isolated pulsar (a rapidly rotating neutron star) about 500 light-years from Earth in the constellation Gemini, unique in that about 99 percent of its radiation is in the gamma-ray region of the spectrum. Geminga is also a weak X-ray emitter, but it was not identified in visible light (as a 25th-magnitude object) until nearly two decades after its discovery in 1972. It is the only known pulsar not detected at radio wavelengths. It pulsates with a period of 0.237 second, has a radius of about 10 km (6 miles), and probably originated in a supernova explosion about 300,000 years ago.