Rick Robinson

Aug 17th 2020

The Shapley Curtis Debate of 1920, or How Big Is the Universe?


A little over 100 years ago, the National Academy of Sciences hosted a debate about a very expansive topic. In fact, the issue under debate was as large as any topic could be: the great architecture of the universe.

The Shapley-Curtis debate, as Astronomy reports, was held on April 26, 1920, at the Smithsonian Institution’s Baird Auditorium. In astronomy circles, however, it goes by a different name: the Great Debate.

Clash of the Titans

The debate pitted the distinguished astronomer Heber Curtis against the much younger iconoclast Harlow Shapley, who was looking to make a name for himself (and win the directorship of prestigious Harvard College Observatory).

But what makes the Great Debate a true classic in the history of science is the way it ended. At the time, its outcome seemed inconclusive, even a bit anticlimactic. But new evidence, discovered a few years later, would show that Shapley and Curtis were both half right — and half wrong.

More than that, the great architecture of the universe turned out to be vastly larger and more complex than anyone had imagined.

Discovering the Galaxy

Since Galileo, astronomers knew that the visible Milky Way, evenly dividing the night sky, was made up of enormous numbers of distant stars. This indicated that stars were not just scattered randomly through space. Instead, the distribution of stars seemed to hint at some overall structure, more or less disk-shaped.

By 1920, astronomers had come to grips with interstellar distances. They knew that even the closest neighboring stars were thousands of times further away than the solar system’s planets — and that the most distant stars they could observe must be thousands of times even more remote.

The conventional wisdom, championed in the Great Debate by Curtis, was that what we now call the Milky Way galaxy was about 10,000 to 30,000 light-years across, according to Science News. The sun, Curtis argued, lay fairly close to the center, as shown by the fact that we see roughly a comparable number of stars when we look toward opposite regions of the sky.

Shapley challenged this conventional wisdom, arguing that the galaxy must be at least 10 times larger and about 300,000 light-years across, based on the complexity of its structure and the enormous numbers of very faint stars that formed the visible Milky Way.

Moreover, said Shapley, while the distribution of (relatively nearby) stars might be similar in multiple directions, very distant clusters and nebulae appeared to be heavily concentrated in the general direction of the constellation Sagittarius. This indicated that the sun lay not near the center of the overall system, but well in its outskirts.

(Handily, NASA provides online transcripts of the original presentations in the Shapley-Curtis debate.)

Spiral Nebulae and Island Universes

One class of distant objects did not quite fit Shapley’s pattern: hazy swirls known at that time as spiral nebulae. The largest and brightest lay in the northern constellation Andromeda, but no existing telescope could fully resolve their details, and no one was entirely sure what to make of them.

Curtis argued that because their spectra resembled those of stars, they might be very distant clusters of stars. So distant, in fact, that they were probably not part of the great Milky Way structure at all, but lay somewhere far outside of it.

Some astronomers had been suggesting this for decades, and had coined the term “island universes” to describe them. The implication was that they had roughly the same relationship to the Milky Way that offshore islands did to a continent.

Shapley rejected this idea. In his conception, the Milky Way was simply too vast to leave room for “island universes” or anything else beyond it. Shapley’s galaxy was the universe.

An Inconclusive Debate, Then Stunning New Evidence

The Shapley-Curtis debate, as it turned out, failed to settle the size and shape of the universe. Curtis, a more experienced public speaker, was seen as having the upper hand on points, but not enough to put Shapley away. (And as Astronomy notes, Shapley did get the job as head of Harvard Observatory.)

Then, two years later, new observations of the spiral nebula in Andromeda blew up the whole debate. Astronomer Edwin Hubble (for whom the Hubble Space Telescope is named) succeeded in taking images that resolved the Andromeda object into stars — enormous numbers of them, very faint and very, very far away.

According to Science News, a disappointed Shapley remarked that Hubble’s discovery “destroyed my universe.” However, the Andromeda galaxy, as we now know it, helped to confirm that the Milky Way was much larger and more complex than most astronomers, including Curtis, had thought. (Though at 100,000 light-years across, the Milky Way isn’t as large as Shapley thought.) Moreover, as Shapley had argued, the sun lies far from the center.

As for the universe as a whole, it turned out to be really big, billions of light-years across, thousands of times larger than anyone at the Shapley-Curtis debate would have guessed. Half right and half wrong, the contestants in the debate had only just begun to unfold the immensity of the universe.