One of the fastest moving stars ever seen is challenging theories to explain its blistering speed. The cosmic cannonball, a neutron star known as RX J0822-4300, was discovered with NASA's Chandra X-ray Observatory. Astronomers used five years of Chandra observations to show that the rogue star is careening away from the Puppis A supernova remnant, leftovers of a star that exploded about 3,700 years ago. The neutron star is racing out of our Milky Way Galaxy at about 3 million mph (4.8 million kph). "Just after it was born, this neutron star got a one-way ticket out of the galaxy,"...