LONDON (Reuters) - Supernovae, powerful and violent explosions of stars at the end of their lives, hold the key to a 10 billion-year-old mystery -- the origins of cosmic dust, astronomers said Wednesday. Scientists long believed that cosmic dust, the fine solid particles in space that are the building blocks of planets, was made in relatively cool, slow-burning normal stars and released in a stellar wind. But astronomers in England and Wales say they have discovered that some supernovae spew out huge amounts of dust, suggesting they are the source of the first cosmic particles in the Universe. "The origin...