The trachea, bronchi, and bronchioles have been compared to the trunk, branches, and twigs of a tree. What other analogy can you think of to describe these structures?

The physical conditions that are necessary to support nuclear fusion and formation of stars are;

On the lower temperature and  increased gravitational attraction, it is more likely for stars to be formed and nuclear fusion occurs.

Stars form in cold, dense quarters of space called molecular clouds. When the force of gravity stretching in on the cloud is greater than the intensity of internal pressure pushing out, the cloud collapses into a protostar.