On 12 February 2006, amateur astronomers reported that a faint star in the constellation of ********* had suddenly become clearly visible in the night sky without the aid of a telescope. Records show that this so-called recurrent nova, RS ******* , has previously reached this level of brightness five times in the last 108 years, most recently in 1985. The latest explosion has been observed in unprecedented detail by an armada of space- and ground-based telescopes. is just over 5,000 light years away from Earth. It consists of a white dwarf star (the super-dense core of a star, about the size of the Earth, that has reached the end of its main hydrogen-burning phase of evolution and shed its outer layers) in close orbit with a much larger red giant star. The two stars are so close together that hydrogen-rich gas from the outer layers of the red giant is continuously pulled onto the dwarf by its high gravity. After around 20 years, enough gas has been accredited that a runaway thermonuclear explosion occurs on the white dwarf's surface. In less than a day, its energy output increases to over 100,000 times that of the Sun, and the accredited gas (several times the mass of the Earth) is ejected into space at speeds of several thousand km per second. Five explosions such as this per century can only be explained if the white dwarf is near the maximum mass it could have without collapsing to become an even denser neutron star. What is also very unusual in RS Oph is that the red giant is losing enormous amounts of gas in a wind that envelops the whole system. As a result, the explosion on the white dwarf occurs "inside" its companion's extended atmosphere and the ejected gas then slams into it at very high speed. "In 2006, our first observations with the UK's MERLIN system were made only four days after the outburst and showed the radio emission to be much brighter than expected," added Dr. Eyres. "Since then it has brightened, faded, then brightened again. With radio telescopes in Europe, North America and Asia now monitoring the event very closely, this is our best chance yet of understanding what is truly going on."

Credit:: David A. Hardy