On November 23, 1924, American astronomer Edwin Hubble published an article asserting that the Andromeda Nebula was not part of the Milky Way, but rather an entirely separate galaxy.
This marked the first confirmation of the existence of extragalactic galaxies, reshaping humanity’s understanding of the universe.
Milestones in Understanding the Universe
The Chinese idiom “there is always a greater sky beyond” aptly describes the limitless nature of human understanding. In astronomy, this "sky" refers to the universe, the vast expanse that astronomers strive to comprehend. Over millennia, three major breakthroughs have defined humanity’s exploration of the cosmos:
1. In 1543, Polish astronomer Nicolaus Copernicus proposed the heliocentric model, placing the Sun, not Earth, at the center of the solar system.
2. In 1785, British astronomer William Herschel constructed the first model of the Milky Way.
3. In 1923, Edwin Hubble confirmed the existence of galaxies beyond the Milky Way.
In Copernicus’ time, the known universe was limited to the solar system. Herschel expanded this vision by confirming the Milky Way's structure as a vast assembly of stars spanning approximately 100,000 light-years. But what lay beyond the Milky Way remained a mystery.
The Debate Over Nebulae
As early as the mid-17th century, astronomers noticed faint, cloud-like objects in the night sky, which they named "nebulae." Philosophers began questioning whether the Milky Way constituted the entirety of the universe. In 1750, British thinker Thomas Wright speculated that some nebulae might be vast star systems similar to the Milky Way. Five years later, German philosopher Immanuel Kant explicitly suggested the existence of countless such systems, later termed "extragalactic galaxies."
While these ideas were revolutionary, they lacked scientific evidence. At the time, astronomers were unsure of the true nature of nebulae or their position relative to the Milky Way.
Edwin Hubble - The Andromeda Nebula Debate
Video by 1 Million Heroes
Herschel’s Observations
William Herschel attempted to resolve the nebulae mystery through observation. He theorized that nebulae could be classified based on whether they could be resolved into individual stars through telescopes. If resolvable, they were likely star systems, or galaxies. If not, they were thought to be clouds of gas and dust within the Milky Way.
Herschel’s observations, however, revealed a mix of results. Some nebulae could be resolved into stars, but others remained diffuse and unresolvable. This left Herschel perplexed. His classification method was also flawed, as what astronomers called "nebulae" included three distinct types of objects: star clusters, gas and dust clouds within the Milky Way, and galaxies beyond it. With the telescopes of the time, distinguishing between these was nearly impossible.
The Key Breakthrough: Measuring Distances
The nature of nebulae remained unresolved for over a century. In 1920, the U.S. National Academy of Sciences held a debate on "The Scale of the Universe," featuring astronomers Harlow Shapley and Heber Curtis. The debate centered on whether nebulae were part of the Milky Way or external galaxies. Determining the distances to these objects was critical. If nebulae were significantly farther than the Milky Way's size, they had to be external galaxies. Otherwise, they were within the Milky Way.
The debate ended inconclusively, highlighting the need for better observational tools to measure distances to nebulae accurately.
The Role of Advanced Telescopes
Advances in telescope technology paved the way for definitive answers. In 1917, the Mount Wilson Observatory in the United States completed a 2.54-meter reflecting telescope, the largest of its time. On October 5, 1923, Edwin Hubble used this telescope to observe the Andromeda Nebula. High-resolution photographs revealed individual stars at the nebula’s edges, including Cepheid variable stars. These stars, whose luminosity is linked to their periodic brightness changes, enabled Hubble to calculate their distances.
Through meticulous analysis, Hubble determined that the Andromeda Nebula was approximately 1 million light-years away (modern estimates place it at 2.4 million light-years). This distance far exceeded the size of the Milky Way, confirming that the Andromeda Nebula was an external galaxy. Hubble renamed it the Andromeda Galaxy, solidifying the existence of extragalactic galaxies.
Hubble’s Legacy
Hubble’s groundbreaking discovery was the culmination of years of meticulous work and scientific curiosity. He had previously categorized nebulae into "galactic" and "non-galactic" types but remained cautious in his conclusions. Only after determining the Andromeda Galaxy's distance did he confidently confirm the existence of extragalactic galaxies.
Today, it is known that the universe contains over 100 billion galaxies, which form the basic structural units of the cosmos. The study of galaxies has become a central field in modern astronomy, contributing to our understanding of the universe's origins, evolution, and structure.
