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Week of October 25, 1999

We've come a long way from cluster to cluster! We started with associations, then to constellations, from there to open clusters and then to globular clusters. The next step, though, is a big one.

Back in the 1800s, Charles Messier was a French comet hunter. He kept finding fuzzy objects in the sky using his telescope, and decided to make a list of them so that he wouldn't confuse these objects with comets. Some of these objects turned out to be star clusters of various types, and other were true nebulae, or clouds of glowing gas. There were others, though, that appeared to be spiral in nature through a big telescope, and they remained at the center of controversy for one hundred and fifty years.

Basically, no one knew what they were. Even as late as the 1920s, astronomers thought that the Milky Way-- the collection of stars, gas and dust that we see in the sky-- was all there was to the Universe. Therefore, by their reasoning, these spiral nebulae must be part of our own Milky Way. Supporting their claim was the fact that these spiral nebulae showed no stars in them, even after using the best telescopes of the time to study them. On the opposing side, spectra taken of these nebulae did not look like other nebulae at all, but instead looked like the spectra of stars, as if the nebulae themselves were composed of many different types of stars. This was an important point, as gas shows a very different type of spectrum than a star.

Things came to a head in 1920, when the two views were debated in what is now called ''The Great Debate''. An established and respected astronomer, Heber Curtis, defended the idea that the nebulae were ``island universes'', clusters of stars and gas in their own right like the Milky Way. A young upstart, Harlow Shapely (pronounced shap-lee) took the opposing side, that these were just small objects inside the Milky Way. The debate was complicated and detailed, and when it was over, ironically, most people thought that Shapely had won. Galaxies come in many different types: spiral like our own, while others are elliptical. A minority are irregular, with no distinct shape, and even fewer are truly peculiar: they have definite shapes, but they are very odd, like ring shaped or galaxies with long arcs coming from them. Galaxies can contain from a million to a trillion stars, with the Milky Way weighing in at something like a half trillion. Here we are, only 80 or so years after the Great Debate, and we know far more about the ``nebulae'' then the two debaters might have ever guessed. And even Curtis might be surprised to know that the Universe is far larger then even he imagined... but that is best left to next week's Snack.

For more info about the Great Debate, including a transcript, go to the Great Debates in Astronomy webpage. It's an interesting read!

©2008 Phil Plait. All Rights Reserved.

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