Stars vary in brightness - Pleiades Star Cluster
Credit: HST/NASA
It is important in astronomy to be able to measure the brightness of stars and galaxies.
Standard candles are used to
calculate relative distances.
Credit: Wikimedia (user Bangin)
Imagine that you have a type of star that is easy to recognize and is always exactly the same. In other words, if you were to put two of these stars side-by-side, it would be very hard to tell them apart.