Algol: Wikis


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Beta Persei A/B/C
Position Beta Per.png
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]
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
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
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 Charles 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.


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.

Orbital Elements of the Algol System
Components Semimajor axis Ellipticity Period Inclination
A—B[7] 0.00218″ 0.00 2.87 days 97.69°
(AB)—C[8] 0.09461″ 0.225 680.05 days 83.98°

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]



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: Algol symbol (Agripe 1531).svg

Modern fiction

  • 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 referred as the only star visible in Hell in the novel "God's Demon" by artist Wayne Barlowe.
  • In Star Trek: The Next Generation, "Algolians" were an alien species who appeared in the episodes "Ménage à Troi" and "Qpid."


  1. ^ a b c d Database entry for Algol A, SIMBAD. Accessed online February 9, 2008.
  2. ^ Database entry for Algol B, SIMBAD. Accessed online February 9, 2008.
  3. ^ Beta Persei, American Association of Variable Star Observers.
  4. ^ "John Goodricke, The Discovery of the Occultating Variable Stars". 2003-08-06. Retrieved 2006-07-31. 
  5. ^ Pickering, Edward C. (1881). "Dimensions of the Fixed Stars, with especial reference to Binaries and Variables of the Algol type". Astronomical register 50: 253–256. doi:10.1007/BF00215914. 
  6. ^ A. H. Batten (1989). "Two Centuries of Study of Algol Systems". Space Science Reviews 50 (1/2): 1–8. doi:10.1007/BF00215914. 
  7. ^ L. A. Molnar, R. L. Mutel (1996). "Dynamical Evolution of the Algol Triple System". Bulletin of the American Astronomical Society 28: 921. doi:10.1007/BF00215914. 
  8. ^ W.I. Hartkopf, B.D. Mason (2006-07-30). "Sixth Catalog of Orbits of Visual Binary Stars". U.S. Naval Observatory. Retrieved 2006-07-31. 
  9. ^ Pustylnik, Izold (1995). "On Accretion Component of the Flare Activity in Algol". Baltic Astronomy 4: 64–78. doi:10.1007/BF00215914. 
  10. ^ 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): 760–768. doi:10.1046/j.1365-8711.1998.01539.x. 
  11. ^ Blue, Charles E. (2002-06-03). "Binary Stars "Flare" With Predictable Cycles, Analysis of Radio Observations Reveals". National Radio Astronomy Observatory. Retrieved 2006-07-31. 
  12. ^ Garcia-Sanchez, J.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Weissman, P. R.; Latham, D. W. (August 25, 1997). "A Search for Stars Passing Close to the Sun". The First Results of Hipparcos and Tycho. Kyoto, Japan: IAU. Retrieved 2007-06-01. 
  13. ^ 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: 1042–1055. doi:10.1086/300723. 
  14. ^ a b c d e Allen, Richard Hinckley (1963). Star Names: Their Lore and Meaning (revised edition). Dover. pp. 332–33. ISBN 0-486-21079-0. OCLC 637940 185804232 637940. , also online on Bill Thayer's site Lacus Curtius: Star Names: Their Lore and Meaning
  15. ^ Tyson, Donald; Freake, James (1993). Three Books of Occult Philosophy. Llewellyn Worldwide. ISBN 0875428320. OCLC 41597186 26634250 41597186. 
  16. ^ Searles B (1988). Films of Science Fiction and Fantasy. New York: Harry N. Abrams. pp. 116–17. ISBN 0-8109-0922-7. 

External links

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

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ALGOL, the Arabic name (signifying "the Demon") of ,6 Persei, a star of the second magnitude, noticed by G. Montanari in 1669 to fluctuate in brightness. John Goodricke established in 1782 the periodicity of its change in about 2 d 21 h, and suggested their cause in recurring eclipses by a large dark satellite. Their intermittent character prompted the supposition. The light of Algol remains constant during close upon 56 hours; then declines in 62 hours (approximately) to nearly one-fourth its normal amount, and is restored by sensibly the same gradations. The amplitude of the phase is 1�1 magnitude; and the absence of any stationary interval at minimum proves the eclipse to be partial, not annular. Its conditions were investigated from photometric data, by Professor E. C. Pickering in 1880;1 and "their realization was finally demonstrated by Dr H. C. Vogel's spectroscopic measures in 1889.2 Previously to each obscuration, the star was found to be moving rapidly away from the earth; its velocity then diminished to zero pari passu with the loss of light, and reversed its direction during the process of recovery. Algol, in fact, travels at the rate of 26.3 miles a second round the centre of gravity of the system which it forms with an invisible companion, while the two together approach the sun with an unvarying speed of 2.3 miles per second. The elements of this disparate pair, calculated by Dr Vogel on the somewhat precarious assumption that its dark and bright members are of equal mean density, are as follows: Diameter of Algol. .. 1,061,000 English miles.

� Satellite. .. 8 34,3 00 � Distance from centre to centre. 3,230,000 Mass of Algol. s solar mass.

� Satellite .

Mean density. ... about a solar.

The plane of the joint orbit, in which no deviation from circularity has yet been detected, nearly coincides with the line of sight. The period of Algol, as measured by its eclipses, is subject to complex irregularities. It shortened fitfully by eight seconds between 1790 and 1879; soon afterwards, restoration set in, and its exact length in 1903 was 2 d 20h 48' n 568, being only two seconds short of its original value. By an exhaustive discussion, Dr S. Chandler ascertained in 1888 the compensatory nature of these disturbances; 3 and he afterwards found the most important among several which probably conspire to produce the observed effects, to be comprised in a period of 15,000 light-cycles, equivalent to 118 years. 4 An explanatory hypothesis, propounded by him in 1892, 5 is still on its trial. The system of Algol, according to this view, is triple; it includes a large, obscure primary, round which the eclipsing pair revolves in an orbit somewhat smaller than that of Uranus, very slightly elliptical, and inclined 20° to the line of sight, the periodic time being 118 years. The alternate delay and acceleration of the eclipses are then merely apparent; they represent the changes in the length of the light-journey as the stars perform their wide circuit. If these suppositions have a basis of reality, the proper motion of Algol should be disturbed by a small, but measurable undulation, corresponding to the projection of its orbit upon the sky; and although certainty on the point cannot be attained for some years to come, Lewis Boss regarded the evidence available in 1895 as tending to confirm Dr Chandler's theory.6 Proceedings Amer. Acad. vol. xvi. p. 27.

Astr. Nach. No. 2947. a Astr. Journal, No. 165.

Ibid. No. 509. b Ibid. Ibid. No. 343.

A rival interpretation of the phenomena it dealt with was put forward by F. Tisserand in 1895.1 It involved the action of no third mass, but depended solely upon the progression of the line of apsides in a moderately elliptical orbit due to the spheroidal shape of the globes traversing it. Inequalities of the required sort in the returns of the eclipses would ensue; moreover, their duration should concomitantly vary with the varying distance from periastron at the times of their occurrence. It is a moot question whether changes of the latter kind actually occur. When they are proved to do so, Tisserand's hypothesis will hold the field.

Algol gives a helium-spectrum which undergoes no alteration at minimum. Hence the light from the marginal and central portions of the disc is identical in quality, and the limb can be little, if at all, darkened by the" smoke-veil "absorption conspicuous in the sun. The rays of this star spend close upon a century in travelling hither. Dr Chase's measures with the Yale heliometer indicated for it, in 1894, a parallax of about o" � 035; 2 and it must, accordingly, be of nearly four times the total brightness of Sirius, while its aerial lustre exceeds seventyfold that of the solar photosphere. Variables of the Algol class are rendered difficult to discover by the incidental character of their fluctuations. At the end of 1905, however, about 37 had been certainly recognized, besides some outlying cases of indeterminate type, in which continuous occultations by two bright stars, revolving in virtual contact, are doubtfully supposed to be in progress. (A. M. C.)

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Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also ALGOL



Wikipedia has an article on:



From Arabic الغول (al-ghūl) "the ghoul".

Proper noun




  1. (astronomy): An eclipsing binary star in the constellation of Perseus; Beta (β) Persei. It represents the eye of Medusa, whose head is being held by Perseus.


  • Chinese: 大陵五


  • Anagrams of agllo
  • Gallo

Simple English

Algol, also known as the Demon Star, is a bright star in the Perseus constellation. It is one of the best known double stars that can only be seen in spectroscopes, the first star of its type discovered, and one of the first variable stars to be discovered before it explodes. Algol is made of three stars (Beta Persei A, B and C) and the brightest is Persei A. These stars partly eclipse each other every 2 days, 20 hours and 49 minutes for 10 hours.[1]


  1. Beta Persei, American Association of Variable Star Observers.


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