Cosmic Rays Tracked To 'Cocoon' In Star Nursery
Astrophysicists think they may have partly solved the mystery of where cosmic rays come from.
Cosmic rays are beams of high-energy charged particles, mostly protons and heavier nuclei -- atoms that have had their electrons stripped off -- that travel at extremely high speeds through the universe, including our galaxy.
Scientists have had great difficulty figuring out where cosmic rays originate and how they are accelerated to such high speeds, although they believe supernova explosions play a role.
Recently, light left by passing cosmic rays has been tracked back to a hollow space in a nebula called Cygnus X where many new stars are born, reports a study published in Science.
An analysis of the light suggests similar structures, known as superbubbles, may be a major source of cosmic rays.
Isabelle Grenier, who co-authored the new study, is an astrophysicist the University of Paris Denis Diderot. She said the fact that cosmic rays are made of charged particles is what makes them hard to track. Unlike light, they are deflected in different directions by magnetic fields and don't move in a straight line that can be traced directly from its source.
"They would just go round in circles and change directions and wander around between the source and us," she said in an interview with CBC's Quirks & Quarks set to air Saturday, "and in the end we would receive them from almost every direction on Earth."
That makes it impossible to track cosmic rays directly, Grenier said, but the particles do leave behind a "wake" of light such as gamma rays when they interact with matter such as the gases that exist between stars.
Those light rays do travel in straight lines and can be traced back to their source.
Using the Fermi Gamma Ray Space Telescope, Grenier and her colleagues found that a region in the constellation Cygnus, about 4,500 light-years away within our own Milky Way galaxy, was glowing with gamma rays.
That area is described in the paper as a "cocoon" about 163 light years across within a gas-filled hollow called a superbubble. Such superbubbles are carved out by strong stellar winds blasting from massive stars and stellar explosions nearby.
A closer examination of the light showed it had extremely high energies of up to 100 billion electron volts. That indicated that the cosmic rays that produced the light were near their source and the place where they had been accelerated to high energies.
The hollow where the cosmic rays are trapped is turbulent and filled with violent stellar winds travelling from nearby stars at roughly 1,000 or 2,000 kilometres per second, Grenier said.
"Think of a whirpool," she added. "They went round in circles and couldn't find a way out very easily."
The researchers estimate the cosmic rays may remain trapped in the "whirlpool" for 100,000 years. While that may seem like a long time, Grenier noted that it isn't much on the scale of the cosmic rays' estimated lifetime of 10 million years.
One thing the researcher still don't know is what accelerated the trapped cosmic rays in the first place. Grenier said it might have been a recent supernova explosion a few thousand years ago. Or it could be the whirlpool itself.
"Perhaps what we're seeing here is another entirely new method of accelerating cosmic rays," she said.