What is Variable star?
[[File:ESO_-_Model_Image_of_Cepheid_L_Carinae_(by).jpg|thumb|240px|[[Cepheid]] L Carinae]] A '''variable star''' is a [[star]] that is sometimes [[wikt:bright|brighter]] or [[wikt:dim|dimmer]]. Most stars have only very small differences in brightness, like the [[Sun]]; but some stars can vary hugely. There are generally two types of variable stars: * A star being [[eclipse]]d, where the changes in brightness are from [[perspective]], are called extrinsic variables. For example, in a [[solar eclipse]] the moon gets between the sun and Earth. Some [[binary star]] systems sometimes show eclipses, as do planetary systems with high-mass planets such as gas giants. * Stars whose brightness actually changes are called intrinsic variables. These stars appear to get bigger and smaller over time. Some pulsate at a constant rate, some do not. All of them change the rate at which energy is put out, which changes their appearance to us. == Intrinsic variable stars == There are a number of different types of variable stars. *Intrinsic variable stars: variation caused by changes in the physical properties of the stars themselves.<ref name=asteroseismology>{{cite journal|year=1994|title=Asteroseismology|journal=Annual Review of Astronomy and Astrophysics|volume=32|page=37|doi=10.1146/annurev.aa.32.090194.000345|bibcode=1994ARA&A..32...37B|last1=Brown|first1=Timothy M.|last2=Gilliland|first2=Ronald L.}}</ref><ref>Cox, John P., ''Theory of Stellar Pulsation'', Princeton, (1980)</ref><ref name=pulsating>{{cite web|url=http://www.star.ac.za/course-resources/local/david-buckley/var2.pdf|title=Variable Stars II - Pulsating Stars|last1=Kilkenny|first1=Dave|access-date=2013-03-25|archive-date=2013-01-23|archive-url=https://web.archive.org/web/20130123225336/http://www.star.ac.za/course-resources/local/david-buckley/var2.pdf|url-status=dead}}</ref> Three subgroups: **Pulsating variables: stars whose [[radius]] expands and contracts as part of their natural evolutionary ageing processes. **Eruptive variables: stars who experience eru