The brightest flash ever detected was observed on 27 December 2004. Its source was a magnetar, a form of neutron star with a powerful magnetic field and mass greater than the sun condensed into an area the size of a small city. With emissions primarily in the gamma ray portion of the spectrum, this explosion released more energy in a tenth of a second than the Sun produces in 100,000 years.
The magnetar is located 50,000 light years away, about half the distance across the galaxy. If the explosion, not just the brightest flash of the century, but possibly the last thousand years of galactic history - occurred within 10 light years of the Earth, it could have stripped off the atmosphere and caused a mass extinction.
The precise cause of the explosion is still unknown. Imagine a sphere 20 km (12 mi) across, so massive that each teaspoonful of its material weighs two million tons, rotating once every 7.5 seconds, with a magnetic field so strong it can wipe a credit card at the distance of Venus orbit. These types of object pushes the extremes of physics so closely that we only have limited knowledge about them.
A "starquake" - an internal reorganization of matter - might have produced the explosion, or a magnetic reconnection, a scenario in which a magnetic field abruptly realigns, releasing the brightest flash the galaxy has seen in years. The brightest flash could have even come from the neutron star collapsing into an even smaller, denser hypothetical body, the so-called quark star.
Even though this explosion was the brightest flash ever observed, you wouldn't be able to see it with the naked eye, as it originates primarily in the gamma ray spectrum. This is to be expected, because gamma rays are the distinct type of radiation created by particles on the scale of the atomic nucleus, like the neutrons of which a neutron star is made. Visible light is emitted on the scale of molecules, emanated prominently in familiar chemical reactions. How ironic that the universe's brightest flash would not feel so bright to us, unless it were so close that it energized our atmosphere.