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Algol

Coordinates:Sky map03h08m10.1315s,+40° 57′ 20.332″
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This article is about the star. For the programming languages, seeALGOL.For other uses, seeAlgol (disambiguation).

Algol
Location of β Persei (circled)
Observation data
EpochJ2000EquinoxJ2000
Constellation Perseus
Right ascension 03h08m10.13245s[1]
Declination +40° 57′ 20.3280″[1]
Apparent magnitude(V) 2.12[2](- 3.39[3])
Characteristics
Spectral type Aa1:B8V[4]
Aa2:K0IV[4]
Ab:F1V[5](kA4hA9.5mF0:[6])
U−Bcolour index −0.37[2]
B−Vcolour index −0.05[2]
Variable type EA/SD[3]
Astrometry
Radial velocity(Rv)3.7 km/s
Proper motion(μ)RA:2.99[1]mas/yr
Dec.:−1.66[1]mas/yr
Parallax(π)34.7 ± 0.6mas[7]
Distance94 ± 2ly
(28.8 ± 0.5pc)[7]
β Per Aa1
Absolute magnitude(MV)−0.07[8]
β Per Aa2
Absolute magnitude(MV)2.9[8]
β Per Ab
Absolute magnitude(MV)2.3[8]
Orbit[9]
Primaryβ Per Aa1
Companionβ 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[9]
Primaryβ Per A
Companionβ 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°
Periastronepoch(T)2446927.22
Argument of periastron(ω)
(primary)		310.02°
Details
β Per Aa1
Mass3.17 ± 0.21[9]M☉
Radius2.73 ± 0.20[9]R☉
Luminosity182[8]L☉
Surface gravity(logg)4.0[10]cgs
Temperature13,000[10]K
Rotational velocity(vsini)49[11]km/s
Age570[8]Myr
β Per Aa2
Mass0.70±0.08[9]M☉
Radius3.48±0.28[9]R☉
Luminosity6.92[8]L☉
Surface gravity(logg)3.5[10]cgs
Temperature4,500[10]K
β Per Ab
Mass1.76±0.15[9]M☉
Radius1.73±0.33[9]R☉
Luminosity10.0[8]L☉
Surface gravity(logg)4.5[10]cgs
Temperature7,500[10]K
Other designations
Algol, Gorgona, Gorgonea Prima, Demon Star, El Ghoul, β Persei, β Per, 26 Persei,BD+40°673,FK5111,GC3733,HD19356,HIP14576,HR936,PPM45864,SAO38592.
Database references
SIMBADdata

Algol/ˈælɡɒl/,[12]designatedBeta Persei(β Persei,abbreviatedBeta Per,β Per), known colloquially as theDemon Star,is a brightmultiple starin theconstellationofPerseusand one of the first non-novavariable starsto be discovered. Based uponparallaxmeasurements, Algol is located 94light-years(29parsecs) from Earth.[7]

Algol is athree-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'smagnitudeis 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 staroccultsthe fainter secondary is very shallow and can only be detected photoelectrically.[13]

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

Observation history[edit]

The Algol system on 12 August 2009. This is aCHARAinterferometer image with1/2-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.
Light curve of the Algol recorded by NASA'sTransiting 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.[14][15] [16]

The association of Algol with a demon-like creature (Gorgonin the Greek tradition,ghoulin the Arabic tradition) suggests that its variability was known long before the 17th century,[17]but there is still no indisputable evidence for this.[18]The Arabic astronomeral-Sufisaid nothing about any variability of the star in hisBook of Fixed Starspublished c.964.[19]

The variability of Algol was noted in 1667 by Italian astronomerGeminiano Montanari,[20]but the periodic nature of its variations in brightness was not recognized until more than a century later, when the British amateur astronomerJohn Goodrickealso proposed a mechanism for the star's variability.[21][22]In May 1783, he presented his findings to theRoyal 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 theCopley Medal.[23]

In 1881, the Harvard astronomerEdward Charles Pickeringpresented evidence that Algol was actually an eclipsing binary.[24]This was confirmed a few years later, in 1889, when the Potsdam astronomerHermann Carl Vogelfound periodicdoppler shiftsin the spectrum of Algol, inferring variations in theradial velocityof this binary system.[25]Thus, Algol became one of the first knownspectroscopic binaries.Joel Stebbinsat theUniversity of Illinois Observatoryused 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.[26]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.[27]

System[edit]

Algol Aa2 orbits Algol Aa1. This animation was assembled from 55 images of theCHARA interferometerin the near-infrared H-band, sorted according to orbital phase. Because some phases are poorly covered, Aa2 jumps at some points along its path.
interpolation
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.[28]From the point of view of the Earth, Algol Aa1 and Algol Aa2 form aneclipsing binarybecause theirorbital planecontains theline of sightto the Earth. The eclipsing binary pair is separated by only 0.062astronomical 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 mutualorbital periodof 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. TheWashington Double Star Cataloglists them as Aa1, Aa2, and Ab, with two very faint stars B and C about onearcmindistant. A further five faint stars are also listed as companions.[29]

The close pair consists of a B8main sequencestar and a much less massive K0subgiant,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 anAm star,but this is now considered doubtful.[5][30]

Studies of Algol led to theAlgol paradoxin the theory ofstellar 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 themain sequence,but the less massive Algol Aa2 is asubgiant starat a later evolutionary stage. The paradox can be solved bymass transfer:when the more massive star became a subgiant, it filled itsRoche 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.[31]The gas flow between the primary and secondary stars in Algol has been imaged using DopplerTomography.[32][33]

This system also exhibitsx-rayandradio wave[34]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.[35]The radio-wave flares might be created by magnetic cycles similar to those ofsunspots,but because the magnetic fields of these stars are up to ten times stronger than the field of theSun,these radio flares are more powerful and more persistent.[36][37]The secondary component was identified as the radio emitting source in Algol usingVery-long-baseline interferometryby Lestrade and co-authors.[4]

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 ofΔP/P≈10−5via theApplegate mechanism.[38]Mass transferbetween the components is small in the Algol system[39]but could be a significant source of period change in otherAlgol-type binaries.

The distance to Algol has been measured using very-long baselineinterferometry,giving a value of 94light-years.[7]About about 7.3 million years ago it passed within 9.8 light-years of theSolar System[40]and itsapparent magnitudewas about −2.5, which is considerably brighter than the starSiriusis today. Because the total mass of the Algol system is about 5.8 solar masses, at the closest approach this might have given enoughgravitytoperturbtheOort cloudof the Solar System somewhat and hence increase the number ofcometsentering the inner Solar System. However, the actual increase in net cometary collisions is thought to have been quite small.[41]

Names[edit]

Algol is a bright star in the constellation of Perseus (upper right).

Beta Perseiis the star'sBayer designation.

The official name Algol[edit]

The nameAlgolderives fromArabicرأس الغولraʾs al-ghūl:head (raʾs) of the ogre (al-ghūl) (see "ghoul").[42]The English name Demon Star was taken from the Arabic name.[43]In 2016, theInternational Astronomical Unionorganized aWorking Group on Star Names(WGSN)[44]to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[45]included a table of the first two batches of names approved by the WGSN; which includedAlgolfor this star. It is so entered on the IAU Catalog of Star Names.[46]

Ghost and demon star[edit]

Algol was calledRōsh ha Sāṭānor "Satan's Head" in Hebrew folklore, as stated byEdmund Chilmead,who called it "Divels head" orRosch hassatan.ALatinname for Algol from the 16th century wasCaput Larvaeor "the Spectre's Head".[43]HipparchusandPlinymade this a separate, though connected, constellation.[43]

First star of Medusa's head[edit]

Earlier the name of the constellationPerseuswasPerseus and Medusa's Headwhere an asterism representing the head of Medusa after Perseus has cut it off already known in ancient Rome.[47]Medusa is agorgonso the star is also calledGorgonea Primameaning the first star of the gorgon.[43]

Chinese names[edit]

InChinese,Đại lăng(Dà Líng), meaningMausoleum,refers to an asterism consisting of β Persei,9 Persei,τ Persei,ι Persei,κ Persei,ρ Persei,16 Perseiand12 Persei.Consequently, theChinese namefor β Persei itself isĐại lăng ngũ(Dà Líng wu,English: The Fifth Star of Mausoleum.).[48]According to R.H. Allen the star bore the grim name ofTseih SheTích thi(Zhi Shī), meaning "Piled up Corpses"[43]but this appears to be a misidentification, andDié Shīis correctlyπ Persei,which is inside the Mausoleum.[49]

Cultural significance[edit]

See also:Algol in astrology
The constellation Perseus and Algol, the Bright Star in the Gorgon's head
Johannes Hevelius, Uranographia, 1690
Johannes Hevelius,Uranographia,1690

Historically, the star has received a strong association with bloody violence across a wide variety of cultures. In theTetrabiblos,the 2nd-century astrological text of the Alexandrian astronomerPtolemy,Algol is referred to as "theGorgonofPerseus"and associated with death by decapitation: a theme which mirrors the myth of the heroPerseus's victory over the snake-hairedGorgonMedusa.[50]In theastrologyoffixed stars,Algol is considered one of theunluckieststars in the sky,[43]and was listed as one of the 15Behenian stars.[51]

See also[edit]

References[edit]

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External links[edit]

Wikisourcehas the text of the1911Encyclopædia Britannicaarticle "Algol".
  • "Algol 3".SolStation.Retrieved31 July2006.
  • "4C02517".ARICNS. 4 March 1998. Archived fromthe originalon 10 February 2006.Retrieved31 July2006.
  • "Algol".Alcyone ephemeris.Retrieved8 June2006.
  • Bezza, Giuseppe."Al-ghûl, the ogre".Translated by Daria Dudziak. Cielo e Terra.Archivedfrom the original on 20 June 2006.Retrieved8 June2006.


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