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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