun". The First Results of Hipparcos and Tycho. Kyoto, Japan: IAU. Retrieved 1 June 2007.
- J. García-Sánchez, R.A. Preston, D.L. Jones, P.R. Weissman (1999). "Stellar Encounters with the Oort Cloud Based on Hipparcos Data". The Astronomical Journal 117 (2): 1042–55. Bibcode:1999AJ....117.1042G. doi:10.1086/300723.
- P. Kunitzsch & T. Smart, Short Guide to Modern Star Names and Their Derivations (Wiesbaden: Otto Harrassowitz, 1986), p 49.
- Allen, R. H. (1963) . Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 331. ISBN 0-486-21079-0.
- (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 11 日
- Ian Ridpath's Star Tales – Perseus
- Robbins, Frank E. (ed.) 1940. Ptolemy Tetrabiblos. Cambridge, Massachusetts: Harvard University Press (Loeb Classical Library). ISBN 0-674-99479-5, IV.9, p.435.
- Henry Cornelius Agrippa. Three Books of Occult Philosophy. Lyons, 1531/33. Llewellyn reprint, 1993; tr. J. Freake (1651), ed. D. Tyson, p.411.
- "HD 19356 -- Eclipsing Binary of Algol type". SIMBAD. Retrieved 31 July 2006.
- "Beta Persei (Algol)". AAVSO. January 1999. Archived from the original on July 8, 2006. Retrieved 31 July 2006.
- "Algol 3". SolStation. Retrieved 31 July 2006.
- "4C02517". ARICNS. 4 March 1998. Archived from the original on 10 February 2006. Retrieved 31 July 2006.
- "Algol". Alcyone ephemeris. Retrieved 8 June 2006.
- Kaler, Jim. "Algol (Beta Persei)". Stars. Archived from the original on 17 June 2006. Retrieved 8 June 2006.
- Bezza, Giuseppe [translated by Daria Dudziak]. "Al-ghûl, the ogre.". Cielo e Terra. Archived from the original on 20 June 2006. Retrieved 8 June 2006.
- Did the ancient egyptians record the period of the eclipsing binary Algol - the Raging one? (arXiv:1204.6206)