Hubble's main interest was the study of the galaxies. His work led to the classification of galaxies, the island universe theory, and Hubble's Law. Galaxies are grouped by shape into spirals, ellipticals, and irregulars.
Photos. From left to right, the M31, M87 and M82 galaxies. To learn more, check out our pages about spirals, ellipticals and irregulars. Courtesy of Carina Software, 12919 Alcosta Boulevard, Suite #7, San Ramon, CA 94583, (510) 355-1266. Used with permission.
The island universe theory tells us that there are galaxies outside of our own Milky Way. At one time, when the size and extent of the universe was not clear, it was thought that the other galaxies near the Milky Way were actually part of our galaxy. Hubble proved, though, that these groups of stars were in fact their own islands of stars.
Hubble's Law helps us find the age of the universe. If we know how fast galaxies are moving away from us and how distant the farthest ones are, then we can determine how long it has taken the universe to expand to the size we see.
The island universe theory and Hubble's Law were the inspirations for the construction, launch, and deployment of a Space Telescope. Such an instrument would allow us to observe objects well beyond our galaxy and see back in time 14 billion years. It might help us find the age of the universe.