Algol
0Algol, by Wikipedia https://en.wikipedia.org/wiki?curid=1394 / CC BY SA 3.0
#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
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.
The Algol system on 12 August 2009.
This is a CHARA interferometer image with 1/2-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'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.
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 variabili...
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