Black Widow Spider
This constellation, named for a spider that sometimes consumes its mate, represents many binary star systems in our galaxy that produce gamma rays. Some of these systems pair a normal star with a pulsar, and in some cases the pulsar's high-energy emissions are slowly destroying its companion star.
A pulsar is a compact, rapidly spinning and highly magnetized neutron star; it's the crushed core left behind when a massive star explodes. Finding a pulsar in a binary system suggests that the companion survived the supernova explosion that formed the pulsar, but such systems may also form when a solitary pulsar is drawn in and captured into orbit by the gravitational pull of a star.
When Fermi sees gamma rays from binaries, it's likely a pulsar is involved. Pulsars emit regular pulses of light at different energies, including gamma rays. They also generate high-energy outflows, or "winds," of accelerated electrons and other particles.
In black widow binaries, a low-mass companion star orbits so closely that it takes the full force of the pulsar's wind. Where the wind meets the star's atmosphere, it heats and disperses the gas. Whittling away at its companion over millions to billions of years, the pulsar wind can dissolve the entire star. Fermi has detected nearly all known black widow binaries, and most members of a related type of system — called redbacks, after an Australian cousin of the black widow — were discovered by observing Fermi sources with radio telescopes.
The power for a pulsar's emissions comes from the neutron star's rapidly spinning magnetic field. Over time, as solitary pulsars wind down, their emissions fade. But if a faded pulsar is a member of a binary system, it can be rejuvenated by a stream of matter pulled off of its companion. The stream's impact gradually causes the pulsar's spin to increase. The pulsar can spin much faster than when it was born, with rotational periods dwindling to 10 milliseconds or less, which equals tens of thousands of revolutions per minute. These millisecond or "recycled" pulsars make up about half of the gamma-ray pulsars seen by Fermi.