One of the brightest gamma-ray sources, lying in the constellation Gemini; the name is a contraction of ‘Gemini gamma-ray source’. It was detected in 1972 by the satellite SAS-2, but not until 1988 was a faint optical counterpart of 25th magnitude identified. Geminga has since been found to be pulsating in X-rays and gamma rays every 0.237 s, suggesting that it is a spinning neutron star. In its gamma-ray, X-ray, and optical properties it is similar to the Vela Pulsar, although it emits no detectable radio waves. It is the nearest pulsar, about 800 l.y. away, and is travelling at over 400 000 km/h perpendicular to our line of sight. Geminga is thought to be a runaway star from the Orion OB1a association.