Image: Cassiopeia A
NASA / CXC / MIT / UMass Amherst
The blue, wispy arcs in this image of Cassiopeia A show where the acceleration is taking place in an expanding shock wave generated by the supernova explosion. The red and green regions show material from the destroyed star that has been heated to millions of degrees by the explosion.
updated 11/15/2006 5:24:12 PM ET 2006-11-15T22:24:12

New clues that cosmic rays, high-energy particles that travel through space and bombard the Earth, are generated by shock waves in supernova remnants were revealed by a new study using NASA’s Chandra X-ray Observatory.

Cosmic rays are composed of high-energy electrons, protons and ions. Scientists used the Chandra observatory to study the X-rays emitted by the electrons (the only one of the particles that emits X-rays) from cosmic rays emanating from Cassiopeia A, a supernova remnant that was first spotted 325 years ago.

Scientists have long theorized that the high-energy shock waves of exploded stars called supernovae were among “the few places in the galaxy that have enough energy to accelerate these particles,” said Michael Stage, an astronomer at the University of Massachusetts at Amherst.

They theorize that the particles, contained by magnetic fields on either side of the shock wave, bounce back and forth across the shock, eventually energizing the electrons to very high energies.

“The electrons pick up speed each time they bounce across the shock front, like they’re in a relativistic pinball machine,” said Glenn Allen, a team member from the Massachusetts Institute of Technology. “The magnetic fields are like the bumpers, and the shock is like a flipper.”

One important key for this theory to work, Stage says, is that the acceleration of the particles should come close to the theoretical maximum rate. The images of X-ray radiation from Cassiopeia A allowed the scientists to map out the accelerations of the electrons and provided “direct evidence that [this maximum] is reached for electrons,” Stage told

Stage says that it is likely that protons and ions would be accelerated in the same way as the electrons.

By looking at the X-ray images, scientists also observed that the highest-energy electrons accelerated very quickly to their energies, whereas lower-energy electrons took much longer to accelerate.

Acceleration of charged particles also occurs in shocks in Earth’s magnetosphere and in jets produced by supermassive black holes.

“Explaining where cosmic rays come from helps us to understand other mysterious phenomena in the high-energy universe,” Stage said.

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