  We present a detailed observational and theoretical study of a ~3 hr long
X-ray burst (the ``super burst'') observed by the Rossi X-ray Timing Explorer
(RXTE) from the low mass X-ray binary (LMXB) 4U 1820-30. This is the longest
X-ray burst ever observed from this source, and perhaps one of the longest ever
observed in great detail from any source. We show that the super burst is
thermonuclear in origin. The level of the accretion driven flux as well as the
total energy release of ~1.5 x 10^{42} ergs indicate that helium could not be
the energy source for the super burst. We outline the physics relevant to
carbon production and burning on helium accreting neutron stars and present
calculations of the thermal evolution and stability of a carbon layer and show
that this process is the most likely explanation for the super burst. We show
that for large columns of accreted carbon fuel, a substantial fraction of the
energy released in the carbon burning layer is radiated away as neutrinos, and
the heat that is conducted from the burning layer in large part flows inward,
only to be released on timescales longer than the observed burst. Thus the
energy released possibly exceeds that observed in X-rays by more than a factor
of ten. Spectral analysis during the super burst reveals the presence of a
broad emission line between 5.8 - 6.4 keV and an edge at 8 - 9 keV likely due
to reflection of the burst flux from the inner accretion disk in 4U 1820-30. We
believe this is the first time such a signature has been unambiguously detected
in the spectrum of an X-ray burst.
