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Narrator: About two or three times a century, a massive in our galaxy

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explodes. The star's core may survive as a

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

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as swiftly expanding debris behind a powerful shockwave.

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As the supernova remnant grows, it sweeps up interstellar gas and

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gradually decelerates. Yet even thousands of years later,

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its imprint on the galaxy remains impressive. Exploding stars

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

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

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electrons and atomic nuclei are boosted to such high speeds has been an

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enduring mystery. Now, observations of two supernova

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

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Because cosmic rays carry electric charge, their

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direction changes as they travel through magnetic fields. By the time the particles

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reach us, their paths are completely scrambled. We can't trace them back to

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

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which is where Fermi comes in. The interaction of high energy particles

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with light and ordinary matter can produce gamma rays, the most powerful form of light.

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Unlike cosmic rays, gamma rays travel to us straight from their sources.

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In 1949,

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physicist Enrico Fermi worked out what he called "magnetized clouds"

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could accelerate cosmic rays. Later studies showed that a

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

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remnants. Confined by a magnetic field,

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high-energy particles move around randomly. Sometimes they cross the shock

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

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energy. After dozens to hundreds of crossings, the particle

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is moving near the speed of light and is finally able to escape. If the

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supernova remnant resides near a dense molecular cloud, some of those

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

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But electrons and protons make gamma rays in different ways.

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Cosmic ray electrons do so when they're deflected by passing near

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the nucleus of an atom. Accelerated protons

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

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neutral pion. These pions quickly decay into a pair of gamma

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rays. At their brightest, both types

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of emission look very similar. Only with sensitive measurements at lower

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gamma-ray energies can scientists determine which process is responsible.

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Now, Fermi observations have done just that.

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They conclusively show these supernova remnants are accelerating protons.

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When they strike protons in nearby molecular clouds, they produce pions...

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and ultimately the gamma-ray emission Fermi sees.

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NASA's Fermi has detected gamma rays from many more supernova remnants,

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but the jury is still out on whether accelerated protons are always responsible

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and what their maximum energies may be. Nevertheless, the Fermi

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

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in establishing just where they arise. Something that would satisfy,

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but certainly not surprise, the original Fermi.

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Music fades