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{{short description|Eclipsing variable star in the constellation Perseus}} | |||
{{Otheruses}} | |||
{{about|the star|the programming languages|ALGOL|other uses}} | |||
{{Starbox begin | | |||
{{good article}} | |||
name=Beta Persei A/B/C }} | |||
{{Use British English|date=February 2017}} | |||
{{Starbox image | | |||
{{Starbox begin | |||
image=] | | |||
|
| name = Algol | ||
}} | |||
{{Starbox observe | | |||
{{Starbox image | |||
epoch=J2000 | | |||
| image = | |||
ra={{RA|03|08|10.1315}} | | |||
{{Location mark | |||
dec={{DEC| +40|57|20.332}} | | |||
|image = Perseus constellation map.svg | |||
appmag_v=2.12 | | |||
|float = center | |||
constell=] }} | |||
|alt = | |||
{{Starbox character | | |||
|label = | |||
class=B8V/K02IV/A5V | | |||
|position = right | |||
b-v=-0.05 | | |||
|width = 280 | |||
u-b=-0.37 | | |||
|mark = Red circle.svg | |||
variable=] }} | |||
|mark_width = 12 | |||
{{Starbox astrometry | | |||
|mark_link = Algol | |||
radial_v=3.7 | | |||
|x = 644 | |||
prop_mo_ra=2.39 | | |||
|y = 663 | |||
prop_mo_dec=-1.44 | | |||
}} | |||
parallax=35.14 | | |||
|caption = Location of β Persei (circled) | |||
p_error=0.90 | | |||
}} | |||
absmag_v=-0.15 }} | |||
{{Starbox |
{{Starbox observe | ||
| epoch = J2000 | |||
mass=3.59/0.79/1.67 | | |||
| ra = {{RA|03|08|10.13245}}<ref name=hipparcos/> | |||
radius=2.3/3.0/0.9 | | |||
| 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> | |||
luminosity=98/3.4/4.1 | | |||
| 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>) | |||
temperature=12,000/4,500/8,500 | | |||
| constell=] | |||
metal=''Not available'' | | |||
}} | |||
rotation=65 km/s. | | |||
{{Starbox character | |||
age=< 3 × 10<sup>8</sup> }} | |||
| engvar = en-UK | |||
{{Starbox catalog | | |||
| 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>)}} | |||
names=Algol, Gorgona, Gorgonea Prima, Demon Star, El Ghoul, 26 Per, ] 9110, ] 936, ] +40°673, ] 19356, ] 646.00, ] 38592, FK5 111, Wo 9110, ADS 2362, WDS 03082+4057A, ] 14576. }} | |||
| b-v=−0.05<ref name=ducati/> | |||
| u-b=−0.37<ref name=ducati/> | |||
| variable=]<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 = β Per 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 = β Per Aa2 | |||
| absmag_v = 2.9<ref name=soderhjelm/> | |||
}} | |||
{{Starbox astrometry|no_heading=y | |||
| component1 = β Per Ab | |||
| absmag_v = 2.3<ref name=soderhjelm/> | |||
}} | |||
{{Starbox orbit | |||
| reference = <ref name=baron/> | |||
| primary = β Per Aa1 | |||
| name = β Per 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 = β Per A | |||
| name = β Per B | |||
| 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 = β Per 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 = β Per 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 = β Per 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 Prima, Demon Star, El Ghoul, β Persei, β Per, 26 Persei, ]+40°673, ] 111, ] 3733, ] 19356, ] 14576, ] 936, ] 45864, ] 38592. | |||
}} | |||
{{Starbox reference | |||
|Simbad = Algol | |||
}} | |||
{{Starbox end}} | {{Starbox end}} | ||
'''Algol''' |
'''Algol''' {{IPAc-en|ˈ|æ|l|g|ɒ|l}},<ref>{{OED|Algol}}</ref> designated '''Beta Persei''' ('''β Persei''', abbreviated '''Beta Per''', '''β Per'''), known colloquially as the '''Demon Star''', is a bright ] in the ] of ] and one of the first non-] ]s to be discovered. | ||
Algol is a ], 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 ] 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 ] 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> | |||
==History== | |||
The name Algol means "demon star," (from ] '''الغول''' ''al-ghūl'', "the ]") which was probably given due to its peculiar behavior.<ref>{{cite book | first=Richard Hinckley | last=Allen | title=Star Names: Their Lore and Meaning (revised edition) | publisher=Dover | year=1963 | id=ISBN 0486210790 }}</ref> In the ] Perseus, it represents the eye of the ] ]. | |||
Algol gives its name to its class of eclipsing variable, known as ]s. | |||
], Algol is considered the most ] star in the sky. In the ] it was one of the 15 ], associated with the ] and ], and marked with the ] sign ]. | |||
==Observation history== | |||
The variability of Algol was first recorded in ] by ], but it is probable that this property was noticed long before this time. The first person to propose a mechanism for the variability of this star was the British amateur astronomer ]. In May, 1783 he presented his findings to the ], 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 ]. <ref>{{cite web | date = 2003-08-06 | url = http://www.surveyor.in-berlin.de/himmel/Bios/Goodricke-e.html | title = John Goodricke, The Discovery of the Occultating Variable Stars | language = English | accessdate = 2006-07-31 }}</ref> | |||
] 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.]] | |||
] (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> | |||
In 1881, the Harvard astronomer ] 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 | year=1881 | pages=253-256 | url=http://adsabs.harvard.edu/abs/1881AReg...19..253. }}</ref> This was confirmed a few years later, in 1889, when the Potsdam astronomer ] found periodic ]s in the spectrum of Algol, infering variations in ] a this binary system. <ref>{{cite journal | author = A. H. Batten | title=Two Centuries of Study of Algol Systems | journal=Space Science Reviews | year=1989 | volume=50 | issue=1/2 | pages=1-8 | url=http://adsabs.harvard.edu/abs/1989SSRv...50....1B }}</ref> Thus Algol became one of the first known ]. | |||
<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 (] in the Greek tradition, ] 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 ] said nothing about any variability of the star in his '']'' published c.964.<ref>{{cite web| url = http://www.ianridpath.com/startales/perseus.html#algol| title = Ian Ridpath's ''Star Tales'' – Perseus}}</ref> | |||
==Star system== | |||
As an eclipsing binary, it is actually two stars in close orbit around one another. Because the ] coincidentally contains the ]'s line of sight, the dimmer star (Algol B) passes in front of the brighter star (Algol A) once per orbit, and the amount of light reaching Earth is temporarily decreased. To be more precise, however, Algol happens to be a triple star system: the eclipsing binary pair is separated by only 0.062 ], while the third star (Algol C) is at an average distance of 2.69 AU from the pair and the mutual ] 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. | |||
The variability of Algol was noted in 1667 by Italian astronomer ],<ref>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 ().</ref> but the periodic nature of its variations in brightness was not recognized until more than a century later, when the British amateur astronomer ] also proposed a mechanism for the star's variability.<ref>.</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 ], 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 ].<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> | |||
{| class="wikitable" style="margin: 1em auto 1em auto" | |||
|+ ''Orbital Elements of the Algol System'' | |||
!Components | |||
!] | |||
!] | |||
!] | |||
!] | |||
|- | |||
!A—B<ref>{{cite journal | author = L. A. Molnar, R. L. Mutel | title=Dynamical Evolution of the Algol Triple System | journal=Bulletin of the American Astronomical Society | year=1996 | volume=28 | pages=921 | url=http://adsabs.harvard.edu/abs/1996AAS...188.6014M }}</ref> | |||
|align="center"| 0.00218″ | |||
|align="center"| 0.00 | |||
|align="center"| 2.87 days | |||
|align="center"| 97.69° | |||
|- | |||
!(AB)—C<ref>{{cite web | author = W.I. Hartkopf, B.D. Mason | date = 2006-07-30 | url =http://ad.usno.navy.mil/wds/orb6.html | title = Sixth Catalog of Orbits of Visual Binary Stars | publisher = U.S. Naval Observatory | language = English | accessdate = 2006-07-31 }}</ref> | |||
|align="center"| 0.09461″ | |||
|align="center"| 0.225 | |||
|align="center"| 680.05 days | |||
|align="center"| 83.98° | |||
|} | |||
In 1881, the Harvard astronomer ] 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 ] found periodic ]s in the spectrum of Algol, inferring variations in the ] 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 ]. ] at the ] 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, </ref> | |||
Studies of Algol led to the '''Algol paradox''' in the theory of ]: 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 ], while the less massive Algol B is a ] at a later evolutionary stage. The paradox can be solved by ]: when the more massive star became a subgiant, it filled its ], 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 actually be seen.<ref>{{cite journal | first=Izold | last=Pustylnik | title=On Accretion Component of the Flare Activity in Algol | journal=Baltic Astronomy | year=1995 | volume=4 | pages=64-78 | url=http://adsabs.harvard.edu/abs/1995BaltA...4...64P }}</ref> | |||
==System== | |||
This system also exhibits variable activity in the form of ] and ] flares. The former is thought to be caused by the magnetic fields of the AB components interacting with the mass transfer.<ref>{{cite journal | author=M.J. Sarna, S.K. Yerli, A.G. Muslimov | title=Magnetic activity and evolution of Algol-type stars - II | journal=Monthly Notices of the Royal Astronomical Society | year=1998 | volume=297 | Issue=3 | pages=760-768 | url=http://adsabs.harvard.edu/abs/1998MNRAS.297..760S }}</ref> The radio emissions may be created by magnetic cycles similar to ], but, as the magnetic fields around these stars are up to ten times stronger than that of the ], these radio flares are more powerful and longer lasting.<ref>{{cite web | last = Blue | first = Charles E. | date = 2002-06-03 | 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 | language = English | accessdate = 2006-07-31 }}</ref> | |||
] 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.]] ] | |||
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 Aa1 and Algol Aa2 form an ] because their ] contains the ] to the Earth. The eclipsing binary pair is separated by only 0.062 ]s (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 ] 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 ] lists them as Aa1, Aa2, and Ab, with two very faint stars B and C about one ] 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> | |||
Algol is 92.8 ]s from Earth; however, about 7.3 million years ago it passed within 9.8 light years and its ] was approximately −2.5, considerably brighter than ] 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 slightly perturb the ]'s ] and increase the number of ]s entering the inner solar system. However, the actual increase in net cratering rate is believed to have been quite small.<ref>{{cite journal | author = J. García-Sánchez, R.A. Preston, D.L. Jones, P.R. Weissman | title=Stellar Encounters with the Oort Cloud Based on Hipparcos Data | journal=The Astronomical Journal | year=1999 | volume=117 | pages=1042-1055 | url=http://www.journals.uchicago.edu/AJ/journal/issues/v117n2/980216/980216.html }}</ref> | |||
The close pair consists of a B8 ] star and a much less massive K0 ], 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 ], 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 ] in the theory of ]: 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 ], but the less massive Algol Aa2 is a ] at a later evolutionary stage. The paradox can be solved by ]: when the more massive star became a subgiant, it filled its ], 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 ].<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 ] and ]<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=] | 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 ]s, but because the magnetic fields of these stars are up to ten times stronger than the field of the ], 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 ] 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}} ≈ {{val|e=−5}} via the ].<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> ] 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 ]. | |||
] (bottom middle), Algol Aa2 (right) and the ] ] (left).]] | |||
The distance to Algol has been measured using very-long baseline ], giving a value of 94 ]s.<ref name=xu/> About 7.3 million years ago it passed within 9.8 light-years of the ]<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 ] was about −2.5, which is considerably brighter than the star ] 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 ] to ] the ] of the Solar System somewhat and hence increase the number of ]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== | |||
] | |||
''Beta Persei'' is the star's ]. | |||
===The official name Algol=== | |||
The name ''Algol'' derives from ] {{lang|ar|رأس الغول}} ''raʾs al-ghūl'' : head (''raʾs'') of the ogre (''al-ghūl'') (see "]").<ref>P. Kunitzsch & T. 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= | url-access=registration }}</ref> In 2016, the ] organized a ] (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 ], who called it "Divels head" or ''Rosch hassatan''. A ] name for Algol from the 16th century was ''Caput Larvae'' or "the Spectre's Head".<ref name=allen1899/> ] and ] 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 ] so the star is also called ''Gorgonea Prima'' meaning the first star of the gorgon.<ref name=allen1899/> | |||
===Chinese names=== | |||
In ], {{lang|zh|大陵}} ({{lang|zh-Latn|Dà Líng}}), meaning '']'', refers to an asterism consisting of β Persei, ], ], ], ], ], ] and ]. Consequently, the ] 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}} {{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 ], 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 ] (α Persei) at 1.9 and ] (γ ]) at 2.2, with whom it forms a right triangle. During eclipses it dims to 3.4, making it as faint as nearby ] 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 | |||
|} | |||
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}} | |||
{{Infobox writer | |||
|name = The constellation Perseus and Algol, the Bright Star in the Gorgon's head | |||
|image = Perseus Hevelius.jpg | |||
|imagesize = 300px | |||
|caption = ], ''Uranographia'', 1690}} | |||
Historically, the star has received a strong association with bloody violence across a wide variety of cultures. In the '']'', the 2nd-century astrological text of the Alexandrian astronomer ], Algol is referred to as "the ] of ]" and associated with death by decapitation: a theme which mirrors the myth of the hero ]'s victory over the snake-haired ] ].<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 ] of ], Algol is considered one of the ] stars in the sky,<ref name=allen1899/> and was listed as one of the 15 ].<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== | |||
* ], egyptologist who contributed to understanding Ancient Egypt and the star | |||
==References== | ==References== | ||
{{Reflist}} | |||
<div class="references-small"> | |||
<references/> | |||
</div> | |||
==External links== | ==External links== | ||
{{EB1911 poster|Algol}} | |||
* {{cite web | url = http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=Beta+Persei&NbIdent=1&Radius=10&Radius.unit=arcmin&CooFrame=FK5&CooEpoch=2000&CooEqui=2000&output.max=all&o.catall=on&output.mesdisp=N&Bibyear1=1983&Bibyear2=2006&Frame1=FK5&Frame2=FK4&Frame3=G&Equi1=2000.0&Equi2=1950.0&Equi3=2000.0&Epoch1=2000.0&Epoch2=1950.0&Epoch3=2000.0 | title = HD 19356 -- Eclipsing binary of Algol type | publisher = SIMBAD| language = English | accessdate = 2006-07-31 }} | |||
* {{cite web |
* {{cite web | url = http://www.solstation.com/stars2/algol3.htm | title = Algol 3 | publisher = SolStation | access-date = 31 July 2006}} | ||
* {{cite web | date = |
* {{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. |
* {{cite web | url = http://www.alcyone.de/SIT/mainstars/SIT000666.htm | title = Algol | publisher = Alcyone ephemeris | access-date = 8 June 2006}} | ||
* {{cite web | |
* {{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}} | |||
* {{cite web | url = http://www.alcyone.de/SIT/mainstars/SIT000666.htm | title = Algol | publisher = Alcyone ephemeris | language = English | accessdate = 2006-06-08 }} | |||
* {{cite web | last = Kaler | first = Jim | url = http://www.astro.uiuc.edu/~kaler/sow/algol.html | title = Algol (Beta Persei) | publisher = Stars | language = English | accessdate = 2006-06-08 }} | |||
* {{cite web | author = Bezza, Giuseppe | url = http://www.cieloeterra.it/eng/eng.articoli.algol/eng.algol.html | title = Al-ghûl, the ogre. | publisher = Cielo e Terra | language = English | accessdate = 2006-06-08 }} | |||
{{Stars of Perseus}} | |||
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Latest revision as of 00:22, 8 December 2024
Eclipsing variable star in the constellation Perseus This article is about the star. For the programming languages, see ALGOL. For other uses, see Algol (disambiguation).
Location of β Persei (circled) | |
Observation data Epoch J2000 Equinox J2000 | |
---|---|
Constellation | Perseus |
Right ascension | 03 08 10.13245 |
Declination | +40° 57′ 20.3280″ |
Apparent magnitude (V) | 2.12 (- 3.39) |
Characteristics | |
Spectral type | Aa1: B8V Aa2: K0IV Ab: F1V (kA4hA9.5mF0:) |
U−B colour index | −0.37 |
B−V colour index | −0.05 |
Variable type | EA/SD |
Astrometry | |
Radial velocity (Rv) | 3.7 km/s |
Proper motion (μ) | RA: 2.99 mas/yr Dec.: −1.66 mas/yr |
Parallax (π) | 34.7 ± 0.6 mas |
Distance | 94 ± 2 ly (28.8 ± 0.5 pc) |
β Per Aa1 | |
Absolute magnitude (MV) | −0.07 |
β Per Aa2 | |
Absolute magnitude (MV) | 2.9 |
β Per Ab | |
Absolute magnitude (MV) | 2.3 |
Orbit | |
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° |
Orbit | |
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) (primary) | 310.02° |
Details | |
β Per Aa1 | |
Mass | 3.17 ± 0.21 M☉ |
Radius | 2.73 ± 0.20 R☉ |
Luminosity | 182 L☉ |
Surface gravity (log g) | 4.0 cgs |
Temperature | 13,000 K |
Rotational velocity (v sin i) | 49 km/s |
Age | 570 Myr |
β Per Aa2 | |
Mass | 0.70±0.08 M☉ |
Radius | 3.48±0.28 R☉ |
Luminosity | 6.92 L☉ |
Surface gravity (log g) | 3.5 cgs |
Temperature | 4,500 K |
β Per Ab | |
Mass | 1.76±0.15 M☉ |
Radius | 1.73±0.33 R☉ |
Luminosity | 10.0 L☉ |
Surface gravity (log g) | 4.5 cgs |
Temperature | 7,500 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 45864, SAO 38592. | |
Database references | |
SIMBAD | data |
Algol /ˈælɡɒl/, 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.
Observation history
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.
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.
System
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.
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.
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 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.
The distance to Algol has been measured using very-long baseline interferometry, giving a value of 94 light-years. 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.
Names
Beta Persei is the star's Bayer designation.
The official name Algol
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.
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". Hipparchus and Pliny made this a separate, though connected, constellation.
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. Medusa is a gorgon so the star is also called Gorgonea Prima meaning the first star of the gorgon.
Chinese names
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.
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 Andromedae) at 2.2, with whom it forms a right triangle. During eclipses it dims to 3.4, making it as faint as nearby ρ Persei at 3.3.
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 |
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: Algol in astrologyThe constellation Perseus and Algol, the Bright Star in the Gorgon's head | |
---|---|
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. 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.
See also
- Jaana Toivari-Viitala, egyptologist who contributed to understanding Ancient Egypt and the star
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External links
- "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.
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- Algol variables
- Bayer objects
- B-type main-sequence stars
- Flamsteed objects
- Henry Draper Catalogue objects
- Hipparcos objects
- K-type subgiants
- Perseus (constellation)
- Stars with proper names
- Triple star systems
- Astronomical objects known since antiquity
- Bright Star Catalogue objects
- Durchmusterung objects
- Am stars
- F-type main-sequence stars