Variable stars

Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups. During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and cepheid-like stars, and long-period variables such as Mira.[137] Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[137] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars. Cataclysmic or explosive variables undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[4] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[138] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[137] Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[137] A notable example of an eclipsing binar

is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days. A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to magnetic reconnection in the atmospheres of the stars. The brightness increase is across the spectrum, from X rays to radio waves. The first known flare stars (V1396 Cygni and AT Microscopii) were discovered in 1924. However, the best-known flare star is UV Ceti, discovered in 1948. Today similar flare stars are classified as UV Ceti type variable stars (using the abbreviation UV) in variable star catalogs such as the General Catalogue of Variable Stars. Flares can happen once every few days, [1][dubious discuss] or, as in the case of Barnard's Star, much less frequently. Most flare stars are dim red dwarfs, although recent research indicates that less massive brown dwarfs might also be capable of flaring. The more massive RS Canum Venaticorum variables (RS CVn) are also known to flare, but it is understood that these flares are induced by a companion star in a binary system which causes the magnetic field to become tangled. Additionally, nine stars similar to the Sun have also been seen to undergo flare events.[2] It has been proposed that the mechanism for this is similar to that of the RS CVn variables in that the flares are being induced by a companion, namely an unseen Jupiter-like planet in a close orbit.