A variable star's apparent brightness changes over time as seen from Earth. There are 2 basic types:
intrinsic variables whose luminosity actually changes eg. it periodically swells and shrinks.
extrinsic variables, whose apparent brightness changes are due to changes in the amount of their light that can reach Earth eg. it has an orbiting companion that sometimes eclipses it.
Intrinsic variables
Pulsating variable stars - there are many types and they usually pulsate because they are at the end of their lives and are unstable. Same vary by 100 times and they vary over days to years.
Eruptive variable stars - there are more likely to have very irregular cycles. They include protestors (in the process of becoming main sequence stars), giants and supergiants (which lose their matter relatively easily and experience eruptions) and white dwarfs that are part of a binary system (as they take matter from their companion via eruptions).
Extrinsic variables
About half of visible stars are part of multiple star systems.
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