1) Astronomer 2) Stargazer 3) Uranologist 1) Lemaitre 
• (n.) One who describes the universe; one skilled in cosmology.
Cos·mol'o·gist noun One who describes the universe; one skilled in cosmology. 
[n] - an astronomer who studies the evolution and space-time relations of the universe
cosmologist Someone who studies the physical universe which is considered a totality of phenomena in time and space.
noun an astronomer who studies the evolution and space-time relations of the universe