Optical Transient SCP 06F6
This pair of NASA Hubble Space Telescope pictures shows the appearance of a mysterious burst of light that was detected on February 21, 2006, brightened over 100 days, and then faded into oblivion after another 100 days. The source of the outburst remains unidentified.
The event was detected serendipitously in a Hubble search for supernovae in a distant cluster of galaxies. The light-signature of this event does not match the behavior of a supernova or any previously observed astronomical transient phenomenon in the universe.
Astronomers do not know the object's distance, so it can either be in our Milky Way galaxy or at a great astronomical distance. The optical spectrum of the object contains absorption features that have not yet been identified. This may represent a previously undetected class of transient phenomenon in the universe.