As part of the ESSENCE supernova search, astronomers have discovered a distant star that exploded when its centre became so hot that matter and anti-matter particle pairs were created.

The star, known as Y-155, was once a magnificent 200 times the mass of our Sun, but around seven billion years ago it became unstable, forcing a runaway thermonuclear reaction that ended in a cataclysmic explosion visible halfway across the Universe.

Astronomer Peter Garnavich from the University of Notre Dame, and colleagues, spotted Y-155 with the National Optical Astronomy Observatory’s (NOAO) four metre Blanco telescope at Cerro Tololo in Chile during the last few weeks of the six year-long ESSENCE supernova search that concluded at the end of 2007.

“ESSENCE found many explosions in our six years of searching, but Y-155 stood out as the most powerful and unusual of all our discoveries” says Dr. Garnavich. Over 200 weaker supernovae events were also discovered. “In our images, Y-155 appeared a million times fainter than the unaided human eye can detect, but that is because of its enormous distance. If Y-155 had exploded in the Milky Way it would have knocked our socks off.”

Using the Keck telescope in Hawaii, the Magellan telescope in Chile and the MMT telescope in Arizona to follow up the discovery, astronomers found that the wavelengths of light emitted from the supernova were stretched (redshifted) by 80 percent due to the expansion of the Universe, and that at its peak, Y-155 was generating energy at a rate 100 billion times greater than the Sun’s output. In order to do this, it must have synthesized between six and eight solar masses of radioactive nickel.

“The thermonuclear runaway experienced by the core of Y-155 is similar to that seen in the explosions of white dwarf stars as Type Ia supernovae, but with a far greater amount of power,” says team member Alex Filippenko. A normal Type Ia supernova produces about one tenth as much radioactive nickel.

The team suggest that the star falls into the category of 'pair-unstable' supernovae – those that become unstable through the production of matter and anti-matter particle pairs. Most stars bigger than eight times the mass of the Sun die in core-collapse supernova or directly form a black hole, but at the largest scale – stars with 150 to 300 solar masses – pair-instability is thought to operate.

Massive stars like these are thought to form in pristine gas not polluted with elements heavier than hydrogen and helium. Small galaxies tend to have a low abundance of heavy atoms, and imaging with the Large Binocular Telescope shows that Y-155 originates from a very low mass host galaxy.

“Though the first massive stars ever to have existed in the Universe have not yet been found, they might eventually be detected through their incredibly powerful pair-instability explosions like Y-155,” says Filippenko.

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