How are cepheid variables used?

Cepheid variables are a type of star that have a regular pulsating action.

They are used to measure galactic distances. Because the luminosity (the amount of light emitted by a star) is directly related to the pulse period, distances can be estimated with great accuracy.

For example, if we observe a cepheid variable at a distance of 10 light years, and then we observe another cepheid variable that is a quarter as bright but has the same period (and so therefore the same luminosity), and we know that light decreases as distance increases squared, so we can reasonably estimate the more distant star is 20 light years distant.

While measuring stars themselves is helpful, it's when the cepheid variables are a part of a larger structure that they really become helpful, because then the entire structure can be worked with and analyzed more fully.

Cepheid variables have a predictable relationship between luminosity and pulsation period, which allows astronomers to calculate their distances based on their observed brightness. They are used as standard candles to measure distances in astronomy, particularly in determining the distances to galaxies.