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.
This image is one of the most photogenic examples of the many turbulent stellar nurseries the NASA/ESA Hubble Space Telescope has observed during its 30-year lifetime. The portrait features the giant nebula NGC 2014 and its neighbour NGC 2020 which together form part of a vast star-forming region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, approximately 163 000 light-years away. This NASA/ESA Hubble Space Telescope image of the cluster Westerlund 2 and its surroundings has been released to celebrate Hubble’s 25th year in orbit and a quarter of a century of new discoveries, stunning images and outstanding science. The image’s central region, containing the star cluster, blends visible-light data taken by the Advanced Camera for Surveys and near-infrared exposures taken by the Wide Field Camera 3. The surrounding region is composed of visible-light observations taken by the Advanced Camera for Surveys.The narrow galaxy elegantly curving around its spherical companion in this image is a fantastic example of a truly strange and very rare phenomenon. This image, taken with the NASA/ESA Hubble Space Telescope, depicts GAL-CLUS-022058s, located in the southern hemisphere constellation of Fornax (The Furnace). GAL-CLUS-022058s is the largest and one of the most complete Einstein rings ever discovered in our Universe. The object has been nicknamed by the Principal Investigator and his team who are studying this Einstein ring as the “Molten Ring”, which alludes to its appearance and host constellation. First theorised to exist by Einstein in his general theory of relativity, this object’s unusual shape can be explained by a process called gravitational lensing, which causes light shining from far away to be bent and pulled by the gravity of an object between its source and the observer. In this case, the light from the background galaxy has been distorted into the curve we see by the gravity of the galaxy cluster sitting in front of it. The near exact alignment of the background galaxy with the central elliptical galaxy of the cluster, seen in the middle of this image, has warped and magnified the image of the background galaxy around itself into an almost perfect ring. The gravity from other galaxies in the cluster is soon to cause additional distortions. Objects like these are the ideal laboratory in which to research galaxies too faint and distant to otherwise see.The NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 observed Saturn on 20 June 2019 as the planet made its closest approach to Earth this year, at approximately 1.36 billion kilometres away.This Hubble image gives the most detailed view of the entire Crab Nebula ever. The Crab is among the most interesting and well studied objects in astronomy. This image is the largest image ever taken with Hubble’s WFPC2 camera. It was assembled from 24 individual exposures taken with the NASA/ESA Hubble Space Telescope and is the highest resolution image of the entire Crab Nebula ever made.This image, captured with the NASA/ESA Hubble Space Telescope, is the largest and sharpest image ever taken of the Andromeda galaxy — otherwise known as M31. This is a cropped version of the full image and has 1.5 billion pixels. You would need more than 600 HD television screens to display the whole image. It is the biggest Hubble image ever released and shows over 100 million stars and thousands of star clusters embedded in a section of the galaxy’s pancake-shaped disc stretching across over 40 000 light-years. This image is too large to be easily displayed at full resolution and is best appreciated using the zoom tool.How did the first stars and galaxies form? The NASA/ESA/CSA James Webb Space Telescope is already providing new insights into this question. One of the largest programs in Webb’s first year of science is the JWST Advanced Deep Extragalactic Survey, or JADES, which will devote about 32 days of telescope time to uncover and characterize faint, distant galaxies. While the data are still coming in, JADES already has discovered hundreds of galaxies that existed when the Universe was less than 600 million years old. The team also has identified galaxies sparkling with a multitude of young, hot stars. This infrared image shows a portion of an area of the sky known as GOODS-South, which has been well studied by the NASA/ESA Hubble Space Telescope and other observatories. More than 45,000 galaxies are visible here. Using these and other data, the JADES team has discovered hundreds of galaxies that existed when the Universe was less than 600 million years old. The sheer number of these galaxies was far beyond predictions from observations made before Webb’s launch. The team also has identified galaxies that existed during a time known as the Epoch of Reionization, when the Universe underwent a transformation from opaque to transparent. Many of these galaxies shown unusually strong emission line signatures due to the creation of multitudes of hot, massive stars. In this image, blue, green, and red were assigned to Webb’s NIRCam (Near-Infrared Camera) data at 0.9, 1.15, and 1.5 microns; 2.0, 2.77, and 3.55 microns; and 3.56, 4.1, and 4.44 microns (F090W, F115W, and F150W; F200W, F277W, and F335M; and F356W, F410M, and F444W), respectively.An orange glow radiates from the centre of NGC 1792, the heart of this stellar forge. Captured by the NASA/ESA Hubble Space Telescope, this intimate view of NGC 1792 gives us some insight into this galactic powerhouse. The vast swathes of tell-tale blue seen throughout the galaxy indicate areas that are full of young, hot stars, and it is in the shades of orange, seen nearer the centre, that the older, cooler stars reside. Nestled in the constellation of Columba (The Dove), NGC 1792 is both a spiral galaxy, and a starburst galaxy. Within starburst galaxies, stars are forming at comparatively exorbitant rates. The rate of star formation can be more than 10 times faster in a starburst galaxy than in the Milky Way. When galaxies have a large resevoir of gas, like NGC 1792, these short lived starburst phases can be sparked by galactic events such as mergers and tidal interactions. One might think that these starburst galaxies would easily consume all of their gas in a large forming event. However, supernova explosions and intense stellar winds produced in these powerful starbursts can inject energy into the gas and disperse it. This halts the star formation before it can completely deplete the galaxy of all its fuel. Scientists are actively working to understand this complex interplay between the dynamics that drive and quench these fierce bursts of star formation.All Image Credits: ESA/Hubble
