The Hubble "tuning fork" diagram shows the many types of galaxies and their classifications.
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Galaxies

The introduction of telescopes to the study of astronomy opened up the universe, but it took some time for astronomers to realize how vast the universe could be. Telescopes revealed that our night sky was not only populated with stars, but with other, more nebulous objects. Some of these objects were nebulae within our Galaxy, the Milky Way. As telescopes became more powerful, it was possible to see that some of the nebulae had a spiral-like structure. These were also believed to be part of our Galaxy and thus relatively nearby.

In 1920, two important astronomers, Harlow Shapley and Heber D. Curtis, held a great debate about the nature of these "spiral nebulae". Were they objects within the Milky Way, or were they communities of stars distinct from our Galaxy? Edwin Hubble studied these "spiral nebulae" and found that they were composed of stars, and thus resolved the debate. These nebulae were not nebulae at all, but galaxies! Suddenly, our universe was much bigger. We realized that our Galaxy was just one of many billions of galaxies in the universe.

Hubble continued to study galaxies his entire career, and we owe much of our understanding of galaxies to him. His observations led to the current classification of galaxies as spirals, ellipticals, or irregulars, and to our knowledge that the appearance of these galaxies depends both on our perspective, and on the forces which form and power galaxies.

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