How are cepheid variable stars found?
It changes from bright to dim. (in a cycle)
When astronomers look at the sky they saw a star that is very bright. After a few days the bright star will not be as bright as the one they saw. After some days, the star will again be very bright.
It is called Cepheid variables because the first pulsating variable star was discovered in a group stars called Cepheus.
Look at this one, as you can see the star in the middle is a cepheid variable star. On April 28 it is brighter and on May 9 it it somehow faded. And then bright again...