The Great Debate over the Spiral Nebulae


By Emily Levesque, University of Washington

In the early 1900s, a fierce debate was churning in the astronomical community about what spiral nebulae actually were. The argument came to a head in April of 1920 with an event known as the “Great Debate”, a presentation pitting astronomers Harlow Shapley and Heber Curtis against one another at the Smithsonian Museum of Natural History in Washington, DC.

A true image of the great Andromeda Galaxy taken by NASA
The great Andromeda Galaxy is a spiral structure. (Image: NASA/Public domain)

Harlow Shapley’s Argument in the “Great Debate”

Harlow Shapley was convinced that these spiral nebulae were part of the Milky Way and pointed to the nearby Great Andromeda Nebula as evidence. If this nebula were a separate galaxy, he argued, it would imply that it sat more than 100 million light-years from our own, a distance far beyond anything imagined by astronomers at that point in time. 

Image of Harlow Shapley
Harlow Shapley argued that the spiral structures are part of a Milky Way Galaxy. (Image: Unknown/Public domain)

He also mentioned that observers had detected a surprisingly bright burst of light in the Andromeda Nebula in 1885; at the time a flash like this was known as a nova, a term we now use to refer to an eruption from the surface of a nearby star. Astronomers were detecting novae fairly regularly at that time, but this particular nova had been shockingly bright, appearing to briefly outshine the entirety of the Andromeda Nebula. 

Shapley correctly pointed out that if this particular nova had truly been hundreds of millions of light-years away, it implied a brightness and energy release so extreme that it seemed impossible.

Heber Curtis’s Argument

Meanwhile, Heber Curtis argued that Andromeda and the other strange spiral nebula were in fact other galaxies, or island universes. He acknowledged that Andromeda had hosted a very bright nova; in fact, it had hosted quite a number of novae, most much dimmer, and all concentrated in the Great Andromeda Nebula. 

Based on this, he argued that Andromeda must comprise its own collection of stars, with its own age and its own rate of nova events; otherwise, why would so many novae be concentrated in such a small part of the Milky Way? Curtis also pointed to the dark shapes tracing the spiral arms of other nebulae, noting that they looked similar to the dust clouds found in the Milky Way.

Settling the Argument

A portrait of Heber Curtis
Heber Curtis argued that the Great Andromeda is a separate galaxy. (Image: National Institute of Standards and Technology/Public domain)

It became clear that there was only one way to truly settle the argument: astronomers needed a way to measure the distance to the Great Andromeda Nebula. 

A year before the Great Debate, a young astronomer named Edwin Hubble accepted a staff position at Mount Wilson. His thesis research was carried out at George Hale’s Yerkes Observatory, and focused on studying photographic plates of faint nebulae.

Hubble arrived at Mount Wilson after World War I, just as Hale’s infamous 100-inch telescope was being completed and Hubble quickly began to use the 100-inch for his research. On an October night in 1923, Hubble captured a photographic plate image of the Great Andromeda Nebula that revealed a shocking discovery: a Cepheid variable in Andromeda. 

The photographic plate where Hubble made this discovery is preserved in the Carnegie Observatories archives, famous for the bright red “VAR!” scrawled on the plate by Hubble next to the innocuous little point of light that is the Cepheid, clearly illustrating his excitement at what he had found.  He knew that this discovery would be able to settle the debate about Andromeda—and indeed, about all other spiral nebulae—once and for all.

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A Nebula or a Galaxy?

Hubble’s new Cepheid had a long period of 31 days, meaning it must be quite luminous, but it appeared to be extremely faint. Combined, this suggested that the Cepheid was incredibly far away. Ultimately, Hubble measured a distance to the Andromeda Cepheid of more than one million light-years. At that distance, Andromeda—clearly visible as a spiral in astronomers’ observations—must be truly enormous, as large as the Milky Way itself. 

His result proved that the Great Andromeda Nebula was not, in fact, a nebula. It was no cluster of stars and gas within our own Milky Way—it was, instead, an entirely separate galaxy located incredibly far away. Hubble had radically altered astronomers’ understanding of the scale and nature of the cosmos. 

Baad’s Discovery for Determining the Distance to Andromeda

Interestingly, the debate over Andromeda’s distance—and the nature of Cepheid variables—didn’t quite end with Hubble. Nearly two decades after Hubble’s observations, astronomer Walter Baade, discovered that there were actually two classes of Cepheid variables.  

On the Hertzsprung-Russell diagram, classical Cepheids stood out as more massive stars, landing high on the instability strip, while type two Cepheids appeared to be lower-mass and older stars, lying lower on the instability strip. The distinction between the two classes could be determined by combining the stars’ spectral types and pulsation periods.

Baade’s discovery nearly doubled the distance to the Andromeda galaxy, which we now know is about 2.5 million light-years away. This improved understanding of Cepheid variable types also doubled the distance to many other galaxies observed by Hubble, and ultimately doubled Hubble’s estimated size of the entire visible universe.

Common Questions about the Great Debate over the Spiral Nebulae

Q: What was the Great Debate?

In the early 1900s, there was a heated debate in astronomical society about what spiral nebulae really were. This debate arose in April 1920 with an event called the Great Debate. Held at the Smithsonian Institution of Natural History in Washington, DC, the event pitted astronomers Harlow Shapley and Heber Curtis against each other.

Q: Who was Harlow Shapley?

Harlow Shapley was one of the astronomers in the Great Debate and was convinced that spiral nebulae were part of the Milky Way galaxy. He cited the great Andromeda Nebula as evidence. He also argued that if Andromeda were a separate galaxy, it would be more than 100 million light-years from Earth, a distance far beyond what astronomers imagined then.

Q: Who was Heber Curtis?

Heber Curtis was another astronomer at the Great Debate event. He argued that Andromeda and any other strange spiral nebula were in fact galaxies or island worlds. Curtis also pointed to the dark shapes that track the spiral arms of other nebulae, and he found them similar to the dust clouds in the Milky Way galaxy.

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