Most stars are of nearly constant luminosity. Our own Sun is a good example which goes through practically no measurable variation in brightness. There are, however, stars which do vary in brightness, called variable stars. They fall into two main groups:
These are stars which have intrinsic variations in brightness, that is the star itself gets brighter and dimmer. There are many types of intrinsic variables, the main types being:
Extrinsic variables have variations in their brightness, as seen by terrestrial observers, due to some external source. One of the most common reasons for this is the presence of a binary companion star, so that the two together form a binary star. When seen from certain angles, one star may eclipse the other, causing a reduction in brightness.
In some binary stars the components are so close to each other that mass transfer between the components occurs. These systems are called interacting binary stars. The mass flow forms an accretion disk in many cases.
On the receiving star, the extra mass may cause various phenomena seen as nova outbursts, X-ray pulsars or X-ray bursts.
See also Variable star designation.
Intrinsic variables
Extrinsic variables
