  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 stars. 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 others, 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 layers and also identifies the
compact source as a very young (~330-year-old) neutron star.
