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 and one of the first non-nova variable stars to be discovered.
Algol is a three-star system, consisting of Beta Persei Aa1, Aa2, and Ab – in which the hot luminous primary β Persei Aa1 and the larger, but cooler and fainter, β Persei Aa2 regularly pass in front of each other, causing eclipses. Thus Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2.86 days during the roughly 10-hour-long partial eclipses. The secondary eclipse when the brighter primary star occults the fainter secondary is very shallow and can only be detected photoelectrically.
Algol gives its name to its class of eclipsing variable, known as Algol variables.
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, but there is still no indisputable evidence for this. The Arabic astronomer al-Sufi said nothing about any variability of the star in his Book of Fixed Stars published c.964.
The variability of Algol was noted in 1667 by Italian astronomer Geminiano Montanari, 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. 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. 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.
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Listed are the first eclipse dates and times of each month; all times in UT. β Persei Aa2 eclipses β Persei Aa1 every 2.867321 days (2 days 20 hours 49 min); therefore keep adding that much to each date and time to get the following eclipses. For example, the Jan 3, 21h, eclipse will yield consecutive eclipse times on Jan 6, 18h, then Jan 9, 15h, then Jan 12, 12h, etc. (all times approximate).
Algol is a multiple-star system with three confirmed and two suspected stellar components. 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. 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 Ab, with two very faint stars B and C about one arcmin distant. A further five faint stars are also listed as companions.
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 stars, the more massive component Algol Aa1 is still in the main sequence, but the less massive Algol Aa2 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. The gas flow between the primary and secondary stars in Algol has been imaged using Doppler Tomography.
This system also exhibits x-ray and radio wave flares. The x-ray flares are thought to be caused by the magnetic fields of the A and B components interacting with the mass transfer. The radio-wave flares 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. The secondary component was identified as the radio emitting source in Algol using Very-long-baseline interferometry by Lestrade and co-authors.
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. Mass transfer between the components is small in the Algol system but could be a significant source of period change in other Algol-type binaries.
Algol is 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.
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"). The English name Demon Star was taken from the Arabic name. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 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.
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". 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, the Chinese name for β Persei itself is 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 積屍 (Zhi Shī), meaning "Piled up Corpses" but this appears to be a misidentification, and Dié Shī is correctly π Persei, which is inside the Mausoleum.
The constellation Perseus and Algol, the Bright Star in the Gorgon's head
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. In the astrology of fixed stars, Algol is considered one of the unluckiest stars in the sky, and was listed as one of the 15 Behenian stars.
- Jaana Toivari-Viitala, egyptologist who contributed to understanding Ancient Egypt and the star
- Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
- 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.
- 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.
- 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.
- Soderhjelm, S. (1980). "Geometry and dynamics of the Algol system". Astronomy and Astrophysics. 89 (1–2): 100. Bibcode:1980A&A....89..100S.
- 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 (1): 20. arXiv:1205.0754. Bibcode:2012ApJ...752...20B. doi:10.1088/0004-637X/752/1/20. S2CID 11110989.
- 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 (1): L44–L48. arXiv:1005.0626. Bibcode:2010ApJ...715L..44Z. doi:10.1088/2041-8205/715/1/L44. S2CID 118573026.
- 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.
- "Algol". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- "Beta Persei (Algol)". AAVSO. January 1999. Archived from the original on 8 July 2006. Retrieved 31 July 2006.
- 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. Bibcode:2008CArcJ..18..327P. doi:10.1017/S0959774308000395. S2CID 162969143.
- 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:1204.6206. Bibcode:2013ApJ...773....1J. doi:10.1088/0004-637X/773/1/1. S2CID 119191453.
- 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:1601.06990. Bibcode:2015PLoSO..1044140J. doi:10.1371/journal.pone.0144140. PMC 4683080. PMID 26679699.
- 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.
- Davis, G.A. (1957). "Why did the Arabs Call Beta Persei "al-Ghul"?". Sky and Telescope. 16: 177. Bibcode:1957S&T....16..177D.
- "Ian Ridpath's Star Tales – Perseus".
- 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).
- ADS O.J. Eggen,"An Eighteenth Century Discussion of Algol", The Observatory, 77 (1957), 191–197.
- Goodricke, John (1783-01-01). "XXVI. A series of observations on, and a discovery of, the period of the variation of the light of the bright star in the head of medusa, called algol. In a letter from John Goodricke, Esq. to the Rev. Anthony Shepherd, D. D. F. R. S. and Plumian Professor at Cambridge". Philosophical Transactions of the Royal Society of London. 73: 474–482. doi:10.1098/rstl.1783.0027.
- "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. S2CID 125814848.
- 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.
- Meltzer, Alan S., A "Spectroscopic Investigation of Algol". Astrophysical Journal, vol. 125, (1957), p.359, BibCode:1957ApJ...125..359M
- Edgar, James S. (editor). Observer's Handbook 2020. The Royal Astronomical Society of Canada. ISBN 9-781927-879191.
- Ashford, Adrian (30 July 2006). "The Minima of Algol". Retrieved 2018-12-04.
- Jetsu, L. (2021). "Say Hello to Algol's New Companion Candidates". The Astrophysical Journal. 920 (2): 137. arXiv:2005.13360. Bibcode:2021ApJ...920..137J. doi:10.3847/1538-4357/ac1351. S2CID 239882152.
- 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.
- Pustylnik, Izold (1995). "On Accretion Component of the Flare Activity in Algol". Baltic Astronomy. 4 (1–2): 64–78. Bibcode:1995BaltA...4...64P. doi:10.1515/astro-1995-0106.
- Richards, Mercedes T.; Albright, Geary E.; Bowles, Larissa M. (January 1995). "Doppler tomography of the gas stream in short-period Algol binaries". The Astrophysical Journal. 438: L103. Bibcode:1995ApJ...438L.103R. doi:10.1086/187726. ISSN 0004-637X.
- Richards, Mercedes T.; Agafonov, Michail I.; Sharova, Olga I. (2012-11-20). "NEW EVIDENCE OF MAGNETIC INTERACTIONS BETWEEN STARS FROM THREE-DIMENSIONAL DOPPLER TOMOGRAPHY OF ALGOL BINARIES: β PER AND RS VUL". The Astrophysical Journal. 760 (1): 8. arXiv:1210.0081. Bibcode:2012ApJ...760....8R. doi:10.1088/0004-637X/760/1/8. ISSN 0004-637X. S2CID 118319759.
- Wade, C. M.; Hjellming, R. M. (February 4, 1972). "Radio Stars Beta Persei and Beta Lyrae". Nature. 235 (5336): 270–271. Bibcode:1972Natur.235..270W. doi:10.1038/235270a0. ISSN 0028-0836. S2CID 4222515.
- 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.
- Richards, Mercedes T.; Waltman, Elizabeth B.; Ghigo, Frank D.; Richards, Donald St. P. (August 2003). "Statistical Analysis of 5 Year Continuous Radio Flare Data from β Persei, V711 Tauri, δ Librae, and UX Arietis". The Astrophysical Journal Supplement Series. 147 (2): 337–361. Bibcode:2003ApJS..147..337R. doi:10.1086/375835. ISSN 0067-0049.
- Lestrade, Jean-Francois; Phillips, Robert B.; Hodges, Mark W.; Preston, Robert A. (June 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.
- 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.
- 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/0611855.
- 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.
- 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 978-0-486-21079-7.
- "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
- "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
- "IAU Catalog of Star Names". Retrieved 28 July 2016.
- (in 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 (1898). Three Books of Occult Philosophy. Lyons, 1531/33. Llewellyn reprint, 1993; tr. J. Freake (1651), ed. D. Tyson, p.411.
|Wikisource has the text of the 1911 Encyclopædia Britannica article "Algol".|
- "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.