17/P Holmes was a run-of-the-mill comet in 2007, a faint fuzz ball hardly worth observing since it never got closer to the Sun than twice Earth's distance. But in October of that year the comet had a huge and still poorly understood outburst, brightening by 14 magnitudes--a factor of 400,000! It became visible to the naked eye, and despite being farther away than Mars, it was obvious as a fuzzy disk even without binoculars.

This gorgeous image shows the comet as it passed the bright star Mirfak in Perseus, and is a combination of 40 exposures. I love the beautiful diffraction spikes around Mirfak, and the ghostly glow of the expanding gas cloud from the comet's paroxysms.

And the winner is...