Iron isotope points to recent nearby supernovae

Los Angeles Times : For the last 17 years, the Cosmic Ray Isotope Spectrometer (CRIS) on NASA’s Advanced Composition Explorer spacecraft has been detecting particles approaching Earth. In that time, it has identified more than 30 000 particles of ordinary iron-56 and just 15 particles of radioactive iron-60. But the radioactive isotopes provided enough information for researchers working with CRIS to determine a source. Heavy elements such as iron are created in supernova explosions, but it takes the shock wave of a second supernova to accelerate the atoms and turn them into cosmic rays. The 2.6-million-year half-life of iron-60 allowed researchers to determine that the supernovae that launched the iron particles toward Earth occurred within 2 million years of each other and were relatively nearby. The finding closely matches a separate study that recently suggested a nearby star went supernova roughly 2.3 million years ago.