Complex sentence: In the nineteenth century, modern astronomers began to take an interest in the historic records; which they then identified, compiled, and studied as part of their research on recent novae, comets, and later, the supernovae.
Split into simpler sentences: In the nineteenth century, astronomers began to take an interest in the historic records. They compiled and examined the records as part of their research on recent novae, comets, and later, the supernovae.