Variable Star

A Variable Star 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.

A few of the types:

• Cepheid Variables oscillate between being physically larger and smaller.
• RR Lyrae Variable.
• Protostars.
• 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.
• Flare Star - shows quick variations, e.g., in minutes - generally an M Dwarf (examples: Wolf 359 and Barnard's Star).

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.