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This article is about the star. For other uses, see Algol (disambiguation).
Perseus constellation map.svg
Red circle.svg

Location of β Persei (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Perseus
Right ascension 03h 08m 10.13245s[1]
Declination +40° 57′ 20.3280″[1]
Apparent magnitude (V) 2.12[2] (- 3.39[3])
Spectral type Aa1: B8V[4]
Aa2: K0IV[4]
Ab: A7m[4]
U−B color index −0.37[2]
B−V color index −0.05[2]
Variable type EA/SD[3]
Radial velocity (Rv) 3.7 km/s
Proper motion (μ) RA: 2.99[1] mas/yr
Dec.: −1.66[1] mas/yr
Parallax (π) 36.27 ± 1.40[1] mas
Distance 90 ± 3 ly
(28 ± 1 pc)
β Per Aa1
Absolute magnitude (MV) −0.07[5]
β Per Aa2
Absolute magnitude (MV) 2.9[5]
β Per Ab
Absolute magnitude (MV) 2.3[5]
Primary β Per Aa1
Companion β Per Aa2
Period (P) 2.867328 days
Semi-major axis (a) 0.00215"
Eccentricity (e) 0
Inclination (i) 98.70°
Longitude of the node (Ω) 43.43°
Primary β Per A
Companion β Per B
Period (P) 680.168 days
Semi-major axis (a) 0.09343"
Eccentricity (e) 0.227
Inclination (i) 83.66°
Longitude of the node (Ω) 132.66°
Periastron epoch (T) 2446927.22
Argument of periastron (ω)
β Per Aa1
Mass 3.17 ± 0.21[6] M
Radius 2.73 ± 0.20[6] R
Luminosity 182[5] L
Surface gravity (log g) 4.0[7] cgs
Temperature 13,000[7] K
Rotational velocity (v sin i) 49[8] km/s
Age 570[5] Myr
β Per Aa2
Mass 0.70 ± 0.08[6] M
Radius 3.48 ± 0.28[6] R
Luminosity 6.92[5] L
Surface gravity (log g) 3.5[7] cgs
Temperature 4,500[7] K
β Per Ab
Mass 1.76 ± 0.15[6] M
Radius 1.73 ± 0.33[6] R
Luminosity 10.0[5] L
Surface gravity (log g) 4.5[7] cgs
Temperature 7,500[7] K
Other designations
Algol, Gorgona, Gorgonea Prima, Demon Star, El Ghoul, β Persei, β Per, 26 Persei, BD+40°673, FK5 111, GC 3733, HD 19356, HIP 14576, HR 936, PPM 46127, SAO 38592.
Database references

Algol, designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation of Perseus. The first and best known eclipsing binary and also one of the first (non-nova) variable stars to be discovered. A three-star system - Beta Persei Aa1, Aa2, and Ab - in which the large and bright primary β Persei Aa1 is regularly eclipsed by the dimmer β Persei Aa2. Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2.86 days (2 days, 20 hours and 49 minutes) during the roughly 10-hour-long partial eclipses. There is also a secondary eclipse (the "second minimum") when the brighter star occults the fainter secondary. This secondary eclipse can only be detected photoelectrically.[9]

Algol gives its name to its class of eclipsing variable, known as Algol variables.

Observation history[edit]

The Algol system on 12 August 2009. This is a CHARA interferometer image with 12-milliarcsecond resolution in the near-infrared H-band. The elongated appearance of Algol Aa2 (labelled B) and the round appearance of Algol Aa1 (labelled A) are real, but the form of Algol Ab (labelled C) is an artifact.

An Ancient Egyptian Calendar of Lucky and Unlucky Days composed some 3200 years ago is claimed to be the oldest historical document of the discovery of Algol.[10][11] [12]

The association of Algol with a demon-like creature (Gorgon in the Greek tradition, ghoul in the Arabic tradition) suggests that its variability was known long before the 17th century,[13] but except for the Ancient Egyptian discovery there is still no indisputable evidence for this.[14]

The variability of Algol was noted in 1667 by Italian astronomer Geminiano Montanari,[15] but the periodic nature of its variations in brightness was not recognized until more than a century later, when the British amateur astronomer John Goodricke also proposed a mechanism for the star's variability.[16] 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.[17]

In 1881, the Harvard astronomer Edward Charles Pickering presented evidence that Algol was actually an eclipsing binary.[18] 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.[19] Thus Algol became one of the first known spectroscopic binaries. 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.[20] 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.[21]


Algol Aa2 orbits Algol Aa1. This animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase. Because some phases are poorly covered, Aa2 jumps at some points along its path.
Interpolation of the orbit of Aa2 around Aa1 with focus on Aa1.

From the point of view of the Earth, Algol Aa1 and Algol Aa2 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 Ab) 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 Aa1, Aa2, and Ab are about 4.5 to 1 to 2.

The three components of the bright triple star used to be, and still sometimes are, referred to as β Per A, B, and C. The Washington Double Star Catalog lists them as Aa1, Aa2, and An, with two very faint stars B and C about one arcmin distant. A further five faint stars are also listed as companions.[22]

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, whereas 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.[23]

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.[24] The radio-wave emissions might be created by magnetic cycles similar to those of sunspots, but because 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.[25]

Magnetic activity cycles in the chromospherically active secondary component induce changes in its radius of gyration that have been linked to recurrent orbital period variations on the order of ΔP/P ≈ 10−5 via the Applegate mechanism.[26] Mass transfer between the components is small in the Algol system[27] but could be a significant source of period change in other Algol-type binaries.

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[28] 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.[29]


Beta Persei is the star's Bayer designation. The name Algol derives from Arabic رأس الغول raʾs al-ghūl : head (raʾs) of the ogre (al-ghūl) (see "ghoul").[30] The English name "Demon Star" is a direct translation of this.[31] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[32] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[33] included a table of the first two batches of names approved by the WGSN; which included Algol for this star. It is so entered on the IAU Catalog of Star Names.[34]

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".[31] Hipparchus and Pliny made this a separate, though connected, constellation.[31]

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.).[35] According to R.H. Allen the star bore the grim name of Tseih She 叠尸 (Dié Shī), meaning "Piled up Corpses"[31] but this appears to be a misidentification.[36]

Cultural significance[edit]

The constellation Perseus and Algol, the Bright Star in the Gorgon's head
Perseus Hevelius.jpg
Johannes Hevelius, Uranographia, 1690

Historically, the star has received a strong association with bloody violence across a wide variety of cultures. 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's victory over the snake-haired Gorgon Medusa.[37] Astrologically,[clarification needed] Algol is considered one of the unluckiest stars in the sky,[31] and was listed as one of the 15 Behenian stars.[38]

See also[edit]


  1. ^ a b c d e Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752free to read. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  2. ^ a b c Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D. 
  3. ^ a b Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1: 02025. Bibcode:2009yCat....102025S. 
  4. ^ a b c Lestrade, Jean-Francois; Phillips, Robert B.; Hodges, Mark W.; Preston, Robert A. (1993). "VLBI astrometric identification of the radio emitting region in Algol and determination of the orientation of the close binary". The Astrophysical Journal. 410: 808. Bibcode:1993ApJ...410..808L. doi:10.1086/172798. ISSN 0004-637X. 
  5. ^ a b c d e f g Soderhjelm, S. (1980). "Geometry and dynamics of the Algol system". Astronomy and Astrophysics. 89: 100. Bibcode:1980A&A....89..100S. 
  6. ^ a b c d e f g h Baron, F.; Monnier, J. D.; Pedretti, E.; Zhao, M.; Schaefer, G.; Parks, R.; Che, X.; Thureau, N.; Ten Brummelaar, T. A.; McAlister, H. A.; Ridgway, S. T.; Farrington, C.; Sturmann, J.; Sturmann, L.; Turner, N. (2012). "Imaging the Algol Triple System in the H Band with the CHARA Interferometer". The Astrophysical Journal. 752: 20. arXiv:1205.0754free to read. Bibcode:2012ApJ...752...20B. doi:10.1088/0004-637X/752/1/20. 
  7. ^ a b c d e f Zavala, R. T.; Hummel, C. A.; Boboltz, D. A.; Ojha, R.; Shaffer, D. B.; Tycner, C.; Richards, M. T.; Hutter, D. J. (2010). "The Algol Triple System Spatially Resolved at Optical Wavelengths". The Astrophysical Journal Letters. 715: L44. arXiv:1005.0626free to read. Bibcode:2010ApJ...715L..44Z. doi:10.1088/2041-8205/715/1/L44. 
  8. ^ Tomkin, J.; Huisong, T. (1985). "The rotation of the primary of Algol". Publications of the Astronomical Society of the Pacific. 97: 51. Bibcode:1985PASP...97...51T. doi:10.1086/131493. 
  9. ^ "Beta Persei (Algol)". AAVSO. January 1999. Archived from the original on 8 July 2006. Retrieved 31 July 2006. 
  10. ^ Porceddu, S.; Jetsu, L.; Lyytinen, J.; Kajatkari, P.; Lehtinen, J.; Markkanen, T; et al. (2008). "Evidence of Periodicity in Ancient Egyptian Calendars of Lucky and Unlucky Days". Cambridge Archaeological Journal. 18 (3): 327–339. doi:10.1017/S0959774308000395. 
  11. ^ Jetsu, L.; Porceddu, S.; Lyytinen, J.; Kajatkari, P.; Lehtinen, J.; Markkanen, T; et al. (2013). "Did the Ancient Egyptians Record the Period of the Eclipsing Binary Algol - The Raging One?". The Astrophysical Journal. 773 (1): A1 (14pp). arXiv:1204.6206free to read. Bibcode:2013ApJ...773....1J. doi:10.1088/0004-637X/773/1/1. 
  12. ^ Jetsu, L.; Porceddu, S. (2015). "Shifting Milestones of Natural Sciences: The Ancient Egyptian Discovery of Algol's Period Confirmed". PLOS ONE. 10 (12): e.0144140 (23pp). arXiv:1601.06990free to read. Bibcode:2015PLoSO..1044140J. doi:10.1371/journal.pone.0144140. 
  13. ^ 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. 
  14. ^ G.A. Davis, "Why did the Arabs Call Beta Persei "al-Ghul"?", Sky and Telescope, 16 (1957), 177 ADS.
  15. ^ G. Montanari, "Sopra la sparizione d'alcune stelle et altre novità celesti", in: Prose de Signori Accademici Gelati di Bologna (Bologna: Manolessi, 1671), pp. 369-92 (Google books).
  16. ^ ADS O.J. Eggen,"An Eighteenth Century Discussion of Algol", The Observatory, 77 (1957), 191-197.
  17. ^ "John Goodricke, The Discovery of the Occultating Variable Stars". 6 August 2003. Archived from the original on 22 June 2006. Retrieved 31 July 2006. 
  18. ^ 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. 
  19. ^ 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. 
  20. ^ 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: 185–214. Bibcode:1910ApJ....32..185S. doi:10.1086/141796. 
  21. ^ Meltzer, Alan S., A "Spectroscopic Investigation of Algol". Astrophysical Journal, vol. 125, (1957), p.359, BibCode:1957ApJ...125..359M
  22. ^ Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466–3471. Bibcode:2001AJ....122.3466M. doi:10.1086/323920. 
  23. ^ Pustylnik, Izold (1995). "On Accretion Component of the Flare Activity in Algol". Baltic Astronomy. 4 (1-2): 64–78. Bibcode:1995BaltA...4...64P. 
  24. ^ 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. 
  25. ^ 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. 
  26. ^ Applegate, James H. (1992). "A mechanism for orbital period modulation in close binaries". Astrophysical Journal, Part 1. 385: 621–629. Bibcode:1992ApJ...385..621A. doi:10.1086/170967. 
  27. ^ Wecht, Kristen (2006). "Determination of Mass Loss and Mass Transfer Rates of Algol (Beta Persei) from the Analysis of Absorption Lines in the UV Spectra Obtained by the IUE Satellite". arXiv:astro-ph/0611855free to read. 
  28. ^ Garcia-Sanchez, J.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; et al. (25 August 1997). "A Search for Stars Passing Close to the Sun". The First Results of Hipparcos and Tycho. Kyoto, Japan: IAU. Bibcode:1997IAUJD..14E..51G. 
  29. ^ 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. 
  30. ^ P. Kunitzsch & T. Smart, Short Guide to Modern Star Names and Their Derivations (Wiesbaden: Otto Harrassowitz, 1986), p 49.
  31. ^ a b c d e Allen, R. H. (1963) [1899]. Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 331. ISBN 0-486-21079-0. 
  32. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016. 
  33. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016. 
  34. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016. 
  35. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網2006年7月11日
  36. ^ Ian Ridpath's Star Tales – Perseus
  37. ^ Robbins, Frank E. (ed.) 1940. Ptolemy: Tetrabiblos. Cambridge, Massachusetts: Harvard University Press (Loeb Classical Library). ISBN 0-674-99479-5, IV.9, p.435.
  38. ^ 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[edit]

  • "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. 
  • Bezza, Giuseppe. "Al-ghûl, the ogre". Translated by Daria Dudziak. Cielo e Terra. Archived from the original on 20 June 2006. Retrieved 8 June 2006. 

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