Astronomers think they have spied the glowing remnants of a star that blew up surprisingly close to Earth just 700 years ago. Radiation from the star's embers and a rare element made in stellar explosions revealed the previously unknown supernova, according to two reports in tomorrow's issue of Nature. However, the discovery prompts a puzzle: Why are there no historical mentions of the blast, which would have lit the night as brightly as the quarter moon?
Supernovae, the cataclysmic deaths of certain stars, pop off once or twice per century in every galaxy. Johannes Kepler spotted the last supernova visible to the naked eye in our Milky Way in 1604. Such explosions hurl hot clouds of gas into space that astronomers can see thousands of years later in x-rays, visible light, and radio waves. Other telltale signs are intense gamma rays from radioactive elements forged in the infernos--such as titanium-44, a rare isotope that decays to calcium within a few centuries.
These x-rays and gamma rays have betrayed a new supernova remnant in the Southern Hemisphere sky, say two groups from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. The amount of titanium-44 still emitting gamma rays hints that the explosion happened no more than 700 years ago. And the size of the glowing ball of x-rays--four times the width of the full moon--shows the remnant is startlingly close to Earth, just 600 light-years away. "If this was an ordinary supernova, it would have been far brighter than Venus," says astronomer Bernd Aschenbach. However, its southern location would have hidden it from active skywatchers in Europe and perhaps much of China, he notes.
Astrophysicist Stan Woosley of the University of California, Santa Cruz, calls the discovery "very exciting." Further telescope studies could confirm the sighting and perhaps reveal much about the complex physics of supernova explosions, he says. But he's stumped as to why such a dazzling object didn't spark a historic account, even among cultures not known for paying close attention to the stars. "There could be oodles of dust between here and there, or else it may have been a strange supernova that was hard to see in visible light," Woosley suggests.