A new movie from NASA's Chandra X-ray Observatory shows a
sequence of Chandra images of the Crab Nebula, taken over an
interval of seven months. Dramatic variations are seen,
including the expansion of a ring of X-ray emission around the
pulsar (white dot near center) and changes in the knots within
this ring. However, arguably the most striking result of these
observations is the variations that were not observed, or in
analogy with a famous Sherlock Holmes story1, this could be a
case where the fact that the dog that did not bark helps to
solve a mystery. The pulsar at the center of the Crab Nebula is
a neutron star that spins around about 30 times a second. It was
created from a supernova explosion in our galaxy that was
observed by astronomers in China and other countries in the year
1054. As the young pulsar slows down, large amounts of energy
are injected into its surroundings. In particular, a high-speed
wind of matter and anti-matter particles plows into the
surrounding nebula, creating a shock wave t