A variable star (or just a variable) is a star which becomes dimmer
and brighter over time as seen from Earth.
Numerous widely-differing mechanisms produce such variations, and
the study of variable stars is relevant many areas of the science
of the astrophysics of stars.
The variation 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.
Some example variable stars:
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 of variable stars:
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 more-specific types of variable stars:
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.
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.
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.