variable star

A variable star (or just a variable) is a star which becomes dimmer and brighter over time as seen from Earth. This can be due to changes in the star such as brightening or enlarging, or through a change in whatever the light must pass through to reach us. Examples: Achernar, AD Leonis, Algol, DQ Tau, GG Tau, HL Tau, Lalande 21185, Rigel, Ross 154, Ross 248, T Tauri, Vega.

Stars generally are at least slightly variable, such as the 0.1% variability of the Sun over the solar cycle. The term microvariable indicates a star showing very slight variability, e.g., small enough that multiple observations must be made to preclude instrument variability and random errors. The term rotational modulation indicates the repeating pattern of variability that is a signature of starspots. Some general classes:

• pulsating variable star - grow larger and smaller.
• eruptive variable star - stellar flares, etc.
• cataclysmic variable star - significant explosions such as novae.
• rotating variable star - e.g., showing/hiding star spots.
• eclipsing binary.
• semiregular variable.
• irregular variable.
• intrinsic variable - due to internal features (e.g., pulsation).
• extrinsic variable - due to surface or external features (e.g., rotation, shape, binary star appearance).

(The above classes are not distinct.) Variation may be something that occurs in less than the course of a day (intraday variable or IDV) or for some pulsating stars, a variation period longer than 30 or so days (a long period variable or LPV) or even over spans of years such as FU Orionis stars. Some of the specific types of variables:

• Cepheid variable - type of pulsating star (oscillates between being physically larger and smaller).
• W Virginis variable - specific type of Cepheid variable: 10-20 day period, spectral type F6 to K2.
• RR Lyrae variable - somewhat similar to a Cepheid.
• protostars such as T-Tauri star and FU Orionis stars (both sometimes termed Orion variables).
• polar or AM Herculis star - binary star with accretion of mass from a red dwarf to a white dwarf with a significant magnetic field.
• intermediate polar or DQ Herculis star - similar to a polar if the magnetic field is weaker or the stars are further apart.
• BY Draconis variable (or BY Draconis star or just BY) - apparently starspots produce their variability.
• RS Canum Venaticorum variable (or RS Canum Venaticorum star) - like BY Draconis variables that are also close binary stars.
• Alpha² Canum Venaticorum variable (or α² Canum Venticorum variable or ACV) - a type of early star that varies in brightness across its surface, showing variability as it rotates.
• flare star - shows quick variations, e.g., in minutes - generally an M dwarf (examples: Wolf 359 and Barnard's Star).
• W Ursae Majoris variable or eclipsing W Ursae Majoris (EW) - type of eclipsing contact binary of low mass stars.
• Beta Cephei variable (BCEP or BCE or Beta Canis Majoris star) - has a type of small rapid variation.
• Algol variable or eclipsing Algol (EA) - type of eclipsing binary with certain peculiar characteristics.
• Beta Lyrae variable or eclipsing Beta Lyrae (EB) - type of eclipsing binary with two massive stars in contact.
• Gamma Cassiopeiae variable (GCAS) - type of star with long-term variations ascribed to a varying decretion disk.
• PV Telescopii variable (PVTEL) - type of pulsating supergiant.
• R Coronae Borealis variable (RCB) - type of eruptive variable.
• S Doradus variable (SDOR) - type of eruptive variable.
• SX Arietis variable (SXARI) - type of B-type rotating variable.
• SX Phoenicis variable (SXPHE) - type of pulsating variable.
• U Geminorum variable (UG) - type of cataclysmic variable star, aka a dwarf nova.
• Z Andromedae variable (ZAND) - type of eruptive variable.
• Z Andromedae with eclipses (ZANDE) - a Z Andromedae variable that is also an eclipsing binary.
• rotating ellipsoidal variable - close binary star non-spherical through gravitational inter-attraction, showing varying amounts of surface toward us.
• FK Comae Berenices variable - rotating star with large cool areas.

The details of the observed variation offer clues to the sources of variation, often revealing or confirming models of stellar structure for the given star, for its particular variable-star type, and for stars in general. Like binary stars, variable stars have yielded clues regarding stars in general and how they function: some types of variation offer some stellar parameter determination and some types offer information regarding their interiors.