Astronomer Edwin Hubble (1889-1953) played a pivotal role in deciphering the vast and complex nature of the universe. His meticulous studies of spiral nebulae proved the existence of galaxies other than our own Milky Way, paving the way for a revolutionary new understanding that the cosmos contains myriad separate galaxies, or “island universes.”
Image © 2007 USPS. All Rights Reserved.
Most of the great successes of Hubble's career were achieved not at Palomar but at Mt. Wilson. So it is only fitting that the dome of the 100-inch Hooker Telescope be shown behind him. The "spiral nebulae" referred to in the USPS description are what modern astronomers refer to as spiral galaxies.
Back in the 1920s no one was certain if they were part of our own Milky Way Galaxy or separate galaxies unto themselves. In 1923 Hubble used the 100-inch telescope to resolve and measure Cepheid variable stars in the Andromeda Galaxy (M31). Cepheid stars vary in brightness in a regular way that is directly proportional to their brightness. Once it is known how bright they really are and how bright they appear, it is possible to determine their distance.
Hubble found that the Cepheids in M31, and thus the galaxy itself, are 1 million light-years distant. (Modern figures put the distance at 2.9 million light years.) The only conclusion was that M31 and the other "spiral nebulae" were separate galaxies, completely distinct from our Milky Way.
Hubble pushed the bounds of known space even further with his 1929 discovery that the universe is expanding. This had a profound influence on cosmology and was the first clue pointing towards the Big Bang.
Hubble didn't get a lot of observing time at Palomar. The 200-inch telescope project took a long time to complete and Hubble passed away just five years after its dedication
If you are interested in reading about the life of Edwin Hubble I strongly recommend that you track down a copy of Gale E. Christianson's Hubble biography Edwin Hubble: Mariner of the Universe. If you are in a hurry, or just need something more right now you can read his biography posted on the Hubble Space Telescope website.