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Narrator: Nearly a thousand years ago, people saw an exploding star, right here
This is the Crab Nebula, the wreckage of that event.
For most X-ray astronomers, it's the brightest and steadiest beacon in the sky.
But now they realize it's not as steady as they thought.
Several orbiting X-ray observatories have seen unexpected variations.
Most X-ray telescopes don't have sharp enough vision to make images.
Instead, they detect the Crab as a broad source.
From 1999 to 2008, it brightened and faded by as much as
3.5 percent a year. And since 2008, it's 
faded by 7 percent. The Gamma-ray Burst Monitor on NASA's
Fermi satellite detected the decline and Fermi also spotted two gamma-ray
flares at even higher energies. What's going on?
Scientists think the X-rays reveal processes deep within the nebula, in a
region powered by a rapidly spinning neutron star, the core of the star
that blew up. This image from NASA's Chandra X-ray Observatory
shows how complex this inner region is. But figuring out
where the Crab's long-term X-ray changes are taking place will require a
new generation of hard X-ray telescopes.
Once regarded as an unchanging standard, the Crab Nebula flickers from energy
ultimately provided by a long-dead star.
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