Does the Milky Way encompass the entirety of the universe?

Long before Herschel attempted to determine the structure of the Milky Way, faint and unclear clouds of objects known as "nebulae" were spotted in the sky along with stars.

For instance, in 1612, the German astronomer Marius became the first to discover the Great Nebula of the Andromeda Galaxy using his telescope.

In 1750, Wright speculated that some of these nebulae might be colossal star systems similar to the Milky Way.

In 1755, the German Kant made the first definitive statement about countless similar systems of objects existing outside the Milky Way in cosmic space.

He referred to them as Extragalactic Galaxies or simply galaxies, sometimes even calling them "nebulae," citing the Great Nebula of Andromeda as an example.

However, at that time, knowledge about the fine structure and distances of nebulae was lacking, making it impossible to draw any conclusions about them.

Despite ongoing improvements in observation methods after Herschel's time and the involvement of physics in astronomy, the nature of nebulae remained unclear until the early 20th century.

In a 1920 debate at the American Academy of Sciences on "The Scale of the Universe," Curtis argued that some nebulae were extragalactic galaxies, while Shapley took the opposing position, leading to the famous "Shapley-Curtis debate" in astronomy's history. The key to resolving this debate lay in accurately determining the distances to the nebulae.

Astronomers consistently strived to determine the distances of various celestial bodies. Apart from the triangulation method and photometric determination, one method involved using the unique luminosity variation characteristics of variable stars, known as the "ruler of the sky."

Variable stars exhibit regular luminosity variations, and by measuring their light cycle, astronomers could deduce their actual luminosity, enabling distance calculation.

In the early 20th century, the U.S. Mount Wilson Observatory constructed the world's largest 2.5-meter telescope.

On October 6, 1923, U.S. astronomer Hubble used this telescope to photograph the Andromeda Nebula, revealing it as a collection of individual stars at its outer edge.

Hubble discovered several variable stars, and by using them, estimated the Andromeda Nebula to be 2.25 million light-years away, well beyond the Milky Way.

This confirmed the existence of extragalactic star systems, leading to the renaming of the Andromeda Nebula to the Andromeda Galaxy and concluding a long-standing scientific debate.

Herschel's work broadened our understanding of the Milky Way, and Hubble's discovery expanded our knowledge from the stellar world to the galactic realm, taking a significant step forward in mankind's understanding of the Universe.

Despite Earth's apparent lack of uniqueness in the universe, humanity has achieved a remarkable history of civilization on this planet and has finally gained a correct understanding of the universe.