Variable stars are stars that show periodic changes in their luminosity. This could be due to variations in the star’s intrinsic brightness, or it could be because some of the light from the star is blocked from view. This could be due to a less luminous companion star or a planet blocking light from the star as its orbit aligns it between the star and the observer on Earth. When this is a companion star, the system is known as an eclipsing binary.

Cepheid Variable Stars

A cepheid variable is a particularly important type of variable star with a period of variability related to its intrinsic brightness. This means that the star’s brightness can be calculated from measurements of its period and, by comparing this intrinsic brightness to the star’s apparent brightness, an estimate of the star’s distance from Earth can be made.

Cepheid variable stars observed in other galaxies provide an effective “standard candle” method of determining a galaxy’s distance from Earth and were used by Edwin Hubble to demonstrate that the universe is expanding (See Hubble’s Law).