Beta Persei A/B/C
Perseus constellation map.svg
Locator Dot2.gif

The red dot shows the location of Algol in Perseus.
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] /K0IV (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] Algol gives its name to its class of eclipsing variable, known as Algol variables.


Observation history

The variability of Algol was first recorded in 1667 by Geminiano Montanari,[4] 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).[5] 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.[6]

In 1881, the Harvard astronomer Edward Charles Pickering presented evidence that Algol was actually an eclipsing binary.[7] 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.[8] Thus Algol became one of the first known spectroscopic binaries.

Despite many claims in the modern literature that its ancient association 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,[9] there is no conclusive evidence for this.[10]


Computer simulation on how Algol may rotate

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[11] 0.00218″ 0.00 2.87 days 97.69°
(AB)—C[12] 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.[13]

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.[14] 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.[15]

Algol is 92.8 light years from Earth; however, about 7.3 million years ago it passed within 9.8 light years[16] 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.[17]


The name Algol derives from Arabic رأس الغول ra's al-ghūl : head (ra's) of the ogre (al-ghūl) (see "ghoul").[18] The English name Demon Star is a direct translation.[19]

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.[19] Hipparchus and Pliny made this a separate, though connected, constellation.[19]

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'.[19]

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.[20] Astrologically,[clarification needed] Algol is considered the most unfortunate star in the sky.,[19] and was listed as one of the 15 Behenian stars.[21] Historically, the star has received a strong association with bloody violence across a wide variety[which?] of cultures.

See also

  • Algol in fiction


  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. ^ 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).
  5. ^ O.J. Eggen,"An Eighteenth Century Discussion of Algol", The Observatory, 77 (1957), 191-197 ADS.
  6. ^ "John Goodricke, The Discovery of the Occultating Variable Stars". 2003-08-06. Retrieved 2006-07-31. 
  7. ^ 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): 253–256. Bibcode 1881AReg...19..253.. 
  8. ^ 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. 
  9. ^ S.R. Wilk, "Mythological Evidence for Ancient Observations of Variable Stars", The Journal of the American Association of Variable Star Observers, 24 (1996), 129-133 ADS
  10. ^ G.A. Davis, "Why did the Arabs Call Beta Persei "al-Ghul"?", Sky and Telescope, 16 (1957), 177 ADS.
  11. ^ 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. 
  12. ^ 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. 
  13. ^ Pustylnik, Izold (1995). "On Accretion Component of the Flare Activity in Algol". Baltic Astronomy 4 (1-2): 64–78. Bibcode 1995BaltA...4...64P. 
  14. ^ 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. Bibcode 1998MNRAS.297..760S. doi:10.1046/j.1365-8711.1998.01539.x. 
  15. ^ 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. 
  16. ^ 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. 
  17. ^ 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): 1042–1055. Bibcode 1999AJ....117.1042G. doi:10.1086/300723. 
  18. ^ P. Kunitzsch & T. Smart, Short Guide to Modern Star Names and Their Derivations (Wiesbaden: Otto Harrassowitz, 1986), p. 49.
  19. ^ a b c d e Allen, Richard Hinckley (1899). Star-Names and Their Meanings (Star Names: Their Lore and Meaning in the unchanged 1963 Dover reprint). G.E. Stechert (New York). 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
  20. ^ Robbins, Frank E. (ed.) 1940. Ptolemy Tetrabiblos. Cambridge, Massachusetts: Harvard University Press (Loeb Classical Library). ISBN 0-674-99479-5, IV.9, p.435.
  21. ^ Henry Cornelius Agrippa. Three Books of Occult Philosophy.  Lyons, 1531/33. Llewellyn reprint, 1993; tr. J. Freake (1651), ed. D. Tyson, p.411.

External links

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

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