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'smagnitude 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 staroccults the 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.
The Algol system on 12 August 2009. This is aCHARA interferometer 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][17] but the validity of this claim has been questioned.[18]
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,[19] but there is still no indisputable evidence for this.[20] The Arabic astronomeral-Sufi said nothing about any variability of the star in hisBook of Fixed Stars published c.964.[21]
The variability of Algol was noted in 1667 by Italian astronomerGeminiano Montanari,[22] but the periodic nature of its variations in brightness was not recognized until more than a century later, when the British amateur astronomerJohn Goodricke also proposed a mechanism for the star's variability.[23][24] 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.[25]
In 1881, the Harvard astronomerEdward Charles Pickering presented evidence that Algol was actually an eclipsing binary.[26] This was confirmed a few years later, in 1889, when the Potsdam astronomerHermann Carl Vogel found periodicdoppler shifts in the spectrum of Algol, inferring variations in theradial velocity of this binary system.[27] Thus, Algol became one of the first knownspectroscopic binaries.Joel Stebbins at theUniversity 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.[28] 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.[29]
Algol Aa2 orbits Algol Aa1. This animation was assembled from 55 images of theCHARA 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.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.[30] From the point of view of the Earth, Algol Aa1 and Algol Aa2 form aneclipsing binary because theirorbital plane contains theline 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 mutualorbital 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 2.5.
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 Catalog lists them as Aa1, Aa2, and Ab, with two very faint stars B and C about onearcmin distant. A further five faint stars are also listed as companions.[31]
The close pair consists of a B8main sequence star 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 aF1 main-sequence star. It has been classified as anAm star, but this is now considered doubtful.[5][32]
Studies of Algol led to theAlgol paradox in 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 star at 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.[33] The gas flow between the primary and secondary stars in Algol has been imaged using DopplerTomography.[34][35]
This system also exhibitsx-ray andradio wave[36] 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.[37] 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.[38][39] The secondary component was identified as the radio emitting source in Algol usingVery-long-baseline interferometry by 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 ≈ 10−5 via theApplegate mechanism.[40]Mass transfer between the components is small in the Algol system[41] but could be a significant source of period change in otherAlgol-type binaries.
Size comparison between theSun (bottom middle), Algol Aa2 (right) and theblue giantBellatrix (left).
The distance to Algol has been measured using very-long baselineinterferometry, giving a value of 94 light-years.[7] About 7.3 million years ago it passed within 9.8 light-years of theSolar System[42] and itsapparent magnitude was about −2.5, which is considerably brighter than the starSirius is today. Because the total mass of the Algol system is about 5.8 solar masses, at the closest approach this might have given enoughgravity toperturb theOort cloud of the Solar System somewhat and hence increase the number ofcomets entering the inner Solar System. However, the actual increase in net cometary collisions is thought to have been quite small.[43]
The nameAlgol derives fromArabicرأس الغولraʾs al-ghūl : head (raʾs) of the ogre (al-ghūl) (see "ghoul").[44] The English name Demon Star was taken from the Arabic name.[45] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[46] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[47] included a table of the first two batches of names approved by the WGSN; which includedAlgol for this star. It is so entered on the IAU Catalog of Star Names.[48]
Algol was calledRōsh ha Sāṭān or "Satan's Head" in Hebrew folklore, as stated byEdmund Chilmead, who called it "Divels head" orRosch hassatan. ALatin name for Algol from the 16th century wasCaput Larvae or "the Spectre's Head".[45]Hipparchus andPliny made this a separate, though connected, constellation.[45]
Earlier the name of the constellationPerseus wasPerseus and Medusa's Head where an asterism representing the head of Medusa after Perseus has cut it off already known in ancient Rome.[49] Medusa is agorgon so the star is also calledGorgonea Prima meaning the first star of the gorgon.[45]
InChinese,大陵 (Dà Líng), meaningMausoleum, refers to an asterism consisting of β Persei,9 Persei,τ Persei,ι Persei,κ Persei,ρ Persei,16 Persei and12 Persei. Consequently, theChinese name for β Persei itself is大陵五 (Dà Líng wu, English: The Fifth Star of Mausoleum).[50] According to R.H. Allen the star bore the grim name ofTseih She積屍 (Zhi Shī), meaning "Piled up Corpses"[45] but this appears to be a misidentification, andDié Shī is correctlyπ Persei, which is inside the Mausoleum.[51]
The Algol system usually has an apparent magnitude of 2.1, similar to those ofMirfak (α Persei) at 1.9 andAlmach (γAndromedae) at 2.2, with whom it forms a right triangle. During eclipses it dims to 3.4, making it as faint as nearbyρ Persei at 3.3.
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).
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 "theGorgon ofPerseus" and associated with death by decapitation: a theme which mirrors the myth of the heroPerseus's victory over the snake-hairedGorgonMedusa.[54] In theastrology offixed stars, Algol is considered one of theunluckiest stars in the sky,[45] and was listed as one of the 15Behenian stars.[55]
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