A Century Ago: Edwin Hubble’s Revelation That Our Milky Way Is Just One Among Many

If we could travel back 101 years, we would find a period when researchers believed the Milky Way encapsulated the entire universe. However, if we ventured back just 100 years, we would observe most scientists recognizing that this belief was incorrect. In that interim, humanity discovered that the universe is far grander than our Milky Way — the spiral nebulae visible through telescopes turned out to be separate galaxies. The dimensionality of the cosmos expanded tremendously, seemingly overnight.

Historical documentation credits one individual with this revelation: Edwin Hubble. While this is certainly part of the narrative, he could not have achieved his notable breakthrough without the brilliance of his contemporaries who laid the groundwork for his findings.

“It’s easy to idealize Hubble and his discovery of the cosmos beyond the Milky Way galaxy, but his research truly relied on the foundations established by several individuals,” stated Jeff Rich, an astronomer at the Carnegie Science Observatories, during a press conference at the 245th gathering of the American Astronomical Society in Maryland.

That Rich was recounting Edwin Hubble’s story at the January 2025 AAS meeting was significant; it was exactly at the 33rd AAS meeting, a century prior on January 1, 1925, in Washington, D.C., that Hubble’s work was presented publicly for the very first time.

Rich characterizes the revelation of the universe beyond the Milky Way as a discovery centuries in the making, explaining how our comprehension of our existence in the cosmos gradually came together along with new findings. The two individuals whose shoulders Hubble relied upon the most, however, were Henrietta Swan Leavitt and Harlow Shapley.

The most crucial stars in the cosmos

Leavitt served at the Harvard College Observatory as a “computer,” tasked with analyzing photographic plates captured by Harvard’s telescopes. Notably, Leavitt examined images of the Small and Large Magellanic Clouds, identifying 1,800 variable stars within them. Through two papers published by Leavitt in 1908 and 1912, she established that many of these variable stars exhibited a distinct period–luminosity relationship. In essence, she recognized that the duration it took for the stars to regularly pulsate and alternate in brightness as they contracted and expanded was linked to their inherent luminosity.

This discovery was essential. If you identified one of these variables, subsequently termed Cepheid variables, you might not determine its distance immediately, but by analyzing the Cepheid’s periodic variations, you could ascertain its intrinsic luminosity. Consequently, to evaluate how distant the star is, one merely needed to compare its actual luminosity with its apparent dimness in the night sky. Even in contemporary astronomy, Leavitt’s period–luminosity relationship remains a fundamental principle for measuring distances throughout the cosmos.

Meanwhile, given his pivotal role in Hubble’s narrative, it is ironic that Harlow Shapley was skeptical about the existence of anything beyond the Milky Way. At the dawn of the 20th century, telescopes lacked the power to resolve individual stars in other galaxies, rendering spiral galaxies as mere spiral smudges, referred to as spiral nebulas. Shapley believed these spiral nebulas were simply stars forming at the edges of the Milky Way.

Shapley’s objective was to measure the size of the Milky Way — and consequently the universe as he perceived it — by establishing the initial official cosmic distance ladder. The Cepheid variables he discovered in our galaxy constituted the first rung. The subsequent rung comprised RR Lyrae stars, a different type of variable star exhibiting a similar period–luminosity relationship to Cepheids, and whose distances could be calibrated by comparing them with the Cepheid variables. Ultimately, he utilized the RR Lyrae variables to measure the distance to ordinary massive, luminous stars situated at the edges of the Milky Way.

Shapley concluded that the Milky Way spanned 300,000 light years, placing our solar system at 50,000 light-years from its center. Though today we recognize the more precise measurements as 100,000 light-years and 26,000 light-years, respectively, Shapley’s results marked the initial application of a cosmic distance ladder. Shapley even participated in the “Great Debate” alongside fellow astronomer Heber Curtis at the National Academy of Sciences in Washington D.C. in April 1920, debating the nature of spiral nebulas. Curtis contended that these spiral nebulas were their own distinct galaxies but asserted that the Milky Way extended only 10,000 light years across. Shapley countered this argument.

Hubble enters the scene

Edwin Hubble joined the team at Mount Wilson Observatory in California in 1919, mere two years after the observatory’s Hooker Telescope, which was then the largest telescope worldwide, had commenced operations.

“[Hubble’s] breakthrough was made feasible by the 100-inch Hooker telescope at Mount Wilson,” remarked Rich. “Hubble’s discovery was possible because of his access to this cutting-edge technology.”

The Hooker telescope was envisioned by the observatory’s director, George Ellery Hale, designed to, among other objectives, unravel the puzzle of the spiral nebulae, made possible by a generous $45,000 contribution from Californian philanthropist John Hooker.

Before we proceed, let’s acknowledge another significant individual in this tale: Milton Humason. Initially employed as a “mule skinner,” transporting building materials and equipment up Mount Wilson with mules while the observatory was under construction, he later became the janitor of the observatory and subsequently assistant to the astronomers utilizing it. Humason and Hubble became nearly inseparable at the telescope, with Humason making numerous astronomical discoveries independently, despite lacking a Ph.D., and he deserves to share in much of the recognition typically accorded to Hubble.

With everything in place, Hubble and Humason commenced their observations of spiral nebulas using the Hooker telescope. In 1923, they managed to capture an image of the Andromeda spiral nebula, Messier 31, revealing something remarkably significant.

“Hubble was so exhilarated by this image that he inscribed ‘VAR!’ on the black and white glass plate, having noticed evidence of a Cepheid variable,” stated Rich. This Cepheid variable star was subsequently designated as ‘V1’. “He understood, based on the work of Henrietta Leavitt and Harlow Shapley, that this indicated he could measure the distance to a spiral nebula for the very first time.”

And measure it

he did. He computed a range of 930,000 light-years, which is less than half the actual distance of 2.5 million light-years. However, despite the constraints of Hubble’s basic calculations (the cosmic distance ladder is still undergoing refinements even in contemporary studies), it undeniably indicated that the Andromeda spiral resided beyond the limits of the 300,000 light-years that Shapley quantified for the Milky Way. Messier 31 was not merely a spiral nebula; it was a spiral galaxy.

Hubble communicated with Shapley, informing him of his revelation. Upon reading the letter, Shapley remarked, “Here is the letter that obliterated my universe.”

Hubble ‘leaked’ details of his discovery to the New York Times in November 1924, which is why the presentation at the AAS the following January, actually conducted by astronomer Henry Norris Russell and not Hubble himself, was intended as the official announcement — informally, however, people were already aware of it.

In the present day, we take for granted that the cosmos is populated with galaxies, some spiral-shaped like the Milky Way and Andromeda, others massive ellipticals, and some small dwarfs. The latest estimates suggest that up to 2 trillion galaxies could exist within the observable universe. Yet, Rich notes how Hubble’s groundbreaking discovery was indeed relatively recent.

“One hundred years is not a long time,” he remarked. Indeed, there are a handful of individuals older than that, having been born in an era prior to the acknowledgment of other galaxies. “This illustrates just how drastically things have transformed and how swiftly discoveries can arrive.”

Today, the photographic plate capturing the Cepheid variable V1, with Hubble’s hastily written “VAR!” in the corner, serves as a treasured memento of discovery, something a scientific Indiana Jones might seek in a millennium’s time. Fortunately, you need not undertake such a strenuous expedition to find it. Typically secured, the plate has been allowed to breathe and is presently showcased for a few months at the Mapping the Infinite: Cosmologies Across Cultures exhibition at the Los Angeles County Museum.

As for Hubble, his journey didn’t end there. His subsequent tuning-fork diagram illustrating the shapes of galaxies continues to be a fundamental educational resource for astronomers, and despite the evolutionary portrayal of galaxies depicted in the tuning fork being inverted, professional astronomers still adhere to the nomenclature of “early” and “late” galaxies suggested by the tuning fork.

Then, in 1929, Hubble unveiled that nearly all the other galaxies in the cosmos were receding from us, based partially on redshift data by fellow astronomer Vesto Slipher, aligning with theoretical developments by Belgian physicist and priest Georges Lemaître, who formulated what is now referred to as the Hubble–Lemaître law that describes the expansion of the universe.

In just five years, we transitioned from believing the Milky Way was the entirety of existence to discovering an infinite, expanding universe. It marked a significant paradigm shift and, coinciding with Albert Einstein‘s General Theory of Relativity, released in 1915, and around the same period when the world’s most eminent physicists, led by Niels Bohr, were deciphering the domain of quantum physics, it became a cornerstone of a transformative epoch in science that has influenced our current comprehension of the cosmos. With new enigmas such as dark matter, dark energy, the quest for a quantum theory of gravity, the Hubble tension, and the origins of the Big Bang challenging physicists today, now seems an opportune moment for another scientific transformation reminiscent of a century ago.