The Hubble "tuning fork" diagram shows the many types of galaxies and their classifications Click on image for full size (29K JPEG)

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.