The discovery of other galaxies as distinct entities separate from our own Milky Way galaxy is a relatively recent development in the history of astronomy. Here is a brief timeline of the key milestones in the discovery of other galaxies:
1750s: Thomas Wright, a British astronomer, proposed the idea that the Milky Way might be just one among many "island universes" or galaxies. However, his ideas were speculative and not widely accepted at the time.
1920: Edwin Hubble, an American astronomer, made observations using the 100-inch Hooker Telescope at Mount Wilson Observatory in California. He discovered that certain objects, previously classified as nebulae within the Milky Way, were actually separate galaxies located far beyond our own. Hubble's work provided evidence for the existence of other galaxies and expanded our understanding of the universe.
1924: Hubble used the observations of Cepheid variable stars in the Andromeda Nebula (now known as the Andromeda Galaxy) to estimate its distance from Earth. By measuring the apparent brightness and period of these stars, he concluded that the Andromeda Nebula was located far beyond the boundaries of our Milky Way, making it another galaxy.
1929: Hubble made another groundbreaking discovery. He observed that galaxies were receding from us, and the farther a galaxy was from the Milky Way, the faster it appeared to be moving away. This relationship, known as Hubble's Law, provided strong evidence for the expanding universe and helped establish the concept of an ever-expanding cosmos.
Since then, astronomers have continued to study and discover numerous galaxies using increasingly advanced telescopes and instruments. Today, our knowledge of galaxies has grown tremendously, revealing a vast and diverse universe filled with billions of galaxies, each containing billions of stars.