variable star

variable stars

A variable star is a star that changes in brightness. Variable stars fall into two general categories: intrinsic variables, in which physical changes, such as pulsations or eruptions, are involved, and extrinsic variables, in which the light output fluctuates due to eclipses or stellar rotation. Further classification is complex; originally it was based on a star's light curve, amplitude, and periodicity (or lack of it), but is now more closely linked to the physical processes underlying the variability.


The standard reference for classification is the General Catalogue of Variable Stars. Specific types of variable star are often named after a prototype (see variable star naming). Additionally, a star may belong to more than one category of variable. Click on entries below for further details.


Classification of variable stars



An intrinsic variable is a variable star whose changes in brightness are due to actual fluctuations in the luminosity of the star itself, not to external processes or accidents of geometry.


   Pulsating variable

      Cepheid variable (period of 1-70 days; strict period-luminosity relationship)
      RR Lyrae star (period of 0.05-1.2 days; light variations of 0.3-2 mag.)
      RV Tauri star (alternating deep and shallow minima)
      Long-period variable
         Mira variable (giant red variable; well-defined period of 80-1,000 days)
         Semi-regular variable (giant star; periodic with intervals of irregular variation)
      Non-radial pulsating variable
          ZZ Ceti star

    Eruptive variable (flares or shell ejection)

       UV Ceti star (red/orange dwarf prone to sudden flares)
       FU Orionis star (slow rise to long-lasting maximum; development of emission spectrum)
       Gamma Orionis star (fast-spinning blue giant; occasional ejection of ring of matter)
       R Coronae Borealis star (sudden fades followed by slow recovery)
       RS Canum Venaticorum star (close binary with active chromosphere)
       S Doradus star (massive, luminous blue star often with expanding envelope)
       Wolf-Rayet star (luminous)
       Orion variable (associated with nebulosity; often irregular)
          T Tauri star
          YY Orionis star
          Irregular variable

    Cataclysmic variable (outbursts on star's surface or accretion disk, or stellar explosion)

       Supernova (sudden, dramatic, final magnitude increase as result of stellar explosion)
       Nova (fusion explosion increases brightness and then fades)
       Recurrent nova (system that has undergone 2 or more recorded nova-like eruptions)
       U Geminorum star (close binary system with Sun-like star and white dwarf)
          Z Camelopardalis star (no well-defined quiescence; "standstills" of brightness)
          SU Ursae Majoris star (orbital period < 2 hours; two distinct outbursts)
       Magnetic cataclysmic variable (close binary system with highly magnetic white dwarf)
          AM Herculis star (strong circular polarization; dwarf in synchronized rotation)
          DQ Herculis star (similar to AM Her but no synchronized rotation)
       Symbiotic star (semiperiodic nova-like outbursts of up to 3 mag.)
          Z Andromedae star

    X-ray variables (X-ray emissions from binary star, including compact component)


An extrinsic variable is a variable star the light fluctuations of which are not due to changes in the star itself but to external effect such as eclipses, obscuration, rotation, or tidal distortion.


    Eclipsing variable (binary system with an orbital plane lying near the observer's line-of-sight)

       Algol star (spherical components; eclipses identifiable from light curve)
       Beta Lyrae star (tidally-distorted components; continuous changes in brightness)
       W Ursae Majoris star (components almost touching)

    Rotating variable (changing light output due to dark/light spots on star's surface)

       Alpha2 Canum Venaticorum star
       BY Draconis star
       ellipsoidal variable
       FK Comae Berenices star
       SX Arietis star