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15.11 Shapley, Curtis and Hubble

About a hundred years ago, astronomers were becoming increasingly divided over the nature of the "nebulae." The nebulae were fuzzy patches of light — some smooth and featureless and some spiral in form — that were scattered around the sky. William Herschel and Charles Messier had cataloged hundreds of them in the late 18th Century. Many astronomers thought that the nebulae were clouds of gas or star-forming regions distributed within the Milky Way, similar to the Orion Nebula. Others thought that they might be "island universes," or vast collections of stars located far beyond the boundaries of the Milky Way.

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Edwin Hubble. Click here for original source URL.

In April 1920, the National Academy of Sciences held a debate in Washington, D.C. Harvard astronomer Harlow Shapely argued that the nebulae were nearby objects located in the Milky Way. The spiral forms were presumed to represent the swirling material of a star-forming region. By identifying Cepheid variable stars within the globular clusters and using the period-luminosity relation to estimate their intrinsic brightness Shapely was able to measure distances to many globular clusters. He found that the globular clusters traced an enormous sphere in space, with the Sun offset from the center of the sphere. Shapely's Milky Way was so huge that he thought it inconceivable that there might be other objects in space as large as our own galaxy. For Shapely, the Milky Way was the universe.

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Mount Wilson Observatory 100-inch telescope. Click here for original source URL.

The minority view was presented by Heber Curtis from Lick Observatory. Curtis believed that the spiral nebulae in particular could be understood as enormous systems of stars. He pointed out that many of the nebulae resolved into numerous points of light when viewed through a large telescope. It was an unusual situation; scientific issues are rarely decided in a single debate between two people. The result of the debate was inconclusive. There was no convincing evidence in favor of either hypothesis. One young man in the audience would soon provide the evidence. He was a brilliant young astronomer named Edwin Hubble.

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The Andromeda Galaxy. Click here for original source URL.

Edwin Hubble was highly accomplished — he had been a boxer, a Rhodes Scholar, and a lawyer before turning his attention to astronomy. He joined the Mount Wilson Observatory in Southern California just after the First World War. The Observatory had just completed a 100-inch telescope, at the time the world's largest. He quickly focused this big "eye" on the nebulae and took photographic plates of unprecedented depth and resolution. Like others before him, he saw that the nebulae broke up into the combined light of individual stars. Hubble went further, taking photographs of the Andromeda nebula repeatedly over a period of several months. His aim was to discover variable stars embedded within the nebula.

Hubble's observations were immediately successful, although he depended heavily on the skills of Milton Humason to gather his data. Humason had been a mule driver during the construction phase of Mount Wilson Observatory, and he stayed on to become a janitor, then a nigh assistant, and finally Hubble's indispensable right hand man. Hubble identified a number of Cepheid variables in the Andromeda nebula and measured their periods. With the period-luminosity relation, each period could be used to predict luminosity for the Cepheid. Knowing that light dims with the inverse square of increasing distance, Hubble could calculate the distance to the Cepheids, and so to the Andromeda nebula that contained them. He deduced that the nebula was about one million light years away. Hubble published his paper in 1925, and astronomy was changed forever: here was the evidence to prove that many nebulae are enormous systems of stars, as Curtis had argued in the 1920 debate.

The discovery that nebulae are galaxies of stars in 1925 represented a spectacular increase in the size of the known universe. The Milky Way is about 30 kpc across, or a 100,000 light years. Hubble showed that Andromeda is ten times farther away. Soon he found other galaxies that were millions of light years away. His photographs showed galaxies that were even fainter, and possibly at even larger distances. Hubble's work conjured up the possibility of space without end, an unimaginable void filled with countless systems of stars. Welcome to the immense realm of the galaxies.