A Universe Unbound: Commemorating 100 Years Since Edwin Hubble Revealed the Andromeda Galaxy

On November 23, 1924, a pivotal moment in the history of astronomy unfolded. Edwin Hubble, an astronomer at the Mount Wilson Observatory, announced, via the New York Times, evidence that the Andromeda “nebula” was not a part of the Milky Way but a galaxy in its own right. This revelation not only expanded the known scale of the universe but also laid the groundwork for modern cosmology, reshaping humanity’s understanding of the cosmos. A century on, we celebrate his groundbreaking discovery and reflect on its profound legacy.

In the early 20th century, the prevailing view of the universe was vastly limited. Most astronomers believed that the Milky Way was the entirety of space, and celestial objects like Andromeda were thought to be “spiral nebulae,” regions of gas and dust within our galaxy. Despite emerging evidence that hinted at a larger universe, the concept of other galaxies remained speculative.

The debate over the nature of these nebulae reached a fever pitch in the early 1920s. Astronomers like Heber Curtis argued that spiral nebulae were separate galaxies, or “island universes,” while others, including Harlow Shapley, maintained that they were part of the Milky Way. The astronomical community lacked the tools and methods to definitively resolve the issue—until Hubble’s work changed everything.

Edwin Hubble’s journey to uncover the true nature of Andromeda began with the Hooker Telescope at Mount Wilson Observatory in California. At the time, it was the most powerful telescope in the world, capable of unprecedented observational precision.

Hubble applied a key discovery made by astronomer Henrietta Leavitt. Leavitt had identified a relationship between the brightness and periodicity of Cepheid variable stars, which allowed astronomers to measure their distances. By observing Cepheid variables within Andromeda, He determined that the “nebula” was far too distant to be part of the Milky Way—approximately 2.5 million light-years away.

This finding was revolutionary. It confirmed that Andromeda was a galaxy in its own right, similar in scale to the Milky Way. Hubble’s announcement effectively ended the “Great Debate” and unveiled a universe teeming with galaxies.

Hubble’s discovery had immediate and far-reaching implications. For the first time, humanity grasped the staggering scale of the universe, recognising that the Milky Way was just one galaxy among countless others. This realisation shifted the focus of astronomy toward understanding the structure, distribution, and behaviour of galaxies.

The discovery also set the stage for subsequent breakthroughs in cosmology. In 1929, he would go on to show that galaxies are receding from one another, providing the first observational evidence of the universe’s expansion and laying the foundation for the Big Bang theory.

Edwin Hubble’s work has had a lasting influence on generations of astronomers. His insights catalysed research into galaxy formation, the large-scale structure of the cosmos, and the mysterious forces driving the universe’s expansion, such as dark energy.

The Hubble Space Telescope, named in his honour, continues to push the boundaries of our understanding. Launched in 1990, this iconic observatory has captured images of distant galaxies, nebulae, and cosmic phenomena, offering glimpses into the early universe and refining our models of cosmic evolution. The telescope has helped determine the age of the universe: now known to be 13.8 billion years. In 2004, the Hubble Ultra Deep Field was released, allowing astronomers to look even further back in the time of the cosmos (Royal Museums Greenwich).

As we commemorate the centennial of Hubble’s revelation, we are reminded of the enduring power of curiosity and discovery. The astronomer’s breakthrough opened the door to an era of cosmic exploration, paving the way for advanced instruments like the James Webb Space Telescope, launched in December 2021, which now peers deeper into the universe than ever before. Indeed, the telescope has found galaxies that are brighter than expected, which may also be more efficient at creating stars. It also discovered the earliest and most distant galaxy yet, JADES-GS-z14-0 (ESA/WEBB).

The Andromeda Galaxy, once a misunderstood “nebula”, now serves as a vivid symbol of our quest to understand the cosmos. Hubble’s work reminds us that the universe is vast, dynamic, and full of mysteries waiting to be uncovered—a humbling and inspiring legacy that continues to guide humanity’s exploration of the stars.

In commemorating this milestone, we honour not only Hubble but also the countless astronomers, past and present, who have expanded our view of the universe and illuminated our place within it. Here’s to another century of discovery.