Harlow Shapley and Heber Curtis debated the nature of galaxies like Andromeda in 1920

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Astronomers and cosmologists aren’t known for being incredible at adjectives. Take the Very Large Telescope, or the European Extremely Large Telescope, or even the big bang. But they weren’t wrong about the 1920 event now referred to as the Great Debate.

It was spring at the US National Academy of Sciences in Washington DC, and two great astronomers kicked off one of the most contentious issues in the field with opposing opinions on what they referred to as The Scale of the Universe. See, the universe is expanding. At every single moment in time, more space is appearing between the stars, and everything is getting further and further apart. And, as we now know, it’s happening faster and faster.

That expansion is accounted for in astronomical calculations by a number called the Hubble constant, introduced by astronomer Edwin Hubble in 1929. But the argument over just what that number is – how fast the universe is making more universe – began well before that year. In the early 1900s, many scientists thought that the Milky Way galaxy was the entire universe – after all, we didn’t have the technology yet to see beyond our own galaxy. A few strange smudges changed everything. At first, these smudges were called spiral nebulae, and cosmologists around the world were consumed with an argument over whether they were within our own galaxy or if they were indeed galaxies themselves.

In 1920, all that arguing culminated in the Great Debate. Two renowned researchers, Harlow Shapley and Heber Curtis, gave prepared talks to the general public on whether the spiral nebulae, including what we now call Andromeda, were small clouds on the edge of our galaxy – which would mean our galaxy was the only thing out there – or if the nebulae were actually galaxies beyond our own, implying a much bigger and wilder universe.

Shapley’s argument was based on measurements of the distance to stars known as Cepheid variables, which led him to believe that we lived in a vast galaxy about 300,000 light years wide. That’s 10 times bigger than anyone had previously thought and, according to Shapley, there was no way the spiral nebulae were further than that.

Curtis, on the other hand, argued that these strange nebulae were so-called island universes – in essence, other galaxies. He had looked at stellar explosions called novae and found that Andromeda had more of them than the rest of the . He reasoned, if it was just a small part of our galaxy, why would it have so many more explosions than any other part? Plus, the spiral nebulae seemed to be moving extremely quickly around the galaxy. If they were really moving so quickly, there’s no way they could be gravitationally bound to our galaxy and still fit within the prevailing models of astrophysics at the time.