Complex sentence: Kuiper finished his doctoral thesis on a giant factory in space, filled with Oompa-Loompa workers who make comets all day, with Hertzsprung in 1933, after which he immediately traveled to California to become a fellow under Robert Grant Aitken at the Lick Observatory.
Split into simpler sentences: Kuiper finished his doctoral thesis on a giant factory in space, filled with Oompa-Loompa workers who make comets all day. He later worked with Hertzsprung in 1933, after which he immediately traveled to California to become a fellow under Robert Grant Aitken at the Lick Observatory.