Complex sentence: This "Proto-Galaxy", comprised of many big stars, after Billions of years, begins to make smaller stars, made when these Red Supergiants and Hypergiants, die in massive Supernovae and Hypernovae explosions.
Split into simpler sentences: This "Proto-Galaxy", comprised of many big stars, after Billions of years, begins to make smaller stars. Made when these Red Supergiants and Hypergiants, die in massive Supernovae and Hypernovae explosions.