A variable star can be a valuable tool in determining distance to a galaxy. Variable stars are stars that have left the main sequence and are currently in a part of the HR diagram called the instability strip. The star's radius and luminosity change gradually and predictably, going from maximum brightness to minimum brightness and back to maximum brightness in a few days time. A variable star is relatively easy to spot, even at great distance, because of its variable nature. Just as a deer in the woods is more visible when moving, a star is more visible if its appearance changes with time.
Computers can compare star brightnesses and easily point out any changes.
There are two main types of variable stars. They are called RR Lyrae and Cepheid Variables. Once the luminosity is known, and the apparent brightness is measured, the distance may be calculated using the inverse-square law for apparent brightness. It is easier to detect a supernova remnant using radio telescopes than with optical telescopes. First, consider what the supernova remnants are.