Astronomers using the Hubble Space Telescope have zoomed in on the heart of the Crab nebula 7,000 light-years away and have discovered dramatic changes in the nebula from one week to the next. The quick transformations, they say, are probably driven by a pulsar--the rapidly spinning core of an exploded star--visible as the leftmost of the two bright dots in this image. (The other dot is a normal star.) We have these Hubble images of an area 2 or 3 light-years across, and part of the thing is just dancing around, says Paul Scowen, an astronomer at Arizona State University. The ripples spreading out below the pulsar like waves on a lake show where streams of electrons and positrons, shooting away from the pulsar at nearly the speed of light, begin to bunch up along the pulsar’s changing magnetic field. Astronomers had expected the field to be constant and unwavering. The mercurial nature of the Crab pulsar’s magnetic field, says Scowen, is a puzzle.