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.[12]
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.[13][14][15]
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,[16] but there is still no indisputable evidence for this.[17] The Arabic astronomer
al-Sufi said nothing about any variability of the star in his Book of Fixed Stars published c.964.[18]
The variability of Algol was noted in 1667 by Italian astronomer
Geminiano Montanari,[19] 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.[20][21] 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.[22]
In 1881, the Harvard astronomer
Edward Charles Pickering presented evidence that Algol was actually an eclipsing binary.[23] 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.[24] 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.[25]
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.[26]
System
Algol is a multiple-star system with three confirmed and two suspected stellar components.[27] 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.[28]
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.[5][29]
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.[30] The gas flow between the primary and secondary stars in Algol has been imaged using Doppler
Tomography.[31][32]
This system also exhibits
x-ray and
radio wave[33] 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.[34] 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.[35][36] The secondary component was identified as the radio emitting source in Algol using
Very-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 of ΔP/P ≈ 10−5 via the
Applegate mechanism.[37]Mass transfer between the components is small in the Algol system[38] but could be a significant source of period change in other
Algol-type binaries.
Algol is about 92.8
light-years from the Sun, but about 7.3 million years ago it passed within 9.8 light-years of the
Solar System[39] 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.[40]
The name Algol derives from
Arabicرأس الغولraʾs al-ghūl : head (raʾs) of the ogre (al-ghūl) (see "
ghoul").[41] The English name Demon Star was taken from the Arabic name.[42] In 2016, the
International Astronomical Union organized a
Working Group on Star Names (WGSN)[43] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[44] 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.[45]
Ghost and demon star
In Hebrew folklore, Algol was called Rōsh ha Sāṭān or "Satan's Head", 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".[42]Hipparchus and
Pliny made this a separate, though connected, constellation.[42]
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. [46] Medusa is a
gorgon so the star is also called Gorgonea Prima meaning the first star of the gorgon.[42]
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.).[47] According to R.H. Allen the star bore the grim name of Tseih She積屍 (Zhi Shī), meaning "Piled up Corpses"[42] but this appears to be a misidentification, and Dié Shī is correctly
π Persei, which is inside the Mausoleum.[48]
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
GorgonMedusa.[49] In the
astrology of
fixed stars, Algol is considered one of the
unluckiest stars in the sky,[42] and was listed as one of the 15
Behenian stars.[50]
See also
Jaana Toivari-Viitala, egyptologist who contributed to understanding Ancient Egypt and the star
^
abcDucati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0.
Bibcode:
2002yCat.2237....0D.
^
abSamus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: 02025.
Bibcode:
2009yCat....102025S.
^
abcLestrade, Jean-Francois; Phillips, Robert B.; Hodges, Mark W.; Preston, Robert A. (June 1993). "VLBI astrometric identification of the radio emitting region in Algol and determination of the orientation of the close binary". The Astrophysical Journal. 410: 808.
Bibcode:
1993ApJ...410..808L.
doi:
10.1086/172798.
ISSN0004-637X.
^
abFrank, M. G.; Whelan, D. G.; Junginger, J. C. (2022). "Spectral Classification of Algol C". Journal of the American Association of Variable Star Observers (Jaavso). 50 (1): 123.
arXiv:2205.06229.
Bibcode:
2022JAVSO..50..123F.
^
abcdefgSoderhjelm, S. (1980). "Geometry and dynamics of the Algol system". Astronomy and Astrophysics. 89 (1–2): 100.
Bibcode:
1980A&A....89..100S.
^
abcdefghBaron, F.; Monnier, J. D.; Pedretti, E.; Zhao, M.; Schaefer, G.; Parks, R.; Che, X.; Thureau, N.; Ten Brummelaar, T. A.; McAlister, H. A.; Ridgway, S. T.; Farrington, C.; Sturmann, J.; Sturmann, L.; Turner, N. (2012). "Imaging the Algol Triple System in the H Band with the CHARA Interferometer". The Astrophysical Journal. 752 (1): 20.
arXiv:1205.0754.
Bibcode:
2012ApJ...752...20B.
doi:
10.1088/0004-637X/752/1/20.
S2CID11110989.
^Porceddu, S.; Jetsu, L.; Lyytinen, J.; Kajatkari, P.; Lehtinen, J.; Markkanen, T.; et al. (2008). "Evidence of Periodicity in Ancient Egyptian Calendars of Lucky and Unlucky Days". Cambridge Archaeological Journal. 18 (3): 327–339.
Bibcode:
2008CArcJ..18..327P.
doi:
10.1017/S0959774308000395.
S2CID162969143.
^Jetsu, L.; Porceddu, S.; Lyytinen, J.; Kajatkari, P.; Lehtinen, J.; Markkanen, T.; et al. (2013). "Did the Ancient Egyptians Record the Period of the Eclipsing Binary Algol - The Raging One?". The Astrophysical Journal. 773 (1): A1 (14pp).
arXiv:1204.6206.
Bibcode:
2013ApJ...773....1J.
doi:
10.1088/0004-637X/773/1/1.
S2CID119191453.
^Wilk, Stephen R. (1996). "Mythological Evidence for Ancient Observations of Variable Stars". The Journal of the American Association of Variable Star Observers. 24 (2): 129–33.
Bibcode:
1996JAVSO..24..129W.
^Davis, G.A. (1957). "Why did the Arabs Call Beta Persei "al-Ghul"?". Sky and Telescope. 16: 177.
Bibcode:
1957S&T....16..177D.
^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 (
Google books).
^Pickering, Edward C. (1881). "Dimensions of the Fixed Stars, with especial reference to Binaries and Variables of the Algol type". Astronomical Register. 50 (1–2): 253–56.
Bibcode:
1881AReg...19..253.
^Wecht, Kristen (2006). 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 (Thesis).
arXiv:astro-ph/0611855.
Bibcode:
2006PhDT........10W.
^Garcia-Sanchez, J.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; et al. (25 August 1997). "A Search for Stars Passing Close to the Sun". The First Results of Hipparcos and Tycho. Kyoto, Japan: IAU.
Bibcode:
1997IAUJD..14E..51G.