Algol variable

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Algol variables or Algol-type binaries are a class of eclipsing binary stars where the orbital plane of the stars are coincident with the line of sight from Earth.[1] When the cooler component passes in front of the hotter one, part of the latter's light is blocked, and the total brightness of the binary, as viewed from Earth, temporarily decreases. This is the primary minimum of the binary. Total brightness may also decrease, but less so, when the hotter component passes in front of the cooler one; this is the secondary minimum.

The period, or time span between two primary minima, is very regular, being determined by the revolution period of the binary, the time it takes for the two components to once orbit around each other. Most Algol variables are quite close binaries, and therefore their periods are short, typically a few days. The shortest known period is 0.1167 days (~2:48 hours, HW Virginis); the longest is 9892 days (27 years, Epsilon Aurigae).

Component stars of Algol binary systems have a spherical, or slightly ellipsoidal shape. This distinguishes them from the so-called beta Lyrae variables and W Ursae Majoris variables, where the two components are so close that gravitational effects lead to serious deformations of both stars.

Generally the amplitudes of the brightness variations are of the order of one magnitude, the largest variation known being 3.4 magnitudes (V342 Aquilae). The components may have any spectrum, though in most cases the brighter component is found to have B, A, F, or G spectra.

Algol itself, the prototype of this type of variable star, Bayer designation Beta Persei, first had its variability recorded in 1667 by Geminiano Montanari. The mechanism for its being variable was first correctly explained by John Goodricke in 1782.

Many thousands of Algol binaries are now known: the latest edition of the General Catalogue of Variable Stars (2003) lists 3,554 of them (9% of all variable stars).

Designation (name) Constellation Discovery Apparent magnitude (Maximum)[2] Apparent magnitude (Minimum)[3] Range of magnitude Period Subtype Spectral types
(eclipsing components)
ε Aur Auriga J.H. Fritsch, 1821 !B9989284163837 2m.92 !B9986571351968 3m.83 !D0000943106794 0.91 !B9908013071983 27.08 years GS F0 Iab + ~ B5V  
U Cep Cepheus   !B9980904574951 6m.75 !B9977764581143 9m.24 !B9990877172895 2.49 !B9990864931395 2.49305 d  
R CMa Canis Major   !B9982595338251 5m.70 !B9981531212315 6m.34 !D0004462871026 0.64 !B9998725394979 1.13594 d SD triple system
S Cnc Cancer Hind, 1848 !B9978849500308 8m.29 !B9976727222944 10m.25 !B9993270555267 1.96 !B9977503358410 9.48455 d DS  
α CrB (Alphecca or Gemma) Corona Borealis   !B9992070074844 2m.21 (B) !B9991584328143 2m.32 (B) !D0022072749131 0.11 !B9971458364751 17.35991 d DM A0V + G5V  
U CrB Corona Borealis   !B9979639880162 7m.66 !B9978263852883 8m.79 !B9998777823672 1.13 !B9987609882910 3.45220 d SD  
u Her (68 Her) Hercules   !B9984545674175 4m.69 !B9983191720914 5m.37 !D0003856624808 0.68 !B9992816578940 2.05103 d SD  
VW Hya Hydra   !B9976486247428 10m.5 !B9973538252026 14m.1 !B9987190661545 3.6 !B9990080750327 2.69642 d SD  
δ Ori (Mintaka) Orion John Herschel, 1834 !B9992391941709 2m.14 !B9991846351867 2m.26 !D0021202635362 0.12 !B9982538517531 5.73248 d DM O9.5 II + B0.5III  
VV Ori Orion   !B9983304081647 5m.31 !B9982665761077 5m.66 !D0010498221244 0.35 !B9996043293682 1.48538 d KE  
β Per (Algol) Perseus Geminiano Montanari, 1669 !B9992485839113 2m.12 !B9987791700786 3m.39 !B9997609830995 1.27 !B9989466291795 2.86730 d SD B8V + K0IIV prototype, triple system
ζ Phe Phoenix   !B9986364626260 3m.91 !B9985138603039 4m.42 !D0006733445532 0.51 !B9994873141076 1.66977 d DM B6 V + B9 V probable quadruple system
U Sge Sagitta   !B9981359198691 6m.45 !B9977721384532 9m.28 !B9989597232883 2.83 !B9987819408753 3.38062 d SD  
λ Tau Taurus Baxendell, 1848 !B9987850872556 3m.37 !B9986364626260 3m.91 !D0006161861394 0.54 !B9986255378643 3.95295 d DM B3 V + A4 IV triple system
BL Tel Telescopium Luyten, 1935 !B9980413146594 7m.09 !B9979106081274 8m.08 !D0000100503358 0.99 !B9933432734758 778 d GS F4Ib+M one component may be variable
  • DM = A detached main-sequence system. Both components are main-sequence stars and neither fills their inner Roche lobe
  • DS = A detached system with a subgiant. The subgiant does not fill its inner critical surface
  • GS = A system with one or both giant and supergiant components; one of the components may be a main sequence star
  • KE = A contact system of early (O-A) spectral type, both components being close in size to their inner critical surfaces.


  1. ^ "A consideration of close binary systems in relation to light variation.". Report of the South African Association for the Advancement of Science 1: 110–111. 1903. 
  2. ^ (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))
  3. ^ (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))