Music 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. Music (fades) Sound Effect