There are many, depending on the mechanism involved.
Usually a variable star is an evolved star undergoing major structural change. These are stars like RR Lyrae or Delta Cephi, but there are many dozens of types that include Polaris and Betelgeuse.
They can be eclipsing binary stars with an occulting companion star that briefly increases or decreases the total light output from the pair of stars.
A flare star can produce a very bright emission of energy from the surface of the star, and this can cause the brightness of the star to appreciably change, although this is usually a random process.
There are also explosive events such as Novae or Supernovae in which a companion to a star, usually a white dwarf, detonates in a violent explosion. For Novae, the companion white dwarf survives the explosion, but for supernovae the companion does not survive. Other kinds of supernovae involve massive stars and these usually leave behind a neutron star or a black hole.
Stars can also change in brightness in less spectacular ways but these are related to photospheric effects such as sunspots and flares.
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.