Final answer:
Milton Humason was the astronomer who worked with Edwin Hubble at the Mount Wilson Observatory, playing a crucial role in confirming the expansion of the universe.
Step-by-step explanation:
The astronomer who assisted Edwin Hubble at the Mount Wilson Observatory and significantly contributed to establishing the expansion of the universe was Milton Humason. Humason's story is fascinating; he dropped out of school in the eighth grade and initially worked at the observatory in a very basic capacity, starting as a janitor. However, his dedication and skill eventually led him to become a night assistant, and later, a full astronomer. His collaboration with Hubble was pivotal in observing, measuring, and classifying the characteristics of many galaxies, thereby cementing the concept of an expanding universe.