Key points: How it was established that other galaxies are island universes of stars; standard candles and distances; the distance ladder

While the Milky Way was considered to be a thin, distorted disk of stars with the sun near the center in the early 20th century, other galaxies were confused with gaseous nebulae, and were assumed to be part of Milky Way.

The best pictures showed other galaxies to have spiral arms, but they also appeared smooth and were thought to be a special kind of gaseous nebulae, the "spiral nebulae".