Shapley–Curtis debate

The Great Debate, also called the Shapley–Curtis Debate, was held on 26 April 1920 at the U.S. National Museum in Washington, D.C. between the astronomers Harlow Shapley and Heber Curtis. It concerned the nature of so-called spiral nebulae and the size of the Universe. Shapley believed that these nebulae were relatively small and lay within the outskirts of the Milky Way galaxy (then thought to be the center or entirety of the universe), while Curtis held that they were in fact independent galaxies, implying that they were exceedingly large and distant. A year later the two sides of the debate were presented and expanded on in independent technical papers under the title "The Scale of the Universe".

In the aftermath of the public debate, scientists have been able to verify individual pieces of evidence from both astronomers, but on the main point of the existence of other galaxies, Curtis has been proven correct.

The format

The debate was the topic of that year's William Ellery Hale lecture during a meeting of the National Academy of Sciences in the Baird Auditorium of the U.S. National Museum in Washington, D.C.(now the Smithsonian Museum of Natural History). Most of the Academy members in attendance that night were not astronomers. The topics considered for that year's meeting included Einstein's theory of relativity, glaciers, and even zoological or biological subjects, before a debate on "The Distance Scale of the Universe" was chosen. Shapley and Curtis agreed to a format where each would present their opposing views in back to back 40 minute lectures. Shapley worked from a typed script and presented a general background introduction to astronomy before going on to his views on the size of the universe. Curtis worked from a set of notes and presented his lecture points in type written projected photographic slides.

No transcript of the debate exists; its content has been pieced together over the years from Shapley's original annotated typewritten script, Curtis's slides (his script was discarded after the lecture), and both participants' letters.

The debate

Shapley presented the case that the Milky Way is the entirety of the Universe. In his astronomical work he had been coming up with estimates for the size of the galaxy using globular clusters and the Cepheid variables found within them. He presented the audience with a galaxy 300,000 light-years in diameter with the Sun off to one side. He spent most of his lecture describing the vast size of the Milky Way and towards the end argued that "spiral nebulae" such as Andromeda were simply objects on the edge of the Milky Way itself. He backed up this claim by appealing to their relative sizes—if Andromeda and other spiral nebulae were not part of the Milky Way, then, given the vast size of our galaxy, the distance to them would be a span most contemporary astronomers would not accept.

Curtis, on the other hand, contended that Andromeda and other such "nebulae" were separate galaxies, or " island universes" (a term invented by the 18th-century philosopher Immanuel Kant, who also argued that the "spiral nebulae" were extragalactic). He showed that there were more novae in Andromeda than in the Milky Way. From this, he could ask why there were more novae in one small section of the galaxy than the other sections of the galaxy, if Andromeda were not a separate galaxy but simply a nebula within Earth's galaxy. This led to supporting Andromeda as a separate galaxy with its own signature age and rate of nova occurrences. Curtis also noted the large radial velocities of spiral nebulae that suggested they could not be gravitationally bound to the Milky Way in a Kapteyn-model universe. Curtis pointed out a similarity in structure that explained why there were no spiral nebulae visible along the plane of the Milky Way (referred to as the zone of avoidance); both the Milky Way and the spiral nebulae had similar dust clouds along their plane, and that dust in the Milky Way blocked our view of the spiral nebulae.

The followup paper

In a May 1921 issue of the ''Bulletin of the National Research Council'' Harlow Shapley and Heber Curtis sides of the debate were presented and expanded on in independent technical papers under the title "The Scale of the Universe". The published papers each included counterarguments to the position advocated by the other scientist at the 1920 meeting.

Aftermath

Later in the 1920s, Edwin Hubble showed that Andromeda was far outside the Milky Way by measuring Cepheid variable stars, proving that Curtis was correct. It is now known that the Milky Way is only one of as many as an estimated 200 billion () to 2 trillion () or more galaxies in the observable Universe. Also, astronomers generally accept that the nova Shapley referred to in his arguments was in fact a supernova, which does indeed temporarily outshine the combined output of an entire galaxy. On other points, the results were mixed (the actual size of the Milky Way is in between the sizes proposed by Shapley and Curtis), or in favor of Shapley (the Sun was near the center of the galaxy in Curtis's model, while Shapley correctly placed the Sun in the outer regions of the galaxy).

It later became apparent that van Maanen's observations were incorrect—one cannot actually see the Pinwheel Galaxy rotate during a human lifespan.

Other Great Debates

The format of the great debate has been used subsequently to argue the nature of fundamental questions in astronomy. In honor of the first "Great Debate", the Smithsonian has hosted four more events.

Notes

References

Bibliography

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External links

* (The original publication of the debate.)
* (Resources related to the debate at the NASA website)
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{{DEFAULTSORT:Great Debate, The
History of astronomy
Scientific debates
Physical cosmology
1920 in Washington, D.C.
1920 in science
Astronomy in the United States