Fermi's Large Area Telescope has discovered the first gamma-ray pulsar in a galaxy outside the Milky Way. The object sets a new record for the most luminous gamma-ray pulsar known. The pulsar lies in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a small galaxy that orbits our Milky Way and is located 163,000 light-years away. The Tarantula Nebula is the largest, most active and complex star-formation region in our galactic neighborhood. It was identified as a bright source of gamma-rays, the highest-energy form of light, early in the Fermi mission. Astronomers initially attributed this glow to collisions of subatomic particles accelerated in the shock waves produced by supernova explosions but later determined that PSR J0540-6919 is responsible for roughly half the gamma-ray brightness originally attributed to the nebula.
When a massive star explodes as a supernova, the star's core may survive as a neutron star, where the mass of half a million Earths is crushed into a magnetized ball no larger than Washington, D.C. A young isolated neutron star spins tens of times each second, and its rapidly spinning magnetic field powers beams of radio waves, visible light, X-rays, and gamma-rays. If the beams sweep past Earth, astronomers observe a regular pulse of emission and the object is classified as a Pulsar.