Final answer:
A 'standard candle' is a type of celestial object with a known luminosity used by astronomers to measure distances in space by examining the object's apparent brightness from Earth.
Step-by-step explanation:
An object for which we have some means of knowing its true luminosity, so that we can use its apparent brightness to determine distance, is commonly referred to as a standard candle. Astronomers use these celestial objects, which emit a known amount of energy, to measure vast cosmic distances. Variable stars and certain types of supernovae serve as standard candles because their luminosity or changes in luminosity can be used to infer their absolute brightness, allowing us to calculate the distance based on how dim they appear from Earth. The usefulness of standard candles comes from the fact that the apparent brightness of an object decreases with the square of the distance to that object. For instance, if one star is three times farther away than another of the same true brightness, it will appear nine times fainter. Astronomers constantly seek out these standard candles to more accurately measure the distances to distant galaxies.