Click here for a higher-resolution image
Somewhat below the center of this image is small spiral galaxy IC 2497, which lies at a distance of about 660 million light years. Below it is a once-mysterious green "smudge" known as Hanny's Voorwerp after its discoverer, Hanny Van Arkel, who found it in the SDSS sky survey images in 2007. After much research it was found that not only was this a newly-discovered object, it is also a newly-discovered kind of astronomical object, now known as a "quasar ionization echo". Briefly, a quasar "turns on" around a supermassive black hole at the center of a galaxy that happens to be close enough to some ionizable material in space (in this case a cloud of mostly O-III gas). The energy of the quasar ionizes the material so it starts to glow like a nebula, then later the quasar runs out of fuel and "turns off". But the cloud of ionized material continues to glow...hence the name "quasar ionization echo". There are now a few dozen objects like this known in the sky, but this one was the first known example. Hanny's Voorwerp is also notable for being one of the few objects in space that is green. This results from the typical blue-green color of an O-III nebula being redshifted significantly (due to its distance) so that it becomes pure green.
There are numerous small galaxies scattered throughout the image, the most prominent of which is edge-on galaxy FGC 920 in the upper-right corner. A few more galaxies whose distances are known are labelled in the high-resolution image linked to above. This is an 8.5-hour image with 7 hours of L and 30 minutes each of RGB.