If you were to look up into the sky for a few hours, and see 100 stars (or take a picture of 100 stars making up a starfield), why would you see the particular stars that you see out of the billions of stars in the Universe? Include two main reasons. (Thinking about magnitude might help you answer this question.)

1. The time of the year: Because of the revolution of Earth around the Sun the part of the universe (constellations and stars) visible keep changing. So that a particular set of stars can be seen in a particular season only.

2. Local sky condition: Even out of all the stars present in the sky at any given time one can only see a limited no. of stars. This depends on the limiting magnitude for that particular location. So lets say the limiting magnitude (magnitude of the faintest object that can be seen) is 6 for a location. Only the stars which are brighter than magnitude 6 can be seen.