What’s in a date? Strictly speaking, New Year’s Day is just an arbitrary flip of the calendar, but it can also be a cathartic time of reflection and renewal. So it is with one of the most extraordinary dates in the history of science, January 1, 1925. You could describe it as a day when nothing remarkable happened, just the routine reading of a paper at a scientific conference. Or you could recognize it as the birthday of modern cosmology–the moment when humankind discovered the universe as it truly is.
Until then, astronomers had a myopic and blinkered view of reality. As happens so often to even the most brilliant minds, they could see great things but they could not comprehend what they were looking at. The crucial piece of evidence was staring them right in the face. All across the sky, observers had documented intriguing spiral nebulae, swirls of light that resembled ghostly pinwheels in space. The most famous one, the Andromeda nebula, was so prominent that it was easily visible to the naked eye on a dark night. The significance of those ubiquitous objects was a mystery, however.
Some researchers speculated that the spiral nebulae were huge and distant systems of stars, “island universes” comparable to our Milky Way galaxy. But many others were equally convinced that the spirals were small, nearby clouds of gas. In this view, other galaxies–if they existed–were far out of sight, blue whales lurking in the far depths of the cosmos. Or perhaps there were no other galaxies at all, and our Milky Way was all there was: a single system that defined the entire universe. The dispute between the two sides was so intense that it prompted a famous 1920 Great Debate…which ended with an unsatisfying draw.
The correct picture of our place in the universe arrived just a few years later through the work of one of the most famous names in astronomy: Edwin Powell Hubble (no relation!). Starting in 1919, Hubble had established himself as one of the most patient and meticulous observers at Mount Wilson Observatory in California. Mt Wilson, in turn, had just established itself as the premier outpost for astronomical research, home of the just-completed 100-inch Hooker Telescope—then the biggest in the world. It was the perfect combination of the right observer in the right place at the right time.
Always cautious when it came to theory and interpretation, Hubble focused his scientific attention on the spiral nebulae without overtly endorsing the “island universe” interpretation. He preferred to wait until he could be the one to step forward with definitive proof–or disproof, if that’s where the evidence pointed.
In 1922, another important piece of the puzzle fell into place. That year, Swedish astronomer Knut Lundmark observed what he believed were individual stars in the arms of the spiral nebula M33. Shortly after, John Duncan at Mount Wilson spotted dots of light that grew fainter and brighter in the same nebula. Could these be variable stars, similar to ones in the Milky Way but far dimmer owing to their enormous distance?
Sensing the answer was at hand, Hubble stepped up his efforts. He spent long nights on his favorite bentwood chair, guiding the movements of the riveted-steel mount of the Hooker telescope to cancel out Earth’s rotation. The effort paid off with highly detailed, long-exposure images of the Andromeda nebula. The mottled light of the nebula began to resolve itself into a multitude of luminous points, looking not like a smear of gas but like a vast hive of stars.
Clinching proof came in October of 1923, when Hubble spied the telltale flicker of a lone Cepheid variable star in one of Andromeda’s arms. This type of star grows brighter and dimmer in a regular and predictable way, with its intrinsic luminosity directly related to its period of variation. Simply by timing the 31-day cycle of this star as it slowly flickered, Hubble could deduce its distance. His estimate was 930,000 light years–less than half the modern estimate, but a shockingly large number at the time. That distance placed Andromeda, one of the brightest and presumably closest of the spiral nebulae, vastly outside the bounds of the Milky Way.
In principle, the Great Debate was settled then and there. Spiral nebulae were other galaxies, and our Milky Way was just one outpost within a staggeringly vast universe. And yet, still the story was far from over.
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