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.
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.
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.
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.
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.
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: konst.org.ua/en/photohunt/album/34]
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.