101: The Great Debate

This is the first in a new series of blog posts under the title of ‘Galaxies 101’. These posts aim to explore the history and basics of the science of galaxies. I’ll be covering some of people who helped us understand these ‘Island Universes’ as well as some of the basics that would be taught during a first year undergraduate galaxies course at university.

It is fortunate that these posts are beginning in the week of the 90th anniversary of The Great Debate which occurred on April 26th, 1920. The Great Debate – or the Shapely-Curtis Debate – took place at the Smithsonian Museum of Natural History between two eminent astronomers, Harlow Shapley and Heber Curtis. Shapely was arguing that the ‘spiral nebulae’, that were observed at the time, were within our own Galaxy – and that our Galaxy was the Universe. He also argued that the Sun was not at its centre. Conversely, Curtis argued that the Sun was at the centre of our Galaxy but that the ‘spiral nebulae’ were not inside our Galaxy at all. He suggested instead that the Universe was much larger than our Galaxy and that these nebulae were in fact other, ‘island’ universes.

Below is a drawing of the ‘spiral nebula’ M51. This is an observation by Lord Rosse, drawn in 1845 using the 72-inch Birr Telescope at Armagh Observatory in the UK.

M51 drawn by Lord Rosse in 1845

With 90 years of hindsight we can now say that Shapely and Curtis were both right and wrong. The Sun is not at the centre of the Galaxy and the Galaxy is only one of hundreds of billions of galaxies in the Universe. But how was the argument resolved? The answer, in part, comes from a very famous name in astronomy: Hubble.

Less a decade after the Great Debate took place, Edwin Hubble used the largest telescope in the world – the 100-inch Hooker Telescope on Mount Wilson – to observe Cepheid variable stars in the Andromeda Nebula/Galaxy. Cepheid variables are a type of pulsating stars whose pulsation periods are precisely proportional to their luminosities. This makes Cepheid variable stars a ‘standard candle’ – an object where the brightness is a known quantity. If you can observe the apparent brightness of a standard candle, then you can determine its distance by a simple inverse square law. Since Cepheid variable stars have pulse rates proportional to their luminosity, if you can measure the pulse rate of a Cepheid variable anywhere in the Universe, then you can determine how far away it is. This is what Edwin Hubble did in 1925 and he calculated the distance to Andromeda as 1.5 million light years.


At the time, Shapely thought that our Galaxy was around 300,000 light years across and Curtis believed it was around 30,000 light years. Hubble’s measurement placed Andromeda well outside our galaxy and showed that Curtis was correct in thinking that the ‘spiral nebulae’ could indeed be other galaxies. Today we think the Milky Way is about 100,000 light years across and that Andromeda is 2.5 million light years away.

The discoveries of the 1920s started a whole new adventure for astronomy. The Universe had gotten a lot bigger and was about to expand much, much more. It is important to remember that Shapely, although wrong about the nature of the nebulae, did correctly assert that the Sun was not at the centre of the Galaxy. This is the kind of Copernican shift that makes people think about things differently and it is important to realise that the issues discussed during the Great Debate were complex. For our benefit though, the Great Debate is a starting point for exploring the relatively new study of galaxies. Humanity’s view of the Universe, and our place within it, has changed an awful lot since 1920. The study of galaxies has had a lot to do with that.

If you want to read more about the Great Debate, and what else helped to resolve the arguments I can recommend this excellent NASA site. You can also read the published text of the debate online.

[Andromeda image credit: Robert Gendler]

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15 responses to “101: The Great Debate”

  1. Els says :

    Tx Robert
    Great blog.
    I’m looking forward to the next installments

  2. Alice says :

    Oooooooooh . . . .

  3. Jo says :

    Thanks Robert. This is an interesting blog. Really good.

  4. Curtis W. Garrett says :

    How great a shift in our world view. And this is just one of them in the last century.
    I too look forward to the future installments of this topic.

  5. Thomas J says :

    Thanks Robert, for an excelent blog post. I’m looking forward to updates.

  6. Joseph K. H. Cheng says :

    This is a really interesting blog. Thank you for posting.


  7. Jules says :

    This is a really good idea! I look forward to more.

  8. Budgieye says :

    I had been doing some reading on the history of science, and thought the Great Debate would be a good topic, then I found this blog on a Google search, and you had beaten me to it.
    I put a pointer to your blog in
    OOTD Tuesday, June 15, 2010
    Are galaxies made of stars?

  9. Bruno says :

    Good to start from the foundations ..thanks

  10. Tom Rimmer says :

    ok……. …… GO GO POWERRANGERS!

  11. luis carlos says :

    me gus mucho las imagenes de las galaxias

  12. carlos eduardo says :

    me gusto la galaxia y el universo

  13. luis carlos says :

    me encanto las imagenes de las glalaxias

  14. veggy2 says :

    On a point of detail I think you’ll find the Birr 72″ was built by Lord Birr on his own land at Parsonstown 163 miles from Armagh and had nothing to do with A rmagh Observatory.

  15. Ani says :

    Mein größter Traum….Einmal in der Unendlichkeit zu sein! Sucht,sucht und Ihr werdet finden!

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