survive as a neutron star or a black hole, but the rest of it rushes into space as swiftly

expanding debris behind a powerful shockwave. As the supernova remnant grows, it sweeps

up interstellar gas and gradually decelerates. Yet even thousands of years later, its imprint

on the galaxy remains impressive.

Exploding stars and their remnants have long been suspected of producing cosmic rays, some

of the fastest matter in the universe. Where and how these protons, electrons and atomic

nuclei are boosted to such high speeds has been an enduring mystery. Now, observations

of two supernova remnants by NASA's Fermi Gamma-ray Space Telescope provide new insights.

Because cosmic rays carry electric charge, their direction changes as they travel through

magnetic fields.

By the time the particles reach us, their paths are completely scrambled. We can't trace

them back to their sources. So scientists must locate their origins by indirect means,

which is where Fermi comes in. The interaction of high-energy particles with light and ordinary

matter can produce gamma rays, the most powerful form of light. Unlike cosmic rays, gamma rays

travel to us straight from their sources. In 1949, physicist Enrico Fermi worked out

what he called "magnetized clouds" could accelerate cosmic rays. Later studies showed that a variant

of his method, called Fermi acceleration worked especially well in supernova remnants.

Confined by a magnetic field, high-energy particles move around randomly. Sometimes

they cross the shock wave. With each round trip, they gain about 1 percent of their original

energy. After dozens to hundreds of crossings, the particle is moving near the speed of light

and is finally able to escape. If the supernova remnant resides near a dense molecular cloud,

some of those escaping cosmic rays may strike the gas, and produce gamma rays.

But electrons and protons make gamma rays in different ways. Cosmic ray electrons do

so when they're deflected by passing near the nucleus of an atom. Accelerated protons

may collide with an ordinary proton and produce a short-lived particle called a neutral pion.

These pions quickly decay into a pair of gamma rays. At their brightest, both types of emission

look very similar. Only with sensitive measurements at lower gamma-ray energies can scientists

determine which process is responsible.

Now, Fermi observations have done just that. They conclusively show these supernova remnants

are accelerating protons. When they strike protons in nearby molecular clouds, they produce

pions... and ultimately the gamma-ray emission Fermi sees. NASA's Fermi has detected gamma

rays from many more supernova remnants, but the jury is still out on whether accelerated

protons are always responsible and what their maximum energies may be. Nevertheless, the

Fermi team has taken a major step--a century after the discovery of cosmic rays-- in establishing

just where they arise. Something that would satisfy, but certainly not surprise, the original

Fermi.