The Great Debate, also called the Shapley–Curtis Debate, was held on 26 April 1920 at the

Smithsonian Museum of Natural History The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. In 2021, with 7. ...

, between the astronomers

Harlow Shapley Harlow Shapley (November 2, 1885 – October 20, 1972) was an American scientist, head of the Harvard College Observatory (1921–1952), and political activist during the latter New Deal and Fair Deal. Shapley used Cepheid variable stars to estim ...

and

Heber Curtis Heber Doust Curtis (June 27, 1872 – January 9, 1942) was an American astronomer. He participated in 11 expeditions for the study of solar eclipses, and, as an advocate and theorist that additional galaxies existed outside of the Milky Way, wa ...

. It concerned the nature of so-called

spiral nebula Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...

. Shapley believed that these nebulae were relatively small and lay within the outskirts of the

Milky Way galaxy The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

(then thought to be the entire universe), while Curtis held that they were in fact independent galaxies, implying that they were exceedingly large and distant. The two scientists first presented independent technical papers about "The Scale of the Universe" during the day and then took part in a joint discussion that evening. Much of the lore of the Great Debate grew out of two papers published by Shapley and by Curtis in the May 1921 issue of the ''Bulletin of the National Research Council''. The published papers each included counter arguments to the position advocated by the other scientist at the 1920 meeting. 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.

Arguments

Shapley was arguing in favor of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

as the entirety of the universe. He believed that "

spiral nebulae Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''Andromeda were simply part of the Milky Way. He could back up this claim by citing relative sizes—if Andromeda were not part of the Milky Way, then its distance must have been on the order of 10⁸

light years A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

—a span most contemporary

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

s would not accept.

Adriaan van Maanen Adriaan van Maanen (March 31, 1884 – January 26, 1946) was a Dutch–American astronomer. Van Maanen, born into a well-to-do family in Friesland, studied astronomy at the University of Utrecht (earning his Ph.D. in 1911) and worked briefl ...

, a well-respected astronomer of the time, also provided evidence supporting Shapley's argument. Van Maanen claimed he had observed the

Pinwheel Galaxy The Pinwheel Galaxy (also known as Messier 101, M101 or NGC 5457) is a face-on spiral galaxy away from Earth in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781 and was communicated that year to Charles Messier, who ...

rotating, and that if the Pinwheel Galaxy were in fact a distinct galaxy and could be observed to be rotating on a timescale of years, its orbital velocity would be enormous and there would be a violation of the universal speed limit, the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...

. Shapley also backed up his claims with the observation of a

nova A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...

in the Andromeda "nebula" that had briefly outshone the entire nebula, constituting a seemingly impossible output of energy were Andromeda in fact a separate galaxy. 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 Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher and one of the central Enlightenment thinkers. Born in Königsberg, Kant's comprehensive and systematic works in epistemology, metaphysics, ethics, and ...

, who also believed 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 The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

of spiral nebulae that suggested they could not be

gravitationally bound The gravitational binding energy of a system is the minimum energy which must be added to it in order for the system to cease being in a gravitationally bound state. A gravitationally bound system has a lower (''i.e.'', more negative) gravitation ...

to the Milky Way in a Kapteyn-model Universe. In addition, he cited dark lanes present in other galaxies similar to the dust clouds found in Earth's own galaxy, explaining the

zone of avoidance The Zone of Avoidance (ZOA, ZoA), or Zone of Galactic Obscuration (ZGO), is the area of the sky that is obscured by the Milky Way. The Zone of Avoidance was originally called the Zone of Few Nebulae in an 1878 paper by English astronomer Richard ...

. Curtis stated that if van Maanen's observation of the

rotating were correct, he himself would have been wrong about the scale of the universe and that the Milky Way would fully encompass it.

After the debate

Later in the 1920s,

Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an Americans, American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects ...

showed that Andromeda was far outside the Milky Way by measuring

Cepheid variable A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid varia ...

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, proving Curtis the more accurate party in the debate. Also, astronomers generally accept that the nova Shapley referred to in his arguments was in fact a

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...

, 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 (our 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

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.

References

Bibliography

External links

* (The original publication of the debate.) * (Resources related to the debate at the

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

website) * {{DEFAULTSORT:Great Debate, The History of astronomy Scientific debates Physical cosmology 1920 in Washington, D.C. 1920 in science Astronomy in the United States