Posted by on Feb 23, 2024 in Main |

Harlow Shapley and Heber Doust Curtis and the ‘Great Debate’ of 1920

The guest speaker at the February meeting of Keighley Astronomical society held on Thursday 22nd was the popular Mr Rod Hine from Bradford AS.

The subject of his presentation was entitled ‘The Battle for Galaxies’ which Mr Hine went on to explain was to evolve astronomers with strong personalities and big egos arguing about the nature of so-called spiral nebulae and the size of the universe.

Mr Hine started right back as far as we can go to find the earliest recognition of Galaxies, and of course with no telescopes or instruments the earliest galaxy observed was our own Galaxy the Milky Way. The word galaxy was borrowed via French and Medieval Latin from the Greek term for the Milky Way, galaxías (kúklos) γαλαξίας (κύκλος) ‘milky (circle)’, named after its appearance as a milky band of light in the sky. Mr Hine explained the ancient stories that were attached to the Milky Way.

Mr Rod Hine delivering his presentation to Keighley Astronomical society

As civilisation progressed mankind started to ponder on the makeup of the Milky Way. He explained that one of the earliest to suggest that the Milky Way was made up of distant stars was the ancient Greek philosopher Democritus (450 to 370BC). Astronomers and mathematicians in the Islamic world had attempted to measure the size of the Milky Way and they observed other galaxies beyond our own which were described as ‘Small clouds’. Mr Hine stated that it was only when Galileo started pointing telescopes at the night sky around 1610 that he discovered that the Milky Way was made up of a myriad of faint stars.

By the time we reached the late 1700’s observers of the night sky were speculating pretty accurately that the structure of the Milky Way was very much as we understand today. Immanuel Kant in 1755 was the first person to describe nebulae as far distant ‘Island universes’. Also during this time the French astronomer Charles Messier was looking for comets, but all he could see where these fuzzy patches of light so he started to catalogue these objects that were not comets. He published an astronomical catalogue consisting of 110 nebulae and star clusters, which came to be known as the Messier objects, referred to with the letter M and their number between 1 and 110.

The Great Spiral Nebula in the constellation Andromeda (1902 photograph). The Debate was over whether this was a cloud of gas and dust or a distant galaxy.

William Herschel later went on to list 25,000 nebulae. Telescopes improved as the years progressed, so the quality and number of observations improved. It was becoming apparent that there was a difference between galaxies and nebulae.

Simply put, the main difference between galaxies and nebulae are an extreme difference in size, as well as their basic structure. A nebula is a cloud of dust and gas, usually tens to hundreds of light years across. A galaxy is much larger, usually thousands to hundreds of thousands of light years across.

Mr Hine explained that that the most important evidence to indicate the difference between galaxies and nebulae was the science of Spectroscopy.
This is the study of the absorption and emission of light and other radiation by matter. It involves the splitting of light (or more precisely electromagnetic radiation) into its constituent wavelengths (a spectrum), which is done in much the same way as a prism splits light into a rainbow of colours. It was the light being emited by these structures that distinguished the difference between them and also led to the discovery that the universe is expanding.

The Baird Auditorium at the Smithsonian institute in 1920

Mr Hine undertook a pratactical example of how an early spectroscope operated using a glass prism. Now that astronomers could split up light into its componets they could really start to measure things such as distances and how far away they were from us.

In 1864 the spectra obtained by William Huggins of various nebulae established that they are composed mainly of incandescent gas (rather than aggregations of stars), thus settling a long debate over their composition. These discoveries moved on to the question of the size of the universe, and where these objects beyond or within the Milky Way.

Henrietta Swan Leavitt (1868 to 1921) was an American astronomer. Her discovery of how to effectively measure vast distances to remote galaxies led to a shift in the understanding of the nature of the universe. By discovering a relationship for some stars between how bright they appear and how fast they blink, Henrietta Leavitt gave us a tool to gauge the size and expansion rate of the universe.

Harlow Shapley (1885 –1972)

So we arrive after the First World War, when there are those astronomers and scientists who take a conservative view that the Milky Way is the whole of the universe and those who do not and believe the evidence shows the universe to be beyond the Milky Way.

The Great Debate, also called the Shapley–Curtis Debate, was held on 26th April 1920 at the Smithsonian Museum of Natural History, between the astronomers Harlow Shapley and Herber Curtis.

Harlow Shapley was an American scientist, head of the Harvard College Observatory. Shapley used Cepheid variable stars to estimate the size of the Milky Way Galaxy and the Sun’s position within it.

Heber Doust Curtis (1872–1942)

Heber Doust Curtis 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.

The debate 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 (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 counterarguments 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

Later in the 1920s, Edwin Hubble showed that Andromeda was far outside the Milky Way by measuring Cepheid variable stars, as Henrietta Leavitt had done; he proved that Curtis was correct. Mr Hine stated that Hubble upon his discovery rushed a letter to Shapley. Shapley stated, “Here is the letter that has destroyed my universe”.

Edwin Powell Hubble

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 favour of Shapley (the Sun was near the centre of the galaxy in Curtis’s model, while Shapley correctly placed the Sun in the outer regions of the galaxy).

An interesting footnote to the ‘Battle for Galaxies’ was that Edwin Hubble died in 1953. Now for a famous person he had no funeral and his wife refused to tell anyone where his ashes were scattered or where his body was buried.

Mr Rod Hine taking questions from society members

Of the two main personalities involved in the Great debate. Despite having earlier argued strongly against the idea of galaxies other than our Milky Way, Shapley went on to make significant progress in the research of the distribution of galaxies, working between 1925 and 1932. In this time period, with the Harvard College Observatory, he worked to map 76,000 galaxies. One of the first astronomers to believe in the existence of galaxy superclusters, Shapley later discovered a large and distant example, which was later named the Shapley Supercluster.

In 1930 Curtis was appointed director of the University of Michigan observatories, but the shortage of funds following the Great Depression prevented the construction of a large reflector he had designed for the university at Ann Arbor. He contributed to develop the McMath–Hulbert private observatory at Lake Angelus.