6/11/2010: Hydrogen in M51, NGC 5194. Perhaps the first spiral nebula, M51 is a large galaxy, over 60,000 ly across, with a readily apparent spiral structure. M51 is a part of a well-known interacting galaxy pair, its spiral arms and and dust lanes clearly sweeping in front of companion galaxy NGC 5195 (top).
This spectacular spiral galaxy is known to astronomers as Messier 83. Colloquially, it is also called the Southern Pinwheel due to its similarity to the more northerly Pinwheel galaxy Messier 101. NASAs Spitzer Space Telescope shows us, in spectacular detail, the infrared structure of what many think of as our own Milky Way galaxys smaller cousin.