What is a standard candle?

A standard candle is an astronomical object that has a known absolute magnitude (intrinsic brightness). They are extremely important to astronomers since by measuring the apparent magnitude of the object the distance to the source can be determined by using the inverse square law. Examples of such objects are Cepheid variable stars, whose absolute magnitude is proportional to their period of variability and also the Type-1a supernovae, since it is believed that they all have essentially the same peak absolute magnitude.