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.

A few of the types:

• Cepheid variable - oscillates between being physically larger and smaller.
• W Virginis variable - specific type of Cepheid variable: 10-20 day period, spectral class F6 to K2.
• RR Lyrae variable - somewhat similar to a cepheid.
• protostars such as T-Tauri star and FU Orionis stars.
• 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) - 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 - a contact binary in which the smaller companion eclipses.

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. In this sense, variable stars are analogous to binary stars, which also serve as windows into stellar detail.