Astronomers have discovered a distant star that exploded when its center became so hot that matter and antimatter particle pairs were created. The star, dubbed Y-155, began its life around 200 times the mass of our Sun but probably became “pair-unstable” and triggered a runaway thermonuclear reaction that made it visible nearly halfway across the universe. University of Notre Dame professor Peter Garnavich and collaborators discovered the exploding star during the “ESSENCE” supernova search that identified more than 200 weaker stellar explosions. “ESSENCE found many explosions in our 6 years of searching, but Y-155 stood out as the most powerful and unusual of all our discoveries,” said Garnavich.
Y-155 exploded about 7 billion years ago, when the universe was half its current age. Astronomers discovered it in the constellation Cetus the Whale with the National Optical Astronomy Observatory’s (NOAO) 4-meter Blanco telescope at Cerro Tololo in Chile in November 2007 during the last weeks of the 6-year ESSENCE project. The Keck 10-meter telescope in Hawaii, the 6.5-meter Magellan telescope in Chile, and the MMT telescope in Arizona rapidly focused on the new star, revealing that the wavelengths of light emitted from the supernova were stretched or “redshifted” by 80 percent due to the expansion of the universe.
Once astronomers established the distance to the explosion, Garnavich and his collaborators calculated that, at its peak, Y-155 was generating energy at a rate 100 billion times greater than the Sun’s output. To do this, Y-155 must have synthesized between 6 and 8 solar masses of radioactive nickel. It is the decay of radioactive elements that drives the light curves of supernovae. A normal “type Ia” thermonuclear supernova makes about one-tenth as much 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,” said team member Alex Filippenko, who obtained the Keck data.
“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,” said Garnavich. “If Y-155 had exploded in the Milky Way, it would have knocked our socks off.”
Most stars bigger than 8 times the Sun’s mass lose their battle with gravity and produce a “core-collapse” supernova or directly form a black hole. But there is a range of masses, 150 to 300 times the mass of the Sun, where the pair-instability is thought to operate. Such massive stars are expected to form in pristine gas that early generations of stars have not polluted with elements heavier than hydrogen and helium. Deep imaging with the Large Binocular Telescope in Arizona shows that Y-155 originated in a very low-mass host galaxy. On average, small galaxies have a low abundance of heavy atoms, so they are excellent locations for pair-instability explosions. “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,” said Filippenko.
The ESSENCE project was a 6-year NOAO Survey Program led by Christopher Stubbs of Harvard University and included an international team of astronomers from the United States, Germany, Australia, and Chile. The ESSENCE project was designed to precisely map the expansion history of the universe by discovering type Ia supernovae and using them as distance markers. The ultimate goal is to understand the mysterious dark energy that is driving the accelerating expansion.
Garnavich and the ESSENCE collaboration announced the discovery at the 215th meeting of the American Astronomical Society in Washington, D.C.