22. Measuring with the Hubble Constant
This law requires two pieces of information, which is how far a galaxy cluster is from the earth, and then how fast it appears to be moving away.
In order to measure how far away a galaxy is, some astronomers use certain stars as reference points. These types of variable stars are called Cepheid Variables. These stars will change their brightness and can be observed with their pulse rates.
For example, the brighter a star is the slower it pulsates.