Editing Algol (section)

==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>