Early models of the cosmos placed our species at the center. The Earth was the nexus of all existence, and we, the purpose for everything. It was a reasonable assessment, based on the evidence we had. From the perspective of a terrestrial observer, the Sun, Moon, planets, and stars appear to trace paths around us. Upon conducting the simplest inspection of the sky, anyone could see that we are special. We could rejoice: oh, how significant we must be! The entire universe is made for us, and the Earth acts as a pedestal, exhibiting our species at the center. This gratifying discovery was integrated into the language and disseminated by literature and education. It was rarely contested and remained predominant for some 1500 years.

In the mid-16th century, a different hypothesis emerged to explain the motions of the heavens1—that which featured the Sun, rather than the Earth, at the center of the universe. It was widely ignored, disregarded as an outlandish proposal. However, with the advent of the astronomical telescope, evidence began mounting to bolster the notion, challenging the established system. The tenets of geocentrism gradually eroded. Support dwindled as heliocentrism was further substantiated by improved instrumentation. By the nineteenth century, the Earth-centered conception of the universe was dispelled.

We were decommissioned from the leading role at cosmic center stage. The Sun superseded the Earth to hold the foremost position at the center of the universe. Nevertheless, this revision could still be conveyed to preserve the human-centered worldview. It was said that the distance from the Earth to the Sun was trivial when juxtaposed against the immense distances to the stars. By this reasoning, we could claim our displacement to be slight; we are nearly at the center of the universe. Heartened by a sense of significance, we found the cosmos, still, a haven for human pride.

The cosmic model of even just a century ago is hardly recognizable when compared to our modern equivalent. At that time, the Milky Way galaxy was not regarded as merely an occupant of the universe, but rather, the universe itself. The entire cosmos was a sole congregation of stars, surrounded by darkness and void. Some astronomers, though, disputed the prevailing notion as spiral clouds, called nebulae, were more frequently witnessed in the depths of space. They thought the strange mists to be galaxies like the Milky Way—“island universes.” Once again, humanity’s privileged position had come under threat; perhaps we are not central figures in a single-galaxy cosmos, but the inhabitants of a starry isle that is one among a multitude of others. The astronomical community at large, however, considered this absurd. It was unimaginable. Spiral nebulae could not be independent stellar assemblies; they are most certainly local—just solar systems in formation—nested within the Milky Way. Due to a lack of compelling evidence in support, this newfangled theory was dismissed. We could then return with contentment to what was familiar, what was comfortable. We could sleep soundly in the heart of the cosmos.

After a period of dormancy, the controversy surrounding the spiral nebulae resurged in the early 20th century. Heber Curtis was at the forefront of this ideological resurrection. An astronomer at the Lick Observatory in California, he was delegated the task of inquiring into the enigmatic clouds. Although at first he objected to the notion of exterior galaxies, Curtis was convinced otherwise as he accumulated new evidence. During his early research, Curtis imaged nebulae which had been previously examined and noticed an absence of rotational motion (a necessary condition to account for their spiral appearance). If the nebulae were to dwell within the Milky Way, such a rotary movement would be perceptible; if they were extremely large and distant, however, the movement would be too small to discern. Curtis dedicated much of his time during the 1910s to persuading the astronomical community. He lectured frequently, all the while developing and substantiating his arguments. Curtis became synonymous with the island-universe theory—the spokesman, advocating ours is a moderately-sized galaxy, 30,000 light-years wide, in which we are nestled just shy of the center; outside its periphery, there exist other assemblages of stars—a great cosmic citizenry of which the Milky Way is a part.

Curtis’s contentions were not widely accepted. He and his colleagues composed a faction of island-universe proponents, while the majority remained steadfast in opposition. One such contesting astronomer was Harlow Shapley; once a subscriber to the island-universe theory, he rejected the concept, drawing from his own observations. Shapley had measured the distance to all of the known globular clusters—dense collections of stars orbiting the galactic center. The Milky Way, which was thought to span 10,000-30,000 light-years, he deduced was much larger—300,000 light-years across. Shapley then reasoned that such exceptional dimensions of this galaxy preclude the existence of others equal in size. The Milky Way is of such vastness, it must be the primary component of the universe. The spiral nebulae are therefore nearby, settled within the galactic edge. Although the single-unit cosmos persisted, the conventional model did not remain entirely intact. By Shapley’s calculations, our solar system is not ensconced in the cosmic hub, as it had always been; rather, it resides 65,000 light-years distant - an astounding and unforeseen displacement from the center of the universe.

In 1920, these two astronomers, resolute in their respective theories, championing different universes, would come to clash. Curtis and Shapley engaged on the topic of the spiral nebulae at a meeting of the National Academy of Sciences. This historic confrontation was dubbed the “Great Debate.” However, it was really nothing of the sort. The interaction consisted of two successive lectures, given under the title “The Scale of the Universe.” It was just one feature during a three-day event and was hardly publicized. The greatness of the debate, however, evolved so dramatically over time that the encounter has since been reimagined as a fantastic brawl between two giants of astronomy.

Shapley was first at the podium. Rather than directly addressing the spiral nebulae, he relied on implication. If he convinced the audience that the Milky Way is vaster than previously thought, then the nebulae, accordingly, would be considered local members. He, additionally, underscored his discovery that the Sun does not hold a central position in the Milky Way, noting that we had been duped by the appearance of the sky.

Curtis began his lecture with a strike, fervently dismantling Shapley’s argument for the tremendous resizing of the Milky Way. Thereafter, Curtis directed his attention to the spiral nebulae, systematically presenting the array of evidence which he had amassed on the subject.

Ultimately, there was no clear-cut victor. Essentially, nothing changed. Everyone departed with mindsets no different than they had prior. The spiral nebulae remained shrouded in mystery.

Edwin Hubble, a talented and keen astronomer, stepped in soon after, determined to reveal the secret of the spiral nebulae and finally settle the long-fought battle. Hubble studied the Andromeda nebula at the Mount Wilson Observatory with a 100-inch telescope, the largest in the world at the time. He was on the search for novae—stars which rapidly and temporarily increase in luminosity. Novae had been spotted in Andromeda before and used to calculate its distance, but the limited knowledge of the stellar processes caused wildly varying results. Hubble thought that further sightings might advance the investigation of the nebulae and, perhaps, uncover their true nature.

Hubble imaged Andromeda and identified three objects which he suspected to be novae. On the resulting photographic plate, he denoted them: “N.” However, upon consulting earlier photos of the nebula, he noticed that one of the three so-called novae acted atypically. The star was not absent from the archived plates—as would be the case for the transient flare-up of a nova—but rather, the star brightened and dimmed regularly, from one plate to the next. It was, in fact, a variable star; Hubble soon recognized the variable to be a Cepheid. Classified as “standard candles”—astronomical objects with known luminosities—Cepheid variables are precise distance markers, reliable tools for cosmic measurements.2 Elated by this discovery, Hubble crossed out the mistaken “N” and wrote beneath: “VAR” (indicating variable), accompanied by an exclamation point.