Most of the cosmic rays that rain down on our planet today were forged in a nearby supernova that exploded no more than a few million years ago, according to a new study.

The findings, published Thursday in the journal Science, confirm another recent study that suggests a large star in our galactic neighborhood went supernova about 2.3 million years ago.

Working with scientists from NASA, Caltech and Washington University, Martin Israel and study co-leader Robert Binns, both of Washington University in St. Louis, analyzed 17 years of data collected by the Cosmic Ray Isotope Spectrometer instrument aboard NASA’s Advanced Composition Explorer spacecraft.

Specifically, the group was looking for detections of the iron isotope Fe-60. This is a rare form of iron that has a half-life of 2.6 million years. Over the course of 17 years, the CRIS instrument detected 300,000 particles of ordinary iron but just 15 of the radioactive Fe-60.

Still, those 15 particles told scientists a great deal about the origins of cosmic rays, which are individual atom nuclei that have been stripped of their electrons and are flying through space at almost the speed of light.

Researchers used Fe-60 like a clock, to help them place the cosmic rays in time. Once they knew when these particles were made, they could determine where they might have come from.

Advertisement

Heavy elements such as iron are usually synthesized in supernovas – the powerful explosions of massive stars. Studies have shown that it takes the successive explosions of two nearby stars to make a cosmic ray. The first explosion makes the particles. The shock wave from the second accelerates them.

That means that the two supernovas that created the Fe-60 detected by CRIS had to have gone off within about 2 million years of each other. If it had been any longer between the explosions, enough of the Fe-60 would have decayed and researchers would not be able to detect it.

They also knew that the place of the double explosion couldn’t be too far from Earth, or else the Fe-60 would have decayed over the time spent traveling through space.

Scientists concluded that the source of the cosmic rays is probably clusters of massive stars where supernova explosions are expected to occur every few million years.


Only subscribers are eligible to post comments. Please subscribe or login first for digital access. Here’s why.

Use the form below to reset your password. When you've submitted your account email, we will send an email with a reset code.