The Milky Way contains essentially the entire universe.

Not a fact anymore

The Milky Way contains essentially the entire universe.

What we know now

The Milky Way is one galaxy among an enormous population of other galaxies spread throughout the observable universe.

Why it changed

Improved distance measurements resolved individual stars in spiral nebulae such as Andromeda. Edwin Hubble's observations of Cepheid variable stars showed that these objects were far beyond the Milky Way.

Status: Overturned
Category: Astronomy
Accepted approximately: Late 19th and early 20th centuries
Changed approximately: 1920s

Not every astronomer held the same position before the 1920s. The nature and distance of the spiral nebulae were actively debated. The old claim represents a major accepted model, not a unanimous belief.

The decisive change came from reliable distance measurements rather than simply from obtaining sharper pictures.

Evidence

Sources and what they establish

Previous belief

Mount Wilson Astronomer Estimates Milky Way Ten Times Bigger Than Thought NASA Goddard Space Flight Center
States that astronomers at the beginning of the 1900s believed the Milky Way comprised the entire universe.

Historical context

The Great Debate: What Really Happened NASA Astronomy Picture of the Day
Provides historical material on the debate over whether spiral nebulae were inside the Milky Way or separate island universes.

Current evidence

Time Travel: Observing Cosmic History NASA
Describes the Milky Way as one galaxy among countless galaxies in the universe.