Editing Algol

{{short description|Eclipsing variable star in the constellation Perseus}}
{{about|the star|the programming languages|ALGOL|other uses}}
{{good article}}
{{Use British English|date=February 2017}}
{{Starbox begin
 | name = Algol
}}
{{Starbox image
 | image =
{{Location mark
|image = Perseus constellation map.svg
|float = center
|alt =
|label =
|position = right
|width = 280
|mark = Red circle.svg
|mark_width = 12
|mark_link = Algol
|x = 644
|y = 663
}}
|caption = Location of β Persei (circled)
}}
{{Starbox observe
 | epoch = J2000
 | ra = {{RA|03|08|10.13245}}<ref name=hipparcos/>
 | dec = {{DEC|+40|57|20.3280}}<ref name=hipparcos>{{cite journal|bibcode=2007A&A...474..653V|title=Validation of the new Hipparcos reduction|journal=Astronomy and Astrophysics|volume=474|issue=2|pages=653–664|last1=Van Leeuwen|first1=F.|year=2007|doi=10.1051/0004-6361:20078357|arxiv = 0708.1752 |s2cid=18759600}}</ref>
 | appmag_v = 2.12<ref name=ducati>{{cite journal|bibcode=2002yCat.2237....0D|title=VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system|journal=CDS/ADC Collection of Electronic Catalogues|volume=2237|pages=0|last1=Ducati|first1=J. R.|year=2002}}</ref> (- 3.39<ref name=gcvs>{{cite journal|bibcode=2009yCat....102025S|title=VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)|journal=VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S|volume=1|pages=02025|last1=Samus|first1=N. N.|last2=Durlevich|first2=O. V.|year=2009|display-authors=etal}}</ref>)
 | constell=[[Perseus (constellation)|Perseus]]
}}
{{Starbox character
 | engvar = en-UK
 | class = '''Aa1''': B8V<ref name=lestrade1993/><br />'''Aa2''': K0IV<ref name=lestrade1993/><br />'''Ab''': F1V<ref name=frank2022/> {{nowrap|(kA4hA9.5mF0:<ref name=fletcher2964>{{cite journal |bibcode=1964AJ.....69..357F |title=Spectrophotometry of Algol |last1=Fletcher |first1=Emery S. |journal=The Astronomical Journal |year=1964 |volume=69 |page=357 |doi=10.1086/109284 }}</ref>)}}
 | b-v=−0.05<ref name=ducati/>
 | u-b=−0.37<ref name=ducati/>
 | variable=[[Algol variable|EA/SD]]<ref name=gcvs/>
}}
{{Starbox astrometry
 | radial_v = 3.7
 | prop_mo_ra = 2.99<ref name=hipparcos/>
 | prop_mo_dec = −1.66<ref name=hipparcos/>
 | parallax = 34.7
 | p_error = 0.6
 | parallax_footnote = <ref name=xu>{{Cite journal |last1=Xu |first1=Shuangjing |last2=Zhang |first2=Bo |last3=Reid |first3=Mark J. |last4=Zheng |first4=Xingwu |last5=Wang |first5=Guangli |date=2019-04-20 |title=Comparison of Gaia DR2 Parallaxes of Stars with VLBI Astrometry |arxiv=1903.04105 |journal=The Astrophysical Journal |volume=875 |issue=2 |pages=114 |doi=10.3847/1538-4357/ab0e83 |doi-access=free |bibcode=2019ApJ...875..114X |issn=0004-637X}}</ref>

|dist_footnote=<ref name=xu/>
}}
{{Starbox astrometry|no_heading=y
 | component1 = A or Aa1
 | absmag_v = −0.07<ref name=soderhjelm>{{cite journal|bibcode=1980A&A....89..100S|year = 1980|title = Geometry and dynamics of the Algol system|journal = Astronomy and Astrophysics|volume = 89|issue = 1–2|pages = 100|last1 = Soderhjelm|first1 = S.}}</ref>
}}
{{Starbox astrometry|no_heading=y
 | component1 = B or Aa2
 | absmag_v = 2.9<ref name=soderhjelm/>
}}
{{Starbox astrometry|no_heading=y
 | component1 = C or Ab
 | absmag_v = 2.3<ref name=soderhjelm/>
}}
{{Starbox orbit
| reference = <ref name=baron/>
| primary = A or Aa1
| name = B or Aa2
| period =
| period_unitless = 2.867328 days
| axis = 0.00215
| axis_unitless =
| eccentricity = 0
| inclination = 98.70
| node = 43.43
| periastron =
| periarg = <!--Argument of periastron (in degrees), secondary -->
| periarg_primary =
| k1 =
| k2 = <!-- Velocity semi-amplitude (secondary in SB2), in km/s -->
}}
{{Starbox orbit
| reference = <ref name=baron/>
| primary = Aa
| name = Ab
| period =
| period_unitless = 680.168 days
| axis = 0.09343
| axis_unitless =
| eccentricity = 0.227
| inclination = 83.66
| node = 132.66
| periastron = 2446927.22
| periarg = <!--Argument of periastron (in degrees), secondary -->
| periarg_primary = 310.02
| k1 =
| k2 = <!-- Velocity semi-amplitude (secondary in SB2), in km/s -->
}}
{{Starbox detail
 | component1 = A or Aa1
 | mass = 3.17 ± 0.21<ref name=baron>{{cite journal|bibcode=2012ApJ...752...20B|title=Imaging the Algol Triple System in the H Band with the CHARA Interferometer|journal=The Astrophysical Journal|volume=752|issue=1|pages=20|last1=Baron|first1=F.|last2=Monnier|first2=J. D.|last3=Pedretti|first3=E.|last4=Zhao|first4=M.|last5=Schaefer|first5=G.|last6=Parks|first6=R.|last7=Che|first7=X.|last8=Thureau|first8=N.|last9=Ten Brummelaar|first9=T. A.|last10=McAlister|first10=H. A.|last11=Ridgway|first11=S. T.|last12=Farrington|first12=C.|last13=Sturmann|first13=J.|last14=Sturmann|first14=L.|last15=Turner|first15=N.|year=2012|doi=10.1088/0004-637X/752/1/20|arxiv = 1205.0754 |s2cid=11110989}}</ref>
 | radius = 2.73 ± 0.20<ref name=baron/>
 | luminosity = 182<ref name=soderhjelm/>
 | temperature = 13,000<ref name=zavala>{{cite journal|bibcode=2010ApJ...715L..44Z|title=The Algol Triple System Spatially Resolved at Optical Wavelengths|journal=The Astrophysical Journal Letters|volume=715|issue=1|pages=L44–L48|last1=Zavala|first1=R. T.|last2=Hummel|first2=C. A.|last3=Boboltz|first3=D. A.|last4=Ojha|first4=R.|last5=Shaffer|first5=D. B.|last6=Tycner|first6=C.|last7=Richards|first7=M. T.|last8=Hutter|first8=D. J.|year=2010|doi=10.1088/2041-8205/715/1/L44|arxiv = 1005.0626 |s2cid=118573026}}</ref>
 | gravity = 4.0<ref name=zavala/>
 | metal =
 | rotational_velocity = 49<ref name=tomkin>{{cite journal|bibcode=1985PASP...97...51T|title=The rotation of the primary of Algol|journal=Publications of the Astronomical Society of the Pacific|volume=97|pages=51|last1=Tomkin|first1=J.|last2=Huisong|first2=T.|year=1985|doi=10.1086/131493|s2cid=119392903 |doi-access=free}}</ref>
 | age_myr = 570<ref name=soderhjelm/>
}}
{{Starbox detail|no_heading=y
 | component1 = B or Aa2
 | mass = {{val|0.70|0.08}}<ref name=baron/>
 | radius = {{val|3.48|0.28}}<ref name=baron/>
 | luminosity = 6.92<ref name=soderhjelm/>
 | temperature = 4,500<ref name=zavala/>
 | gravity = 3.5<ref name=zavala/>
 | metal =
 | rotation =
 | age =
}}
{{Starbox detail|no_heading=y
 | component1 = C or Ab
 | mass = {{val|1.76|0.15}}<ref name=baron/>
 | radius = {{val|1.73|0.33}}<ref name=baron/>
 | luminosity = 10.0<ref name=soderhjelm/>
 | temperature = 7,500<ref name=zavala/>
 | gravity = 4.5<ref name=zavala/>
 | metal =
 | rotation =
 | age =
}}
{{Starbox catalog
 | names = Algol, Gorgona, Gorgonea&nbsp;Prima, Demon&nbsp;Star, El&nbsp;Ghoul, β&nbsp;Persei, β&nbsp;Per, 26&nbsp;Persei, [[Star catalogue#BD/CD/CPD|BD]]+40°673, [[Fifth Fundamental Catalogue|FK5]]&nbsp;111, [[Boss General Catalogue|GC]]&nbsp;3733, [[Henry Draper catalogue|HD]]&nbsp;19356, [[Hipparcos catalogue|HIP]]&nbsp;14576, [[Bright Star Catalogue|HR]]&nbsp;936, [[PPM Star Catalogue|PPM]]&nbsp;45864, [[Smithsonian Astrophysical Observatory Star Catalog|SAO]]&nbsp;38592.
}}
{{Starbox reference
|Simbad = Algol
}}
{{Starbox end}}
'''Algol''' {{IPAc-en|ˈ|æ|l|g|ɒ|l}},<ref>{{OED|Algol}}</ref> designated '''Beta Persei''' ('''β&nbsp;Persei''', abbreviated '''Beta&nbsp;Per''', '''β&nbsp;Per'''), known colloquially as the '''Demon Star''', is a bright [[multiple star]] in the [[constellation]] of [[Perseus (constellation)|Perseus]] and one of the first non-[[nova]] [[variable star]]s to be discovered.

Algol is a [[triple star|three-star system]], consisting of Beta Persei&nbsp;Aa1, Aa2, and&nbsp;Ab – in which the hot luminous primary β Persei&nbsp;Aa1 and the larger, but cooler and fainter, β Persei&nbsp;Aa2 regularly pass in front of each other, causing eclipses. Thus Algol's [[apparent magnitude|magnitude]] is usually near-constant at&nbsp;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 [[occultation|occults]] the fainter secondary is very shallow and can only be detected photoelectrically.<ref>{{cite web |date=January 1999 | url = http://www.aavso.org/vstar/vsots/0199.shtml | title = Beta Persei (Algol) | publisher = AAVSO  | access-date = 31 July 2006 |archive-url = https://web.archive.org/web/20060708092301/http://www.aavso.org/vstar/vsots/0199.shtml |archive-date = 8 July 2006}}</ref>

Algol gives its name to its class of eclipsing variable, known as [[Algol variable]]s.

==Observation history==
[[File:Algol triple star system imaged with the CHARA interferometer.jpg|thumb|left|upright=1.6|The Algol system on 12 August 2009.  This is a [[CHARA array|CHARA]] interferometer image with {{sfrac|2}}-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.]]
[[File:Algol TESS lightcurve.png|thumb|left|Light curve of the Algol recorded by NASA's [[Transiting Exoplanet Survey Satellite]] (TESS).]]

An ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago is said to be the oldest historical documentation of the discovery of Algol.<ref>{{cite journal|display-authors=6|author=Porceddu, S.|author2=Jetsu, L.|author3=Lyytinen, J.|author4=Kajatkari, P.|author5=Lehtinen, J.|author6=Markkanen, T.|author7=Toivari-Viitala, J.|title=Evidence of Periodicity in Ancient Egyptian Calendars of Lucky and Unlucky Days|journal = Cambridge Archaeological Journal|volume =18|issue=3|date = 2008|pages = 327–339|doi=10.1017/S0959774308000395|bibcode = 2008CArcJ..18..327P |s2cid=162969143|url=https://zenodo.org/record/896419 }}</ref><ref>{{cite journal|display-authors=6|author=Jetsu, L.|author2=Porceddu, S.|author3=Lyytinen, J.|author4=Kajatkari, P.|author5=Lehtinen, J.|author6=Markkanen, T.|author7=Toivari-Viitala, J.|title=Did the Ancient Egyptians Record the Period of the Eclipsing Binary Algol - The Raging One? |journal = The Astrophysical Journal|volume =773|issue=1|date = 2013|pages = A1 (14pp)|doi = 10.1088/0004-637X/773/1/1|arxiv = 1204.6206 |bibcode = 2013ApJ...773....1J |s2cid=119191453}}</ref><ref>{{cite journal|author=Jetsu, L.|author2=Porceddu, S.|title=Shifting Milestones of Natural Sciences: The Ancient Egyptian Discovery of Algol's Period Confirmed|journal = PLOS ONE|volume = 10 | issue = 12 |date = 2015|pages = e.0144140 (23pp)|doi=10.1371/journal.pone.0144140|pmid=26679699|pmc=4683080|arxiv = 1601.06990 |bibcode = 2015PLoSO..1044140J |doi-access=free}}</ref>

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,<ref>{{cite journal|author=Wilk, Stephen R.| title=Mythological Evidence for Ancient Observations of Variable Stars |journal=The Journal of the American Association of Variable Star Observers  |volume= 24 |issue=2 |date=1996 |pages=129–33 |bibcode=1996JAVSO..24..129W}}</ref> but there is still no indisputable evidence for this.<ref>{{cite journal | last1 = Davis | first1 = G.A. | year = 1957 | title = Why did the Arabs Call Beta Persei "al-Ghul"? | journal = Sky and Telescope | volume = 16 | page = 177 | bibcode = 1957S&T....16..177D }}</ref> The Arabic astronomer [[al-Sufi]] said nothing about any variability of the star in his ''[[Book of Fixed Stars]]'' published c.964.<ref>{{cite web| url = http://www.ianridpath.com/startales/perseus.html#algol| title = Ian Ridpath's ''Star Tales'' – Perseus}}</ref>

The variability of Algol was noted in 1667 by Italian astronomer [[Geminiano Montanari]],<ref>G.&nbsp;Montanari, "Sopra la sparizione d'alcune stelle et altre novità celesti", in: ''Prose de Signori Accademici Gelati di Bologna'' (Bologna: Manolessi, 1671), pp.&nbsp;369–92 ([https://books.google.com/books?id=TfgLF4-BsqEC Google books]).</ref> 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.<ref>[http://adsabs.harvard.edu/abs/1957Obs....77..191E ADS O.J.&nbsp;Eggen,"An Eighteenth Century Discussion of Algol", ''The Observatory'', '''77''' (1957), 191–197].</ref><ref>{{Cite journal|last=Goodricke|first=John|date=1783-01-01|title=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|journal=Philosophical Transactions of the Royal Society of London|volume=73|pages=474–482|doi=10.1098/rstl.1783.0027|doi-access=free}}</ref> 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]].<ref>{{cite web | date = 6 August 2003 | url = http://www.surveyor.in-berlin.de/himmel/Bios/Goodricke-e.html | title = John Goodricke, The Discovery of the Occultating Variable Stars  | access-date = 31 July 2006 | archive-url= https://web.archive.org/web/20060622004628/http://www.surveyor.in-berlin.de/himmel/Bios/Goodricke-e.html| archive-date= 22 June 2006 | url-status= live}}</ref>

In 1881, the Harvard astronomer [[Edward Charles Pickering]] presented evidence that Algol was actually an eclipsing binary.<ref>{{cite journal | last = Pickering | first = Edward C. | title=Dimensions of the Fixed Stars, with especial reference to Binaries and Variables of the Algol type | journal=Astronomical Register | date=1881 | pages=253–56 | bibcode=1881AReg...19..253. | issue = 1–2 | volume=50}}</ref> This was confirmed a few years later, in 1889, when the Potsdam astronomer [[Hermann Carl Vogel]] found periodic [[doppler shift]]s in the spectrum of Algol, inferring variations in the [[radial velocity]] of this binary system.<ref>{{cite journal | author = A. H. Batten | title=Two Centuries of Study of Algol Systems | journal=Space Science Reviews  | date=1989 | volume=50 | issue=1/2 | pages=1–8 | bibcode=1989SSRv...50....1B  | doi = 10.1007/BF00215914 | s2cid=125814848 }}</ref> Thus, Algol became one of the first known [[spectroscopic binary|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.<ref>{{cite journal | author = J. Stebbins | title = The Measurement of the Light of Stars with a Selenium Photometer with an Application to the Variation of Algol |journal= Astrophysical Journal | date=1910 | volume=32 | pages=185–214|doi=10.1086/141796 |bibcode=1910ApJ....32..185S| doi-access=free }}</ref> 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.<ref>Meltzer, Alan S., A "Spectroscopic Investigation of Algol". ''Astrophysical Journal'', vol. 125, (1957), p.359, [http://adsabs.harvard.edu/abs/1957ApJ...125..359M BibCode:1957ApJ...125..359M]</ref>

==System==
[[File:Algol AB movie imaged with the CHARA interferometer - labeled.gif|thumb|left|Algol Aa2 orbits Algol Aa1. This animation was assembled from 55 images of the [[CHARA array|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.]] [[File:Algol Bahninterpolation.gif|thumb|left|alt=interpolation|Interpolation of the orbit of Aa2 around Aa1 with focus on Aa1.]]
Algol is a multiple-star system with three confirmed and two suspected stellar components.<ref>{{cite journal|arxiv=2005.13360|last1=Jetsu|first1=L.|title=Say Hello to Algol's New Companion Candidates|journal=The Astrophysical Journal |year=2021|volume=920 |issue=2 |page=137 |doi=10.3847/1538-4357/ac1351 |bibcode=2021ApJ...920..137J |s2cid=239882152 |doi-access=free }}</ref> From the point of view of the Earth, Algol&nbsp;Aa1 and Algol&nbsp;Aa2 form an [[eclipsing binary]] because their [[orbital plane (astronomy)|orbital plane]] contains the  [[line-of-sight propagation|line of sight]] to the Earth. The eclipsing binary pair is separated by only 0.062&nbsp;[[astronomical unit]]s (au) from each other, whereas the third star in the system (Algol&nbsp;Ab) is at an average distance of 2.69&nbsp;au from the pair, and the mutual [[orbital period]] of the trio is 681&nbsp;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.<ref name=wds>{{cite journal|bibcode=2001AJ....122.3466M|title=The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog|journal=The Astronomical Journal|volume=122|issue=6|pages=3466–3471|last1=Mason|first1=Brian D.|last2=Wycoff|first2=Gary L.|last3=Hartkopf|first3=William I.|last4=Douglass|first4=Geoffrey G.|last5=Worley|first5=Charles E.|year=2001|doi=10.1086/323920|doi-access=free}}</ref>

The close pair consists of a B8 [[main sequence]] star and a much less massive K0 [[subgiant]], which is highly distorted by the more massive star.  These two orbit every 2.9 days and undergo the eclipses that cause Algol to vary in brightness.  The third star orbits these two every 680 days and is an A or F-type main sequence star.  It has been classified as an [[Am star]], but this is now considered doubtful.<ref name=frank2022>{{cite journal |bibcode=2022JAVSO..50..123F |title=Spectral Classification of Algol C |last1=Frank |first1=M. G. |last2=Whelan |first2=D. G. |last3=Junginger |first3=J. C. |journal=Journal of the American Association of Variable Star Observers (Jaavso) |year=2022 |volume=50 |issue=1 |page=123 |arxiv=2205.06229 }}</ref><ref name=renson2009>{{cite journal
 | last1=Renson | first1=P. | last2=Manfroid | first2=J.
 | title=Catalogue of Ap, HgMn and Am stars
 | journal=Astronomy and Astrophysics
 | volume=498 | issue=3 | pages=961–966 | date=May 2009
 | doi=10.1051/0004-6361/200810788 | bibcode=2009A&A...498..961R | doi-access=free }}</ref>

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&nbsp;Aa1 is still in the [[main sequence]], but the less massive Algol&nbsp;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.<ref>{{cite journal | first=Izold | last=Pustylnik | title=On Accretion Component of the Flare Activity in Algol | journal=Baltic Astronomy | date=1995 | volume=4 | issue=1–2 | pages=64–78 | bibcode=1995BaltA...4...64P | doi=10.1515/astro-1995-0106 | doi-access=free }}</ref> The gas flow between the primary and secondary stars in Algol has been imaged using Doppler [[Tomography]].<ref>{{Cite journal|last1=Richards|first1=Mercedes T.|last2=Albright|first2=Geary E.|last3=Bowles|first3=Larissa M.|date=January 1995|title=Doppler tomography of the gas stream in short-period Algol binaries|journal=The Astrophysical Journal|language=en|volume=438|pages=L103|doi=10.1086/187726|bibcode=1995ApJ...438L.103R|issn=0004-637X|doi-access=free}}</ref><ref>{{Cite journal|last1=Richards|first1=Mercedes T.|last2=Agafonov|first2=Michail I.|last3=Sharova|first3=Olga I.|date=2012-11-20|title=NEW EVIDENCE OF MAGNETIC INTERACTIONS BETWEEN STARS FROM THREE-DIMENSIONAL DOPPLER TOMOGRAPHY OF ALGOL BINARIES: β PER AND RS VUL|journal=The Astrophysical Journal|volume=760|issue=1|pages=8|doi=10.1088/0004-637X/760/1/8|arxiv=1210.0081|bibcode=2012ApJ...760....8R|s2cid=118319759|issn=0004-637X}}</ref>

This system also exhibits [[x-ray]] and [[radio wave]]<ref>{{Cite journal|last1=Wade|first1=C. M.|last2=Hjellming|first2=R. M.|date=February 4, 1972|title=Radio Stars Beta Persei and Beta Lyrae|journal=Nature|language=en|volume=235|issue=5336|pages=270–271|doi=10.1038/235270a0|bibcode=1972Natur.235..270W|s2cid=4222515|issn=0028-0836}}</ref> 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.<ref>{{cite journal | author=M.J. Sarna | author2=S.K. Yerli | author3=A.G. Muslimov | title=Magnetic Activity and Evolution of Algol-type Stars - II | journal=[[Monthly Notices of the Royal Astronomical Society]] | date=1998 | volume=297 | issue=3 | pages=760–68 | bibcode=1998MNRAS.297..760S | doi=10.1046/j.1365-8711.1998.01539.x | doi-access=free }}</ref> The radio-wave flares might be created by magnetic cycles similar to those of [[sunspot]]s, 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.<ref>{{cite web | last = Blue | first = Charles E. | date = 3 June 2002 | url = http://www.nrao.edu/pr/2002/algol/ | title = Binary Stars "Flare" With Predictable Cycles, Analysis of Radio Observations Reveals | publisher = National Radio Astronomy Observatory  | access-date = 31 July 2006 | archive-url= https://web.archive.org/web/20060702231546/http://www.nrao.edu/pr/2002/algol/| archive-date= 2 July 2006 | url-status= live}}</ref><ref>{{Cite journal|last1=Richards|first1=Mercedes T.|last2=Waltman|first2=Elizabeth B.|last3=Ghigo|first3=Frank D.|last4=Richards|first4=Donald St. P.|date=August 2003|title=Statistical Analysis of 5 Year Continuous Radio Flare Data from β Persei, V711 Tauri, δ Librae, and UX Arietis|journal=The Astrophysical Journal Supplement Series|language=en|volume=147|issue=2|pages=337–361|doi=10.1086/375835|bibcode=2003ApJS..147..337R|issn=0067-0049|doi-access=free}}</ref> The secondary component was identified as the radio emitting source in Algol using [[Very-long-baseline interferometry]] by Lestrade and co-authors.<ref name=lestrade1993>{{Cite journal|last1=Lestrade|first1=Jean-Francois|last2=Phillips|first2=Robert B.|last3=Hodges|first3=Mark W.|last4=Preston|first4=Robert A.|date=June 1993|title=VLBI astrometric identification of the radio emitting region in Algol and determination of the orientation of the close binary|journal=The Astrophysical Journal|language=en|volume=410|pages=808|doi=10.1086/172798|issn=0004-637X|bibcode=1993ApJ...410..808L}}</ref>

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 {{sfrac|ΔP|P}}&nbsp;≈&nbsp;{{val|e=−5}} via the [[Applegate mechanism]].<ref name="Applegate 1992">{{cite journal |last1=Applegate |first1=James H. |title=A mechanism for orbital period modulation in close binaries|journal=Astrophysical Journal, Part 1 |date=1992 |volume=385 |pages=621–629|bibcode=1992ApJ...385..621A|doi = 10.1086/170967 |doi-access=free }}</ref> [[Mass transfer#Astrophysics|Mass transfer]] between the components is small in the Algol system<ref>{{cite thesis |last=Wecht|first=Kristen |arxiv=astro-ph/0611855 |title=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 |date=2006 |bibcode=2006PhDT........10W }}</ref> but could be a significant source of period change in other [[Algol variable|Algol-type binaries]].
[[File:1e9m comparison Gamma Orionis, Algol B, the Sun, and smaller - antialiased no transparency.png|left|thumb|Size comparison between the [[Sun]] (bottom middle), Algol Aa2 (right) and the [[blue giant]] [[Bellatrix]] (left).]]
The distance to Algol has been measured using very-long baseline [[interferometry]], giving a value of 94&nbsp;[[light-year]]s.<ref name=xu/> About 7.3&nbsp;million years ago it passed within 9.8&nbsp;light-years of the [[Solar System]]<ref>{{cite conference
 | display-authors=4| author=Garcia-Sanchez, J.
 | author2=Preston, R. A.
 | author3=Jones, D. L.
 | author4=Lestrade, J.-F.
 | author5=Weissman, P. R.
 | author6=Latham, D. W.
 | title =A Search for Stars Passing Close to the Sun
 | book-title =The First Results of Hipparcos and Tycho
 | publisher =IAU | date =25 August 1997 | location =Kyoto, Japan
 | bibcode =1997IAUJD..14E..51G
 }}<!--| access-date = 1 June 2007--></ref> 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&nbsp;solar masses, at the closest approach this might have given enough [[gravity]] to [[Perturbation (astronomy)|perturb]] the [[Oort cloud]] of the Solar System somewhat and hence increase the number of [[comet]]s entering the inner Solar System. However, the actual increase in net cometary collisions is thought to have been quite small.<ref>{{cite journal | author = J. García-Sánchez | author2 = R.A. Preston | author3 = D.L. Jones | author4 = P.R. Weissman | title=Stellar Encounters with the Oort Cloud Based on Hipparcos Data | journal=The Astronomical Journal | date=1999 | volume=117 | issue = 2 | pages=1042–55 | doi=10.1086/300723 | bibcode=1999AJ....117.1042G| s2cid = 122929693 | doi-access=free }}</ref>

==Names==
[[File:PerseusCC.jpg|thumb|right|upright|Algol is a bright star in the constellation of Perseus (upper right).]]

''Beta Persei'' is the star's [[Bayer designation]].

===The official name Algol===
The name ''Algol'' derives from [[Arabic language|Arabic]] {{lang|ar|رأس الغول}} ''raʾs al-ghūl'' : head (''raʾs'') of the ogre (''al-ghūl'') (see "[[ghoul]]").<ref>P.&nbsp;Kunitzsch & T.&nbsp;Smart, ''Short Guide to Modern Star Names and Their Derivations'' (Wiesbaden: Otto Harrassowitz, 1986), p 49.</ref> The English name Demon Star was taken from the Arabic name.<ref name=allen1899>{{cite book | last=Allen | first=R. H. | date=1963 | orig-year=1899 | author-link=Richard Hinckley Allen | title=Star Names: Their Lore and Meaning | url=https://archive.org/details/starnamestheirlo00alle/page/331 | edition=Reprint | publisher=Dover Publications Inc. | location=New York | isbn=978-0-486-21079-7 | page=[https://archive.org/details/starnamestheirlo00alle/page/331 331] | url-access=registration }}</ref> In 2016, the [[International Astronomical Union]] organized a [[IAU Working Group on Star Names|Working Group on Star Names]] (WGSN)<ref name="WGSN">{{cite web | url=https://www.iau.org/science/scientific_bodies/working_groups/280/ | title=IAU Working Group on Star Names (WGSN)|access-date=22 May 2016}}</ref> to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016<ref name="WGSN1">{{cite web | url=http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin1.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin1.pdf |archive-date=2022-10-09 |url-status=live | title=Bulletin of the IAU Working Group on Star Names, No. 1 |access-date=28 July 2016}}</ref> 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.<ref name="IAU-CSN">{{cite web | url=http://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt | title=IAU Catalog of Star Names |access-date=28 July 2016}}</ref>

===Ghost and demon star===
Algol was called ''Rōsh ha Sāṭān'' or "Satan's Head" in Hebrew folklore, 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".<ref name=allen1899/> [[Hipparchus]] and [[Pliny the Elder|Pliny]] made this a separate, though connected, constellation.<ref name=allen1899/>

===First star of Medusa's head===
Earlier the name of the constellation ''Perseus'' was ''Perseus and Medusa's Head'' where an asterism representing the head of Medusa after Perseus has cut it off already known in ancient Rome.<ref name="vitru9cap4.2">{{cite web | url=https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Vitruvius/9*.html#4.2 | title=Marcus Vitruvius Pollio: de Architectura, Book IX | access-date=8 October 2023}}</ref> Medusa is a [[gorgon]] so the star is also called ''Gorgonea Prima'' meaning the first star of the gorgon.<ref name=allen1899/>

===Chinese names===
In [[Chinese astronomy|Chinese]], {{lang|zh|大陵}} ({{lang|zh-Latn|Dà Líng}}), meaning ''[[Stomach (Chinese constellation)|Mausoleum]]'', refers to an asterism consisting of β Persei, [[9 Persei]], [[Tau Persei|τ Persei]], [[Iota Persei|ι Persei]], [[Kappa Persei|κ Persei]], [[Rho Persei|ρ Persei]], [[16 Persei]] and [[12 Persei]]. Consequently, the [[Chinese star names|Chinese name]] for β Persei itself is {{lang|zh|大陵五}} ({{lang|zh-Latn|Dà Líng wu}}, English: The Fifth Star of Mausoleum.).<ref>{{in lang|zh}} [http://aeea.nmns.edu.tw/2006/0607/ap060711.html AEEA (Activities of Exhibition and Education in Astronomy) <span lang="zh>天文教育資訊網2006年7月11日</span>] {{Webarchive|url=https://web.archive.org/web/20120204114123/http://aeea.nmns.edu.tw/2006/0607/ap060711.html |date=2012-02-04 }}</ref> According to R.H. Allen the star bore the grim name of ''Tseih She'' {{lang|zh|積屍}} ({{lang|zh-Latn|Zhi Shī}}), meaning "Piled up Corpses"<ref name=allen1899/> but this appears to be a misidentification, and ''Dié Shī'' is correctly [[Pi Persei|π Persei]], which is inside the Mausoleum.<ref>{{cite web| url = http://www.ianridpath.com/startales/perseus.htm#chinese| title = Ian Ridpath's ''Star Tales'' – Perseus}}</ref>

==Observing Algol==

The Algol system usually has an apparent magnitude of 2.1, similar to those of [[Mirfak]] (α Persei) at 1.9 and [[Almach]] (γ [[Andromeda (constellation)|Andromedae]]) at 2.2, with whom it forms a right triangle. During eclipses it dims to 3.4, making it as faint as nearby [[Rho Persei|ρ Persei]] at 3.3.
{| class="wikitable"
 |+ Observing Algol's Eclipses in 2025<ref>Edgar, James S. (editor). ''Observer's Handbook 2025''. The Royal Astronomical Society of Canada. {{ISBN|9-781927-879375}}.</ref><ref>{{cite web|url=https://www.skyandtelescope.com/observing/celestial-objects-to-watch/the-minima-of-algol/ |title=The Minima of Algol |last=Ashford |first=Adrian |date= 30 July 2006|accessdate=2024-12-04}}</ref>
 |-
 ! Date
 ! Time
 |-
 ! December 1, 2024
 | 18:52
 |-
 ! January 2, 2025
 | 07:53
 |-
 ! February 2, 2025
 | 20:55 
 |-
 ! March 3, 2025
 | 13:09 
 |-
 ! April 1, 2025
 | 05:22
 |-
 ! May 2, 2025
 | 18:23
 |-
 ! June 3, 2025
 | 07:22 
 |-
 ! July 1, 2025
 | 23:30
 |-
 ! August 2, 2025
 | 12:25
 |-
 ! September 3, 2025
 | 17:35
 |-
 ! October 1, 2025
 | 01:20 
 |-
 ! November 2, 2025
 | 06:23 
 |-
 ! December 3, 2025
 | 19:22
 |}

[[image:Perseus - Prodromus astronomiae 1690 (5590393).jpg |thumb |The constellation Perseus and Algol, the Bright Star in the Gorgon's head ([[Johannes Hevelius]], ''Uranographia'', 1690)]]
Listed are the first eclipse dates and times of each month, with all times in UT. β Persei Aa2 eclipses β Persei Aa1 every 2.867321 days (2 days 20 hours 49 min). To determine subsequent eclipses, add this interval to each listed date and time. For example, the Jan 2 eclipse at 8h will result in consecutive eclipse times on Jan 5 at 5h, Jan 8 at 1h, Jan 10 at 22h, and so on (all times approximate).

==Cultural significance==
{{see also|Stars in astrology#Algol|label 1=Algol in astrology}}
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 (constellation)|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]].<ref>Robbins, Frank E. (ed.) 1940. ''Ptolemy: Tetrabiblos''. Cambridge, Massachusetts: Harvard University Press (Loeb Classical Library). {{ISBN|0-674-99479-5}}, IV.9, p.435.</ref> In the [[astrology]] of [[fixed stars]], Algol is considered one of the [[luck|unluckiest]] stars in the sky,<ref name=allen1899/> and was listed as one of the 15 [[Behenian fixed star|Behenian stars]].<ref name=Tyson>{{cite book|author=Henry Cornelius Agrippa|title=Three Books of Occult Philosophy|year=1898|url=https://archive.org/details/threebooksofoccu00agri}} Lyons, 1531/33. Llewellyn reprint, 1993; tr. J. Freake (1651), ed. D. Tyson, p.411.</ref>

==See also==
* [[Jaana Toivari-Viitala]], egyptologist who contributed to understanding Ancient Egypt and the star
{{clear}}
==References==
{{Reflist}}

==External links==
{{EB1911 poster|Algol}}
* {{cite web | url = http://www.solstation.com/stars2/algol3.htm | title = Algol 3 | publisher = SolStation  | access-date = 31 July 2006}}
* {{cite web | date = 4 March 1998 | url = http://www.ari.uni-heidelberg.de/aricns/cnspages/4c02517.htm | title = 4C02517 | publisher = ARICNS  | access-date = 31 July 2006|archive-url = https://web.archive.org/web/20060210103512/http://www.ari.uni-heidelberg.de/aricns/cnspages/4c02517.htm |archive-date = 10 February 2006}}
* {{cite web | url = http://www.alcyone.de/SIT/mainstars/SIT000666.htm | title = Algol | publisher = Alcyone ephemeris  | access-date = 8 June 2006}}
* {{cite web | author = Bezza, Giuseppe| others = Translated by Daria Dudziak | url = http://www.cieloeterra.it/eng/eng.articoli.algol/eng.algol.html | title = Al-ghûl, the ogre | publisher = Cielo e Terra  | access-date = 8 June 2006 | archive-url= https://web.archive.org/web/20060620085229/http://www.cieloeterra.it/eng/eng.articoli.algol/eng.algol.html| archive-date= 20 June 2006 | url-status= live}}
{{Sky|03|08|10.1315|+|40|57|20.332|93}}

{{Stars of Perseus}}
{{Portal bar|Astronomy|Stars|Outer space}}

{{DEFAULTSORT:Algol}}
[[Category:Algol variables]]
[[Category:Bayer objects|Persei, Beta]]
[[Category:B-type main-sequence stars]]
[[Category:Flamsteed objects|Persei, 26]]
[[Category:Henry Draper Catalogue objects|019356]]
[[Category:Hipparcos objects|014576]]
[[Category:K-type subgiants]]
[[Category:Perseus (constellation)]]
[[Category:Stars with proper names|Algol]]
[[Category:Triple star systems]]
[[Category:Astronomical objects known since antiquity]]
[[Category:Bright Star Catalogue objects|0936]]
[[Category:Durchmusterung objects|BD+40 0673]]
[[Category:Am stars]]
[[Category:F-type main-sequence stars]]