Key Takeaway:
In 1924, American astronomer Edwin Hubble discovered that spiral nebulae, previously believed to be part of the Milky Way, were independent galaxies. This discovery, made by Hubble, shifted our understanding of the cosmos from a singular galaxy to a vast expanse filled with countless others. Henrietta Swan Leavitt’s study of Cepheid variable stars, which changed in brightness, provided an unprecedented measuring tool for determining their absolute distance. Hubble’s breakthrough led to the birth of the Big Bang Theory, which posits that the universe is expanding and that the farther away a galaxy is, the faster it appears to be moving. This concept challenged long-held beliefs about a static cosmos and laid the foundation for one of the most transformative ideas in modern cosmology. Today, we estimate that the universe contains up to two trillion galaxies, a number that underscores the vastness of the cosmos.
A century ago, on a quiet Sunday in late November 1924, an article buried among advertisements in the New York Timesrevealed a discovery that would forever alter humanity’s understanding of the cosmos. The headline boldly proclaimed that spiral nebulae—long believed to be part of the Milky Way—were, in fact, independent galaxies, vast and separate star systems drifting far beyond the confines of our own. This revelation, made by American astronomer Edwin Hubble, was a turning point in astronomy, expanding the known universe from a single galaxy to a vast expanse filled with countless others.
The scientific world was still grappling with the nature of these nebulae. Until Hubble’s breakthrough, astronomers fiercely debated whether these spiraling structures were simply gaseous clouds within the Milky Way or distant, self-contained galaxies. Just four years prior, this argument had culminated in a legendary intellectual clash known as The Great Debate, held in Washington, D.C., between two leading astronomers, Harlow Shapley and Heber Curtis. Shapley argued for a vast Milky Way that could contain these spiral nebulae, while Curtis maintained that these structures were entirely separate galaxies. Though no definitive conclusion was reached that night, the tools needed to settle the debate were already emerging, thanks to the meticulous work of an often-overlooked astronomer: Henrietta Swan Leavitt.
Measuring the Cosmos: The Work That Paved the Way
In the late 19th century, Henrietta Swan Leavitt worked as a “computer” at Harvard College Observatory, analyzing photographic plates taken through telescopes. Among her many contributions, one of the most significant was her study of Cepheid variable stars—stars that periodically change in brightness. While examining images of a neighboring galaxy, the Small Magellanic Cloud, she made a groundbreaking discovery: the rate at which these stars pulsed was directly related to their intrinsic brightness. This relationship, later known as the period-luminosity law, provided a way to determine their absolute distance.
Leavitt’s findings offered astronomers an unprecedented measuring tool. If a Cepheid variable’s brightness and pulsing period were known, its true distance could be calculated. This method would soon revolutionize how scientists gauged cosmic distances. Princeton University’s Shapley refined her work to estimate the size of the Milky Way, confirming it was far larger than previously thought. However, this only fueled the debate—was our galaxy alone in the universe, or just one of many?
Hubble’s Breakthrough: The Universe Expands
Armed with the most advanced telescope of his time—the 100-inch Hooker Telescope at Mount Wilson Observatory in California—Edwin Hubble set out to answer this question once and for all. Peering into the spiral nebulae, he spotted a Cepheid variable star within the Andromeda Galaxy, a celestial body whose pulsing light could serve as a cosmic ruler. By applying Leavitt’s period-luminosity relationship, he determined that Andromeda was far too distant to be part of the Milky Way. It was, in fact, a separate galaxy.
This discovery shattered the prevailing notion of a universe confined to a single galaxy. As Hubble expanded his observations, he found that the same was true for the Messier 33 galaxy and many others. The cosmos was not just the Milky Way; it was a vast ocean of galaxies stretching into infinity. When Shapley, once a staunch defender of the single-galaxy theory, read Hubble’s letter detailing his findings, he is said to have handed it to a colleague with the resigned remark: “Here is the letter that has destroyed my universe.”
A Universe in Motion: The Birth of the Big Bang Theory
Hubble’s work did not stop at proving the existence of other galaxies. Using spectral analysis, astronomers had long observed that galaxies appeared to be moving—some toward us, others away. American astronomer Vesto Slipher, using the Doppler shift technique, had already measured these motions and found that most galaxies seemed to be receding.
Building on Slipher’s data, Hubble compared the distances of galaxies with their velocities. What he discovered was astonishing: the farther away a galaxy was, the faster it appeared to be moving. This finding provided the first empirical evidence that the universe was expanding, a realization that would later give rise to the theory of the Big Bang.
The concept of an expanding universe challenged long-held beliefs about a static cosmos. If galaxies were all moving apart, then at some point in the distant past, they must have been much closer together—perhaps originating from a single point. Although Hubble himself remained cautious about the broader implications, his work laid the foundation for one of the most transformative ideas in modern cosmology.
The Legacy of a Discovery
The impact of Hubble’s discovery was profound. It redefined humanity’s place in the universe, shifting our perspective from a singular, isolated galaxy to a vast and ever-expanding cosmos filled with billions of galaxies. Today, thanks to advances in technology and observational astronomy, we estimate that the universe contains up to two trillion galaxies, an unfathomable number that underscores just how much remains to be explored.
To honor his contributions, NASA launched the Hubble Space Telescope in 1990, a powerful observatory that has since captured some of the most breathtaking images of the cosmos. From the iconic Pillars of Creation to the Hubble Deep Field, these images have continued to expand our understanding of the universe, revealing celestial wonders that Hubble himself could have only dreamed of seeing.
A century after that New York Times article first shared his discovery with the world, Edwin Hubble’s work remains a cornerstone of modern astronomy. His insights not only resolved one of the greatest scientific debates of the 20th century but also opened the door to even deeper questions about the nature of space, time, and our place in the vast, unfolding story of the cosmos.
