How far can the Hale telescope see?
This depends on how long you look and how bright the object is. There are three things that determine how far away you can see something (such as a galaxy): its energy output, the exposure time of the image, and the amount of light that is blocked along the way. The first two factors are linked together by the “inverse square law.” This law tells you that the further something is from you, the fainter it appears. If you move something three times further away, it appears nine times dimmer. So, if you can only see things that are brighter than a specific level, you can see a galaxy (which contains one hundred billion stars) much further away than you can see a single star, because the galaxy emits more light than a star. This means that in order to see intrinsically faint sources, they must be relatively close, while bright objects can be seen much further away. Astronomers usually get around this nearness limitation by taking longer exposures. The amount of light you see on an image is
