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The Curtis-Shapley debate two different views of our Universe

In the early part of the twentieth century there was much disagreement between astronomers about the structure and size of the Universe. The 'leaders' of the two opposing views were two American astronomers, Heber Curtis and Harlow Shapley.

These two men took part in what has come to be known as the 'Great Debate', organised by the National Academy of Sciences on April 26th 1920 at the Smithsonian Institute in Washington D.C.

Some views before 1920

In the years leading up to 1920 Kapteyn completed a huge survey of stars to determine the structure of our galaxy. He did not know anything about the absorption of starlight by dust in the galaxy and decided that our galaxy, and therefore possibly the universe, was like a flat lens with a diameter of 17 kiloparsecs with the Sun apparently close to its centre.

Shapley's view of the Universe

He believed that out galaxy was the whole universe, that it had a diameter 300 000 light years (100 kiloparsecs) and that the Sun was about 20 kiloparsecs from its centre. He argued that the spiral nebulae, then recently discovered with large telescopes, were in fact gas clouds within the galaxy. He based his ideas partly on the asymmetric distribution of the globular clusters. One major flaw in Shapley's argument was that he did not appreciate the effect on interstellar absorption on the apparent brightness of distant stars.

Curtis's view of the Universe

He believed that our galaxy was only part of a much larger Universe. He stated that the galaxy had a diameter 30 000 light years (10 kiloparsecs), only a tenth as large as Shapley's estimate with Sun very near its centre. He suggested that the galaxy was shaped like a flat lens rather like two plates placed face to face. He also argued that the spiral nebulae were 'island universes' outside our galaxy, a suggestion first made by Kant in the mid 18th century. Curtis's ideas were based on star counts and the actual brightenesses of different types of stars.

In the debate at the Smithsonian Shapley spoke first and gave a popular lecture aimed at the audience of mainly non-astronomers while Curtis followed this with a rather more technical delivery. Shapley emphasised his ideas about the model of the galaxy and only mentioned the nature and distance of the spiral nebulae briefly towards the end of his talk. Both men had actually argued from the wrong standpoint with insufficient data.

Discoveries following the Great Debate

In the mid-1920's using the 100 inch Hooker Telescope at Mount Wilson, then the largest telescope in the world, astronomer Edwin Hubble discovered Cepheid variable stars in the Andromeda Galaxy (M31).

Hubble used these Cepheid variables to show that the distance to M31 was greater than even Shapley's estimate of the size our galaxy. He stated that this meant that M31 was a galaxy much like our own and quite separate from it.

In the 1930s, the further discovery of interstellar absorption and an increased understanding of the distances and distribution of globular clusters finally led astronomers to accept that the size of our galaxy had been seriously underestimated and that the Sun was not close to the centre.
This meant that Shapley was proved more correct about the size of our galaxy and the position of the Sun in it, but Curtis was proved correct that our Universe was composed of many more galaxies, and that spiral nebulae were galaxies just like our own.

However the measurements of Cepheid variable stars in the Andromeda nebula made by Hubble in 1923 were still not correct. He estimated them to be at a distance of 978 000 light year (300kpc) whereas the modern value for the distance of the Andromeda galaxy is closer to 2 400 000 light year (740kpc) over twice as far away.

Interstellar absorption

The photograph of street lamps in the fog is a good example of the effect of dust or gas on how bright a source of light appears.

The street lamps are all the same brightness but those further away look dimmer because the some of the light has been absorbed by the fog. In just the same way distant stars in our galaxy look dimmer than they should because of the absorption of light by the gas clouds in the galaxy. Knowledge of this fact would have led Shapley to reduce his idea of the size of the Milky Way galaxy. (Photo: Klyagin Konstantin Nickolayevich)

[You can see similar street lamp images at:]

The next photograph shows the Sombrero galaxy and you can see the dark band of gas along the centre of the galaxy that would absorb starlight. An observer looking along the plane of the galaxy would see the stars on the other side from them dimmed because some of the light coming from them had been absorbed by the gas.


© Keith Gibbs 2020