Messier 51, The Whirlpool Galaxy. The SDSS image of this famous spiral galaxy (interacting with a smaller neighbor at the lower left) occupies about three one-millionths of the total sky area imaged by the SDSS. The SDSS imaging survey detected about 100 million galaxies, most of them much more distant, and thus much smaller and fainter in appearance, than M51. Some of these distant galaxies can be seen as small extended sources on this image, while the sharper, point-like sources are mostly foreground stars in our own Milky Way galaxy. The diameter of M51 is roughly 75,000 light years.
Credit: The Sloan Digital Sky Survey.