100 years ago: Edwin Hubble proved the Milky Way is not the only galaxy in the universe

In early 1920s astronomy, most researchers believed our Milky Way was the entire universe. They assumed those faint spiral patches in the sky were just cloudy formations within our own galaxy.

Little did they know that a major twist was about to happen. After decades of tireless work, one astonishing discovery showed those spiral spots were completely separate galaxies. It changed our sense of place in the cosmos forever.

Jeff Rich from the Carnegie Science Observatories reminds us that science is built on collective efforts, not just the findings of individual heroes. 

“It’s easy to romanticize Hubble and his discovery of the universe beyond the Milky Way galaxy,” said Jeff Rich, in a press conference at the 245th meeting of the American Astronomical Society in Maryland.

It was fitting that Rich shared Hubble’s story at the January 2025 American Astronomical Society (AAS) meeting since, exactly a century earlier, on January 1, 1925, Hubble’s paradigm-shifting findings were first presented at the AAS’s 33rd meeting in Washington, D.C.

Focus on small beginnings

A crucial part of this tale begins with Henrietta Swan Leavitt, who examined variable stars in the Small Magellanic Cloud and Large Magellanic Cloud. She noticed that the brightness of certain stars changed in a predictable cycle.

These stars were eventually called Cepheid variables. Using Leavitt’s period-luminosity relation, astronomers finally had a tool to measure truly vast distances. This laid the groundwork for bigger discoveries.

The puzzle of distance

Around the same time, Harlow Shapley wanted to figure out the size of the Milky Way. He used Cepheid data and another type of pulsating star, RR Lyrae, to form the first cosmic distance ladder.

His results suggested our galaxy was large. Still, he believed these spiral smudges were part of it. He only needed a definitive measurement for one of those smudges in order to test the theory.

The turning point at Mount Wilson

When Edwin Hubble arrived at Mount Wilson Observatory in California, the massive Hooker telescope was at his disposal. That powerful instrument gave him a way to spot individual stars in the Andromeda “nebula.”

Working closely with his colleague Milton Humason, Hubble recorded a Cepheid variable in that distant patch of light. This signaled that Andromeda was far beyond our galaxy.

A letter that changed everything

Hubble calculated an approximate distance to Andromeda of more than 900,000 light-years, though modern estimates hover around 2.5 million light-years. The numbers made one thing clear: the Milky Way was not alone.

“Here is the letter that destroyed my universe,” said Harlow Shapley reportedly, when he realized how this clashed with his earlier picture. 

How far we’ve come

Not long after this, Hubble used redshift data gathered by Vesto Slipher to show that most galaxies are speeding away from us. This observation fits with equations from Georges Lemaître, who framed it in a law describing cosmic expansion.

Fast forward to today, and astronomers estimate the universe contains about 2 trillion galaxies, each with its own unique history. All these galaxies exist beyond the star-filled swirl that we call home.

Shaping the future

A century ago, the Milky Way seemed like everything. Hubble’s work widened our view, influenced Einstein’s thinking on relativity, and fueled modern cosmology.

Scientists still rely on Cepheid variables to check cosmic distances. They use advanced telescopes to sharpen these measurements, hoping to resolve the Hubble tension about the expansion rate of our universe.

Glancing ahead

The evolving story of galaxies, dark matter, dark energy, and other puzzles holds as many surprises as it did a hundred years ago. Observatories worldwide keep searching for signals that might rewrite today’s cosmic beliefs.

Hubble’s discovery shows how quickly new evidence can overturn established ideas. It also reminds us that each generation of astronomers relies on the insights and labor of those who came before.

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