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.
