You may have already noticed this, either by reading Sky & Telescope or by looking out at Perseus in the northeast sky over the past few evenings.
Comet 17P/Holmes suddenly exploded on October 24, and is now visible as a comet to the naked eye. No one’s sure why – it might be breaking up, it might be “outgassing” from melting ice or a gas-filled cavern. Or it might be an omen of doom as predicted by Nostradamus. (My money’s on the ice.)
NASA’s “Astronomy Picture of the Day” has a brief animation showing the sudden increase in brightness.