After more than three years in space, NASA's Fermi Gamma-ray Space Telescope is extending its view of the high-energy sky into a range that to date has been largely unexplored territory. Now, the Fermi team has presented its first "head count" of sources in this new realm.

Fermi's Large Area Telescope (LAT) scans the entire sky every three hours, continually deepening its portrait of the sky in gamma rays, the most extreme form of light. While the energy of visible light falls between about 2 and 3 electron volts, the LAT detects gamma rays with energies ranging from 20 million electron volts (MeV) to more than 300 billion (GeV).

But at higher energies, gamma rays are few and far between. Above 10 GeV, even Fermi's LAT detects only one gamma ray every four months from some sources. The LAT's predecessor, the EGRET instrument on NASA's Compton Gamma Ray Observatory, detected only 1,500 individual gamma rays in this range during its nine-year lifetime, while the LAT detected more than 150,000 in just three years.

Any object producing gamma rays at these energies is undergoing extraordinary astrophysical processes. More than half of the 496 sources in the new census are active galaxies, where matter falling into a supermassive black hole powers jets that spray out particles at nearly the speed of light.