ST. LOUIS — Quark stars, exotic objects that have yet to be directly observed, are part of a new theory to explain some of the brightest stellar explosions recorded in the universe. Super-luminous supernovae, which produce more than 100 times more light energy than normal supernovae and occur in about one out of every 1,000 supernovae explosions, have long baffled astrophysicists. The problem has been finding a source for all of that extra energy. University of Calgary astrophysicists Denis Leahy and Rachid Ouyed think they have a possible source — the explosive conversion of a neutron star into a quark...