The surface of hot neutron stars is covered by a thin atmosphere. If there is accretion after neutron-star formation, the atmosphere could be composed of light elements (H or He); if no accretion takes place or if thermonuclear reactions occur after accretion, heavy elements (for example, Fe) are expected. Despite detailed searches, observations have been unable to confirm the atmospheric composition of isolated neutron stars1. Here we report an analysis of archival observations of the compact X-ray source in the centre of the Cassiopeia?A supernova remnant. We show that a carbon atmosphere neutron star (with low magnetic field) produces a good fit to the spectrum. Our emission model, in contrast with others2, 3, 4, implies an emission size consistent with theoretical predictions for the radius of neutron stars. This result suggests that there is nuclear burning in the surface layers5, 6 and also identifies the compact source as a very young (~330-year-old) neutron sta
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