Complex sentence: Based on this, Olsen was able to estimate that Andromeda was--he subsequently became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.
Split into simpler sentences: Based on this, Olsen was able to estimate that Andromeda was away. As a result, Olsen became became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.