In 1923, the universe was still thought to be a small place. Astronomers believed the Milky Way galaxy was at least tens of thousands light-years across, but they thought it was all there was to the universe — one grand galaxy spiraling in a lonesome ballet.

To be sure, the Milky Way is an enormous place, with at least a hundred billion stars. But Edwin Hubble suspected there was so much more.

Hubble, who would have turned 127 years old this year, was perplexed by Andromeda, a fuzzy spiral region in the night sky.

At the time, the prevailing view was that Andromeda was located inside our own Milky Way. It was assumed to be a nebula, a region of gaseous space where stars are made.

Hubble was joined by a small but growing group of scientists — led by Heber Curtis of the Lick Observatory at the University of California — who were doubtful of this conclusion.

To them, Andromeda was incredibly odd. A large number of supernovae — exploding stars — emanated from it, many more than you’d expect from a similarly sized swath of the night’s sky. What’s more, these numerous supernovae were all very faint, as if they were very far away.

Hubble and Curtis hypothesized Andromeda was its own “island universe,” a self-contained system of stars, much like our own Milky Way. At the time, this was an outrageous idea, and was met with stiff, stubborn resistance by the scientific establishment. It would mean the universe was at least double its imagined size.

In 1920, Harlow Shapley, an astronomer who had just produced the most accurate measurement of the Milky Way to date, engaged Curtis in a fierce “great debate” at the National Academy of Sciences. Shapley held the view that the Milky Way and the Universe were on in the same, and Andromeda was a “truly nebulous object.”

More evidence was needed for either argument. And Hubble was the one to find it.

How a female astronomer helped Hubble make his groundbreaking discovery

In 1923, Hubble was an astronomer at the Mount Wilson Observatory, which was then home to the Hooker telescope, the largest telescope in the world.

To prove Andromeda existed outside the Milky Way, Hubble would need to measure how far away it was from Earth. If it was farther away than the estimated diameter of the Milky Way, then it couldn’t be inside the Milky Way.

Hubble’s work here stands on the back of another great but less well-known astronomer, a woman named Henrietta Leavitt.

Leavitt was an astronomer at the Harvard College Observatory in the early 1900s, where she was employed as a “computer,” a person whose job it was to support the senior astronomers by making mathematical calculations (much like computers do today). In 1908, while making calculations of a class of stars called cepheids, she made a wild discovery.

Cepheids are stars that periodically dim and brighten. Leavitt discovered that the time it took for these stars to cycle could be used to calculate how far away the stars are in the sky. With this discovery, astronomers finally had a yardstick, a way to measure the distance to objects in the cosmos. (Learn more about Leavitt here.)

All Hubble had to do was look for cepheid stars in Andromeda and make the appropriate calculations. Night after night, he took photographs of Andromeda with the enormous telescope, searching for cepheids. In October 1923, he found one, blinking in one of Andromeda’s spiral arms. A week more of observations allowed him to follow Leavitt’s formula and determine its distance.

This is a picture of that star, called “variable number one” — or V1 — captured by Hubble’s namesake telescope in 2011. V1 has been called “the most important star in the history of cosmology."

The light from V1 made it clear: Andromeda was well outside the bounds of the Milky Way. When Shapley received a letter about Hubble’s results, he reportedly said, “Here is the letter that has destroyed my universe.”

Destroyed it, yes. But its shattering also made it larger. The Milky Way was no longer the entire universe. It was more like one tiny grain of sand on a beach; a ballet with an untold number of dancers.

Hubble announced his findings exactly 92 years ago today, and presented the results soon after in a January 1925 at a meeting of the American Astronomical Society. And they shocked the world. As the New York Times declared jubilantly:

This work of Hubble’s was just the beginning of an incredibly fruitful career. He’d go on to develop a system for classifying galaxies and find evidence that the universe was expanding, a finding that shook even Albert Einstein.

Today, the universe keeps growing larger. Astronomers now believe there are some 2 trillion galaxies stretching out over 90 billion light-years. Wherever we look in the cosmos, there are endless mysteries to uncover. Hubble taught us that these mysteries can be bigger and grander than we ever imagined.

As Hubble said in 1948: “Equipped with his five senses, man explores the universe around him and calls the adventure Science.”

There are more adventures to be had.

Correction: This post originally misstated the estimated number of stars in the Milky Way galaxy.