March 21, 2019 by Written by Ashley Hamer
Imagine if you’d lived your whole life indoors, believing your little room was all of reality, only to wake up one morning to learn that there are millions of other buildings just like yours and that the world was much, much bigger than you thought. That’s what happened to planet Earth in 1925 when Edwin Hubble confirmed that the smudges of light in the sky that astronomers had been puzzling over were indeed spirals of stars outside of the Milky Way — their own “island universes,” as some called them (or galaxies, as we call them today). The story of their discovery is one of dramatic intrigue, media sensationalism, and just a little spite.
Small and Close vs. Big and Far
A century ago, astronomers were mired in a fierce debate over what they saw in the night sky. The heavens were dotted with swirling pinwheels of light they called spiral nebulae. Most could only be seen with a telescope, but one, the Andromeda nebula, could be seen with the naked eye if the night was dark enough. What exactly were they?
The answer depended on who you asked. One camp speculated that the nebulae were massive, faraway clusters of stars a lot like our own Milky Way. The other camp thought they were smaller, closer clouds of gas, akin to our modern definition of the word “nebula.” This debate was formalized in what literally came to be called “The Great Debate,” with astronomer Heber Curtis in one corner and Harlow Shapley in the other. One fateful night in 1920, the two men presented their published papers, Heber arguing that the universe is composed of many galaxies like our own while Shapley argued that the entire universe is one big galaxy.
There was no winner that night, and both scientists would prove to be right on some points and wrong on others. But as for the main argument, it would take a few more years before the world had an answer. That would come from a name you’ve certainly heard before: Edwin Hubble, an arch-rival of Shapley who just so happened to be studying the heavens via the world’s biggest stargazer, the 100-inch Hooker Telescope.
“Another Earth in Another Universe”
Previous research had suggested that these nebulae were actually moving away from us at a dramatic speed, and that made Hubble a quiet advocate of the many-galaxies theory. He spent long periods aiming the Hooker Telescope at the Andromeda nebula, hoping to capture extra detail that could reveal its secrets. His patience paid off; images revealed that the nebula was not a cloud of gas, but a cluster of stars.
Then in 1923, Hubble got his smoking-gun evidence: He spotted a Cepheid variable, a particular type of star that shines and dims in a predictable way that can tell you how far away it is. Hubble estimated that this star within the Andromeda galaxy was an eye-watering 930,000 light-years away, farther than anything detected at that time. (We now know that the Andromeda galaxy is actually about 2.5 million light-years away, but still, the estimate did what it needed).
But Hubble was a cautious scientist, and it wasn’t until his discovery leaked to the media and other astronomers suggested he publish his results that he eventually agreed to. On January 1, 1925, the world finally heard about his discovery. The universe was 100,000 times larger than we’d previously believed. Our small place in space got even smaller.
The media paid less attention than you might think, but those who did notice advertised the news in lights. “Offers Proof of Complete Universe Beyond Our Own,”announced one headline in Baltimore’s Evening Sun, atop an article that sensationalized the discovery as “proof that beyond this entire universe … is another Earth in another universe separate from our own.”
As Corey Powell points out in Discover Magazine, it was oddly Shapley, not Hubble, who suggested everyone stop calling these pinwheels “nebulae” or “island universes.” They were just like the Milky Way galaxy, he said, so they should be called “galaxies.” But Hubble had won the fight; he wasn’t about to take suggestions from the losing side. For the rest of his life, he called these objects “extra-galactic nebulae.”
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