Background One of the hardest problems in studying the Universe is figuring out how far away things are. You can't just fly over to another galaxy with a ruler in the morning and be back for lunch.
Light is very important to astronomers.
We can only study the light they have given off. This is important to remember. If a light is giving off lots of energy every second, we say it is very luminous . You've probably noticed that if you get closer and closer to a light, it looks brighter and brighter. Think of a big fire and a small candle, both 5 feet away from you.