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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:
Intrinsic variables
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
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,
List of known variable stars.
External link: The American Association of
Variable Star Observers
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