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.
"It's exciting to see two completely different observations point in the same direction," said Martin Israel, a professor of physics at Washington University in St. Louis and a coauthor of the Science paper. "And the fact that these articles are coming out at the same time is just an accident."
Working with a consortium of scientists from NASA, Caltech and Washington University, Israel and study co-leader Robert Binns, also of Washington University, analyzed 17 years of data collected by the Cosmic Ray Isotope Spectrometer (CRIS) instrument aboard NASA's Advanced Composition Explorer (ACE) 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 (Fe-56) but just 15 of the radioactive Fe-60.
Still, those 15 particles were able to tell scientists a great deal about the origins of cosmic rays, the authors said.
Cosmic rays are individual atom nuclei that have been stripped of their electrons and are flying through space at almost the speed of light.
Because these particles have a positive charge, they generally do not travel in a straight line. Instead, they get kicked around by the tangled web of magnetic fields that exist in the Milky Way galaxy.
"They are moving at high speeds, but they are diffusing out in every direction," Israel said.
That makes cosmic rays challenging to study. Other rays -- like X-rays and gamma rays -- are more well behaved. If you see an X-ray coming from the direction of a certain star, chances are it originated from that star.
But since cosmic rays move about in all different directions, it is hard to tell where they came from.
In the case of the new study, the researchers used Fe-60 kind of like a clock, to help them place the cosmic rays in time. Once they knew when these particles were made, they could then determine where they might have come from.
Heavy elements such as iron are usually synthesized in supernovas -- the powerful explosions of massive stars. In addition, previous 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 the researchers would not be able to detect it.
The researchers 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.
Armed with this information, the scientists concluded that the source of the cosmic rays is probably clusters of massive stars, known as OB associations, where supernova explosions are expected to occur every few million years -- frequently enough to make Fe-60 and send it speeding off before it has time to decay.
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