RX J1856.5-3754 (also called RX J185635-3754, RX J185635-375, and various other designations) is a nearby neutron star. It is believed to have been created by a supernova explosion of its companion star about one million years ago, and is moving 108 km/s across the sky. It was discovered in 1992, and observations in 1996 appeared to confirm that it was a neutron star, the closest neutron star to Earth yet discovered. The distance was thought to be about 150–200 light years away. However, further observations using the Chandra X-Ray Observatory in 2002 appear to show that its distance is greater (about 450 light years). It was once proposed that RX J1856.5-3754 was too small to be a neutron star, and may therefore in fact be a quark star. However, this theory is not currently considered to have much supporting evidence.

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