PARIS (AFP) - The discovery of a supernova only hours after its explosion has probably solved a long-standing mystery on the origin of the brightest known phenomena in the Universe, scientists reported Wednesday. On August 24, scientists witnessed the spectacular eruption of light and energy thrown off by the birth of SN 2011fe, the brightest and at a mere 20.9 million light years away closest-to-Earth supernova in over 25 years. One light year is the distance that light travels in 365 Earth days, about 9.46 trillion kilometres or 5.87 trillion miles. We caught the supernova just 11 hours after it exploded, so soon that we were later able to calculate the actual moment of the explosion within 20 minutes, said Peter Nugent of the US Lawrence Berkeley National Laboratory and lead author of one of two studies, both published in Nature. With this close-up look, we found things nobody had dreamed of, he said in a statement. Brightness thrown off by the violent stellar blast of type 1a supernovas is the yardstick for measuring distances across space. This was crucial in this years Nobel Prize winning discovery that the expansion of the Universe is accelerating and not slowing down, as once thought. Up to now, astronomers had advanced three competing scenarios to explain the origin of 1a supernovas, rare events that only happen once every couple of hundred years in a galaxy such as our own. In all three, a so-called white dwarf an ageing star in decline steals matter, and energy, from a companion star until the white dwarf explodes in a flash a billion times brighter than our Sun. A supernova is so powerful it can outshine an entire galaxy such as the Milky Way, which contains some 200 billion stars, for several weeks. White dwarfs are essentially Earth-sized diamonds. A single tablespoon weighs about ten tonnes. Most white dwarfs die not with a bang but a whimper, gradually leaking away heat over billions of years. But a few interact with other stars to create supernova detonations that are, in essence, colossal thermonuclear explosions.