Complex sentence: Henrietta Swan Leavitt joined the Harvard "computers", a group of women engaged in the production of astronomical data at Harvard; she is instrumental in discovery of the cepheid variable stars, which were evidence for the expansion of the universe.
Split into simpler sentences: Henrietta Swan Leavitt joined the Harvard "computers", a group of women engaged in the production of astronomical data at Harvard. She was instrumental in discovery of the cepheid variable stars, which are evidence for the expansion of the universe.