![Geminga pulsar, imaged in X-ray wavelengths by the Earth-orbiting XMM-Newton X-ray Observatory. The …[Credits : European Space Agency]](http://media-2.web.britannica.com/eb-media/87/98387-003-069D43EC.gif)
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.
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