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Narrator: Nearly a thousand years ago, people saw an exploding star, right here

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This is the Crab Nebula, the wreckage of that event.

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For most X-ray astronomers, it's the brightest and steadiest beacon in the sky.

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But now they realize it's not as steady as they thought.

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Several orbiting X-ray observatories have seen unexpected variations.

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Most X-ray telescopes don't have sharp enough vision to make images.

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Instead, they detect the Crab as a broad source.

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From 1999 to 2008, it brightened and faded by as much as

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3.5 percent a year. And since 2008, it's

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faded by 7 percent. The Gamma-ray Burst Monitor on NASA's

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Fermi satellite detected the decline and Fermi also spotted two gamma-ray

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flares at even higher energies. What's going on?

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Scientists think the X-rays reveal processes deep within the nebula, in a

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region powered by a rapidly spinning neutron star, the core of the star

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that blew up. This image from NASA's Chandra X-ray Observatory

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shows how complex this inner region is. But figuring out

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where the Crab's long-term X-ray changes are taking place will require a

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new generation of hard X-ray telescopes.

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Once regarded as an unchanging standard, the Crab Nebula flickers from energy

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ultimately provided by a long-dead star.