Algol

<a href="/facts/Celestial_coordinate_system/23HhqvHj">Coordinates</a>: <a href="http://www.wikisky.org/?ra=3.1361476388889&de=40.955647777778&zoom=3&show_grid=1&show_constellation_lines=1&show_constellation_boundaries=1&show_const_names=1&show_galaxies=1&img_source=IMG_all">03h 08m 10.1315s, +40° 57′ 20.332″</a>

Algol /ˈælɡɒl/, designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright <a href="/facts/Multiple_star/1vX99BMV">multiple star</a> in the <a href="/facts/Constellation/nBQqgW8A">constellation</a> of <a href="/facts/Perseus_(constellation)/KD8mcCCy">Perseus</a> and one of the first non-<a href="/facts/Nova/H1GcmKXT">nova</a> <a href="/facts/Variable_star/Ac8mp2gy">variable stars</a> to be discovered.
Algol is a <a href="/facts/Triple_star/1vX99BMV">three-star system</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 <a href="/facts/Apparent_magnitude/7pCTF1Me">magnitude</a> 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 <a href="/facts/Occultation/a9Q1KTPe">occults</a> the fainter secondary is very shallow and can only be detected photoelectrically.
Algol gives its name to its class of eclipsing variable, known as <a href="/facts/Algol_variable/4tdb6oIN">Algol variables</a>.

Algol open-in-new

Algol