Calculations show that the star's sudden brightness was clearly visible to the naked eye, but no one reported anything until the European Space Agency's XMM-Newton telescope spotted an unexpected burst of cosmic X-rays. On Oct. 9, 2007, XMM-Newton was turning from one target to another when it passed across a bright source of X-rays that no one was expecting. The source was not listed in any previous X-ray catalog, yet the mysterious object was lighting up XMM-Newton's view of the cosmos. The XMM-Newton team looked up three possible celestial candidates as at this location, including a normally faint star known...