See also: Classification of Variable Stars

Variable stars are stars for which we can detect brightness or color variations on time scales. There are a number of reasons why stars show such a behavior. These variations may be periodic, semi-periodic or irregular. Features like typical time scales, amplitude of the brightness variations, and the shape of the light curve can be obtained from photometric observations, and these parameters let us classify a given star to the proper class.

Variability types are grouped according to the major astrophysical reasons for variability. There are two main families of variable stars: intrinsic variable stars and extrinsic variables. Extrinsic variables vary due to processes external to the star, for example eclipsing binaries or rotational variables. In the case of intrinsic variable stars, parameters like brightness or color vary because of pulsation, flares or explosions. Each family mentioned above is divided on several groups.

For example, pulsating variables swell and shrink due to internal forces. An eclipsing binary will dim when it is eclipsed by a faint companion, and then brighten when the occulting star moves out of the way. Some variable stars are actually extremely close pairs of stars, exchanging mass as one star strips the atmosphere from the other.

Variability types are based on characteristic features of the light curves. Unfortunately, this gives a non-homogeneous results. As it follows, every classification should take into account also physical properties of the star and other parameters like position of the star on H-R diagram, its metalicity, presence or not characteristic spectral lines etc. of the classified star.

Variable stars play a significant role in our understanding of the universe. Historically, Cepheid variables have played a major part in determining distances to far-away galaxies and determining the age of the Universe. Mira-like variables give us a glimpse into the future evolution of stars.

• Why Observe Variable Stars?
• Variable Stars Easy to Observe
• Variable Star Charts
• AAVSO: Variable Star of the Season Archive