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The Great Debate, also called the Shapley–Curtis Debate, was held on 26 April 1920 at the Smithsonian Museum of Natural History, 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 distant nebulae were relatively small and lay within the outskirts of Earth's home galaxy, 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.

Harlow Shapley (left) and Heber Curtis (right).

The Academy selected Harlow Shapley to argue against the existence of external galaxies, also called 'island universes.' Looking into the crowd of a respected scientists from many disciplines, Shapley. realized he had no ammo to directly shoot down the idea. What he did have, however, was a great deal of evidence for his own idea that the Milky Way was ten times bigger Although possibly entertaining, formal debates than anyone had ever imagined.

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.

This Big Galaxy,' as he called it, filled up the universe Judging his evidence for an enormous Milky Way credible Shapley could then tack on the implication that nothing existed outside its boundaries, not even island universes Shapley was inexperienced at the lecture podium but knew that many scientists unfamiliar with astronomy filled the hall. He thus crafted an understandable twenty-page speech to get his point across, not even bringing up the word 'light-year until page seven. His opponent, Heber D Curtis, an experienced orator from the Lick Observatory near San Jose, California, watched from the audience. Curtis followed with a technical and dense speech arguing for the existence of island universes. Many scientists declared him the 'victor of the day by default for the mere fact that he gave an appropriate talk. Such was the so- called "Great Debate," and since Curtis won the day, later generations of scientists looked back at this meeting to be the day external galaxies won official acceptance.

Despite the immortalization of the Shapley-Curtis debate, it was never really a debate to begin with. Here's what Curtis thought would go on:

I agree with [astronomer George Ellery Hale] that it should not be made a formal "debate", but I am sure that we could be just as good friends if we did go at each other "hammer and tongs"... . A good friendly "scrap" is an excellent thing once in a while; sort of clears up the atmosphere. It might be far more interesting both for us and our jury, to shake hands, metaphorically speaking, at the beginning and conclusion of our talks, but use our shillelaghs in the interim to the best of our ability.

The lore of the "Great Debate" began a year after the real event, in 1921, when the Academy published its proceedings. Neglecting to mention that their edition didn't contain the original transcripts, the Academy published new papers that Shapley and Curtis had extensively reworked to seem like a point-counterpoint argument. Except for those few who actually attended the proceedings, most who learned of the debate read the published proceedings. Most received the impression that a true debate occurred, and ended with Curtis' victorious speech.

Although possibly entertaining, formal debates are not the best method of working out scientific conclusions. In formal debates, too often theatrics, personalities and stage presence win over substance. Moreover, formal debates limit participation, evidence and interpretation in ways that the informal ongoing dialogue in scientific circles does not. Finally, acceptance or rejection of scientific ideas is determined over time by the scientific community.

Looking back, both Shapley and Curtis had well-founded udience arguments. The Milky Way was indeed much larger than arguing anybody had ever imagined, and galaxies did indeed exist scientists outside it. The clinching evidence for island universes however, wouldn't arrive until 1924 with the work of Edwin so-Hubble, after whom the Hubble Space Telescope was later named. This story looks at how astronomers concluded that island universes were real, and that the Milky Way was just one of many, many galaxies in the sky debate.

Edwin Hubble.

While the Great Debate might have been over-hyped, it what nonetheless encapsulated a very pressing question. Was the Milky Way the only galaxy in the universe, or were there many more like it? The question was difficult to answer because so many details come into play. As you read, note what questions drive astronomers, move ideas forward, and become points of controversy.

The story begins in the late 1700s. Using ever-improving telescopes, skywatchers viewed planets, comets, asteroids, stars-and an unidentified class of fuzzy objects called nebulae. Meaning 'cloud' in Latin, early astronomers saw nebulae as immense blotches of lightsped against a background of pin-point sized stars. Some astronomers speculated that nebulae comprised many stars, while others proposed some kind of mysterious luminous matter. By the 1840s, Lord Rosse of Ireland used his giant telescopes to discern individual stars within nebulae, but over in Scotland John P. Nichol swore that the clouds were some sort of luminous fluid. As telescopes improved throughout the 1800s and revealed more detail of the nebulae, they were further classified into 'spirals and 'globular clusters' based on their appearance. Yet the fundamental challenge posed by nebulae was to determine whether they were comprised of star-like material or an entirely unknown substance.

Many people wrongly think that production of useful technology is the goal of science. While technologies are often based on scientific understanding, basic science is solely concerned with furthering our understanding of the natural world knowledge for knowledge sake. Yet, science and technology are intricately tied together. New technologies (such as improved telescopes) can increase scientists' ability to make careful observations that might possibly lead to new understandings. And new understandings of the natural world often lead to technological advances that could not have been predicted.

In 1864, William Huggins attached a spectroscope to his telescope and aimed it at a nebula.In brief, a spectroscope is used to measure the emission spectrum from an incandescent object.

Using a spectroscope, astronomers can tell the elemental makeup of celestial objects because each element has a unique spectrum line.Although he was later shown to be mistaken, Huggins wrote the following of his initial observations and thinking.

[I had a feeling] of excited suspense, mingled with a degree of awe, with which, after a few moments of hesitation.I put my eye to the spectroscope.Was i not about to look into a secret place of creation?
I looked into the spectroscope, No s expected!A single bright line only!At first I suspect some displacement of the prism, and that I was looking at a reflection of the illuminated slit from one of its faces.This thought was scarcely more than momentary, then the true interpretation flashed upon me.The light of the nebula wasthe monochromatic, and so, unlike any other light I had yet subjected to prismatic examination, could not be extended out to form a complete spectrum.

The riddle of the nebulae was solved. The answer, which had come to us in the light itself, read: Not an aggregation of stars, but a luminous gas. Stars after the order of our own sun, and of the brighter stars, would give a different spectrum; the light off this nebula had clearly been emitted by a by a luminous gas.

Other astronomers argued that nebulae were made of stars based on an 1885 "nova" in the Andromeda Nebula At the time, no astronomer thought that stars could explode. The word "nova" means "new star" in Latin, and they thought these bright objects to be stellar newborns. Then in 1898, the German astronomer Julius Scheiner used a spectroscope pointed toward the Andromeda Nebula. His observations of the nebula had a similar spectrum to the sun. Considering the visual evidence and the spectroscopic evidence combined, most astronomers then worked with the idea that nebulae were composed of stars.

Andromeda Nebula, as photographed in 1889 by Isaac Roberts.

Observations can be affected by what one thinks is true about nature. As seen above, scientists in the 1880s could make out the Andromeda Nebula pretty well, but not until thirty years later would they think it was an external galaxy.

Many argued that nebulae weren't just composed of stars, but actually in the process of forming stars. Following the 1885 nova, astronomers looked more closely at the spiral nebulae. By 1900, over 100,000 nebulae had been detected. These twisted, whirlpool-like objects seemed as if they would condense into stars or planetary systems over time. If nebulae were just proto-stars, then no need existed to think of them as being outside the Milky Way They could just be the seeds of stars within the Milky Way.

Other astronomers continued to think that nebulae were truly 'island universes' -great collections of stars that reside outside our own galaxy. To determine if nebulae were within our galaxy or external, astronomers focused on the distances to the nebulae. The size of the Milky Way played an important factor in determining the distance to nebulae. For example, astronomer F.W. Very assumed that the Milky Way and the Andromeda Nebula were of a similar size. He arbitrarily assigned each to have a diameter of 120 light-years. By comparing their brightne he concluded the two were about 3800 light-years apart. server Most astronomers, however, felt that the Milky Way was far larger than 120 light-years an accepted consensus around 1910 was about 30,000 light-years in diameter. Efforts continued to determine the sizes of nebulae. The German astronomer, Max Wolf, concluded that most were about 1,000 light-years in diameter. By the beginning of the twentieth century most accepted that nebulae were made of stars, and the question then turned into the location of these nebulae.

By 1920 much accumulated data existed for and against island universes. However, the evidence for each side was less than ideal. For island universe advocates, they could argue that nebulae moved far faster than stars. For Big Galaxy advocates they could argue that collected evidence showed island universes to be physically impossible! The key to solving the island universe debate lay with Cepheid variable stars.

By the early twentieth century had been known that many stars named in respect to their luminosity. Cepheid variables were a class of variables that had a very notable relationship between the period of their variability and their absolute luminosity, This relationship was demonstrated in 1912 after much research and calibration by Henrietta Leavitt of the Harvard College Observatory. The relationship was quite predictable, and because the absolute luminosity was known, these stars could be used as 'standard candles' to determine distance. In 1918, Shapley further studied these stars and began using them as yardsticks to measure distances to globular clusters.

Here's how one could use a Cepheid variable to determine distance. First, begin with the following two definitions. The apparent magnitude is the brightness of a star as seen from earth (a standardized measurement). The absolute magnitude is the apparent magnitude as seen from 10 parsecs (about 30 light-years). Once these are optically measured, distance can be ascertained by this equation:

m – M = 5 log(r/10)

Where m is apparent magnitude, M is absolute and ris distance in parsecs, If, for example, m - M is 0 then r = 10 parsecs, because the logarithm of 0 is 1. If m - M = 5, then r = 100. From 1918 onwards, Shapley had been magnitude calculating the distances to globular clusters one by one, hoping to show that their distances all fit within the Milky Way galaxy.

Enter Edwin Hubble, whose work with Cepheid variables changed the landscape of astronomy. Born in 1889 and student of math and astronomy at the University of Chicago. Hubble accepted a Rhodes Scholarship to Oxford and studied Roman and English law for a time. He served in World War I and then went on to get his doctorate in astronomy from the Verkes Obsenvatory Upon graduating he joined the Mount Wilson Observatory and gained use ofits new 100 inch telescope. Fresh out of school, Hubble had his eyes set on verifying the island universe model. He thought the required evidence could be gathered by locating a variable star within a nebulal which could be used to estimate the distance to the nebula. Frequently checing photographic plates. he realized in 1923 that he had stumbled upon a sight that no astronomer had yet to notice -Cepheid variable stars in the Andromeda Nebula. He wrote to Shapley early in the next year:

You will be interested to hear that I have found a Cepheid variable in the Andromeda Nebula (M31). I have followed the nebula this season as closely as the weather permitted and in the last five months have netted nine novae and two variables.. the distance comes out something over 300,000 parsecs... I have a feeling that more variables will be found by careful examination of long exposures. Altogether the next season should be a merry one and will be met with due form and ceremony.

The photographic plate on which Hubble identified the first Cepheid in M31.

For emphasis, 300,000 parsecs is over 900,000 light- years- over three times as large as Shapley's Big Galaxy Shapley, in turn, remarked in his journal, "Here is the letter that has destroyed my universe."

Hubble published his work in whole on January 1, 1925, and on that day the issue of external galaxies is said to have finally been put to rest. Faced with a catalog of evidence from Hubble and many other astronomers, a consensus emerged that large nebulae were indeed island universes - galaxies unto their own.

You may be wondering about the measurements van Maanens made earlier to determine the internal rotational motions of nebula M101. They turned out to be incorrect. This illustrates the need for caution in interpreting data from difficult observations and the need for multiple tests done by different groups.

The M101 spiral nebula, captured in 1916.

Hubble quietly continued his research on nebulae and by the end of the decade would come to one of the most remarkable conclusions in the history of astronomy that the universe was expanding. In 1924 upon hearing of Hubble's results regarding the distance to the nebula. Curtis remarked

There is a grandeur and majesty in the concept [of island universes] and an agreement with the general cosmical continuity expected on philosophical grounds which is both inspiring and alluring. Few greater concepts have ever been formed in the mind of thinking man than this one, namelythat we, the microbic inhabitants of a minor satellite of one of the millions of suns which form our galaxy, may look out beyond its confines and behold other similar galaxies, tens of thousands of light-years in diameter, each composed, like ours, of a thousand milliorn or more suns, and that, in so doing, we are penetrating the greater cosmos to distances of from half a million to a hundred million lightyears.

With sufficient evidence for the existence of externalca galaxies, a new wave of cosmology began, in which M astronomers dutifully conjured up explanations of theid universe's origins and its grand structure.

Arguments on The Great Debate

Shapley was arguing in favor of the Milky Way as the entirety of the universe. He believed that "spiral nebulae" such as 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 span most astronomers would not accept. Adriaan van Maanen was also providing evidence to Shapley's argument. Van Maanen was a well-respected astronomer of the time who claimed he had observed the Pinwheel Galaxy rotating. 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 clearly be a violation of the universal speed limit, the speed of light. Also used to back up his claims was the observation of a nova 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.

Adriaan van Maanen.

Curtis on the other side 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 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 the Earth's galaxy. This led to supporting Andromeda as a separate galaxy with its own signature age and rate of nova occurrences. He also cited dark lanes present in other galaxies similar to the dust clouds found in the Earth's own galaxy and massive doppler shifts found in other galaxies.

Curtis stated that if van Maanen's observation of the Pinwheel Galaxy 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

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

Due to the work of Edwin Hubble, it is now known that the Milky Way is only one of as many as an estimated 200 billion (2×1011) to 2 trillion (2×1012) or more galaxies (containing more stars than all the grains of sand on planet Earth), 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, 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 (Curtis' galaxy was centered on the Sun, while Shapley correctly placed the Sun in the outer regions of the galaxy).

Stories about the deyelopment of knowledlge concerning the sine of the universe might neatly use fthe Great Debate as the clincher for the ezxistence of external galaxies. Yet, note how this story illustrates that (1) scientific ideas develolp over time, and (2) scientists do not ote on What the natural world is like. Scientists do Sometiten vote on what to call something or how to categorize it, but not how the natural world works whichMuch time (often decades) passes as scientific ideas emerge, develop and are eventually accepted or discarded. 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.


National Academy of Sciences. Available in: Access in: 19/11/2918.

Wikipedia. Available in: Access in: 19/11/2018.

National Science Foundation. Available in: Access in: 19/11/2018.


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