Astro History: The Great Debate

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April 26^th marks the 106th anniversary of a particular event in space science history. It’s not the anniversary of a discovery or a launch or a breakthrough of any sort, but it still has gone down in the annals of astronomy lore as a Very Important Date. On April 26^th, 1920, two of the biggest heavy hitters in astronomical science of the day, Harlow Shapley and Heber Curtis, met in Washington, DC for what is today known simply as The Great Debate.

In this entirely academic undertaking, the two science giants vied against each other over questions that were bothering researchers of the day: what is the actual nature of the universe? How big is it? How big is the Milky Way? And what is the difference between them? 

For modern folks who immerse themselves in space science (guilty) the difference between the Milky Way and the universe is as big as the that between your hometown and the Earth. But for folks who, say, hear the word “centaur” and think of a horse/human hybrid and not a weird outer solar system object, the subject debated by Shapley and Heber in 1920 is not so cut and dried. 

In honor of the anniversary of this event, let’s meet Harlow Shapely and Heber Curtis and find out what it was they were arguing about—and what the guy the Hubble Space Telescope is named for has to do with any of it.

The Great Debaters

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Harlow Shapley was a native of Missouri who dropped out of formal education before he reached middle school. By teaching himself while working, he managed to get back into school in time to earn a diploma and get himself to the University of Missouri. According to the story that gets told (and was related to me in college by my astronomy professor), he didn’t know what he wanted to study so he opened up the course catalog to see what came up first.

The first subject was archaeology. Harlow couldn’t figure out how to pronounce it, so he skipped it. Next up was astronomy. As that offered no pronunciation issues, Shapley enrolled, and eventually became one of the most influential astronomers of his age. After the Great Debate he became the Director of Harvard Observatory and made seminal contributions to space science, not the least of which was serving as graduate advisor to the great Cecilia Payne-Gaposchkin. 

Heber Curtis had a more conventional entry into the world of space science. He came from Michigan, though his astronomy degree eventually came from the University of Virginia. He worked for many years at Lick Observatory in California, once home to the largest telescope in the world. His research focused on a survey of nebular objects, and he is the first to be credited with finding a jet emanating from the core of an active galaxy. Following the Great Debate he served as director of several observatories and even has a small lunar crater named after him.

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The Question

When Shapley and Curtis stepped onto the stage in Baird Auditorium at the Smithsonian Museum of Natural History on April 26^th, 1920, they were there to represent diametrically opposing viewpoints about the size of the universe and the Milky Way’s role within it. Specific to the argument was the nature of so-called spiral nebulas.

Telescopes such as the ones at Lick or Harvard Observatories were, by 1920, well able to make out the fact that there were fuzzy things in the sky with distinctive spiral shapes to them, most prominently the Andromeda Nebula. For centuries the nature of these spiral nebulas had been questioned: how big were they? How far away? Were they, in fact, nebulas within the Milky Way, or were they “island universes” of their own? 

This was what Shapley and Curtis came to debate. Shapley came down on the side that said the Milky Way was everything there was, and that the spiral nebulas were, indeed, strangely-shaped nebulosities within the bounds of the Milky Way, or perhaps just outside it. Curtis took the opposite side, that these spiral nebulas were actually things like the Milky Way at great distances away.

The Arguments

A lot of Shapley’s argument came down to the sizes of things. He argued that observations of globular clusters made the case for a Milky Way that was 300,000 light years across, an unimaginably enormous scale for the minds of 1920. If, he argued, the Andromeda Nebula was actually a whole separate thing like the Milky Way, then to look so small to our eyes it must be millions of light years away—and that was absurd, for the universe was nowhere near so big!

Curtis used measurements of the distribution of the spiral nebulas to argue that they were outside the Milky Way. We don’t, for instance, see any along the cloudy band of the Milky Way, only above and below it, as we would if they were outside our galaxy. Also these spiral nebulas seemed to have a lot of novas in them, and if they’re inside the Milky Way it would be very odd that these spots within our galaxy had so many more novas than other spots.

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Besides, Curtis argued, the Milky Way was nothing like 300,000 light years across. It was, at most, only 30,000 light years across. If the Andromeda Nebula was indeed an identical galaxy of similar size, then it would only be about 500,000 light years away. Quite far, certainly, but not outside the realm of possibility. 

The two debated for over an hour, with each given 40 minutes to make his argument. This was followed a year later by each publishing an expanded account of their arguments. The going impression was that Curtis had made the better argument and won the actual debate, but the showing helped Shapley get hired by Harvard, so he probably wasn’t too broken up about it.

The Truth of the Matter

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Of course, in the end, they were both right and they were both wrong. When it comes to the fundamental question of whether the so-called spiral nebulas were within the Milky Way or were island universes of their own, Curtis proved to be right. But it wasn’t until a fella by the name of Edwin Hubble came along that the argument was truly settled.

In 1924 the man for whom one of our greatest telescopes would eventually be named was able to make more precise (though still very bad by current standards) measurements of the distances to several spiral nebulas, including Andromeda. His calculations placed Andromeda at a distance of nearly a million light years away, with other spiral nebulas being much farther. 

It was impossible, even for the crowd that wanted the Milky Way to be the entire universe, that our galaxy could extend so far. The spiral nebulas had to be their own separate entities entirely outside the Milky Way. I can’t find an exact date when we began to refer to the Andromeda Nebula as the Andromeda Galaxy, but it would likely have been within the years following publication of Hubble’s discovery.

Today we know that Andromeda is about 2.5 million light years away from us. So in the end Shapley wound up having the more correct view of how big things were out beyond the bounds of the Milky Way, though he used it to come to the wrong conclusion. As for the Milky Way itself, both of them were wrong. It’s hard to measure our own galaxy’s size while we’re stuck within it, but today we estimate the Milky Way as being something around 100,000-150,000 light years across—half the size of Shapley’s Milky Way but as much as five times the size of Curtis’s.

It’s a Big Universe Out There

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In the end the cosmos turned out to be far more vast than Shapley or Curtis or anyone of their generation could have believed. One can only imagine how much their minds would have been blown if they could have seen the data being returned by the likes of the Hubble, Webb, or Rubin Observatories.

I like to think sometimes about what current beliefs we have that scientists in a hundred years will look back on and say “how quaint they were”. What theories are being put forth now that we think of as being very “out there” that will turn out to be correct? All the debates around dark energy and the expansion of the universe come to mind.

And of course, sometimes we don’t even know what we don’t know. That will be up to the coming generations to show us. I wish them luck (and healthy debates)!