In which stage do stars spend most of their lifetime?

Stars become the main sequence as they move through a galaxy. During this stage, it will fuse hydrogen atoms and form helium molecules in its core for 90 percent of its life. We are currently in the main sequence of our sun's life cycle.

Explanation:

Earth is not considered a closed system mainly because of two reasons, 
1. mass is lost from the atmosphere into space constantly so gasses so to say keep leaking out.
2. with the advent of the space age rockets and satellites go into space carrying matter from earth which is mostly left there.