  For most of their existence stars are fueled by the fusion of hydrogen into
helium proceeding via two theoretically well understood processes, namely the
$pp$ chain and the CNO cycle. Neutrinos emitted along such fusion processes in
the solar core are the only direct probe of the deep interior of the star. A
complete spectroscopy of neutrinos from the {\it pp} chain, producing about
99\% of the solar energy, has already been performed \cite{bib:Nature-2018}.
Here, we report the direct observation, with a high statistical significance,
of neutrinos produced in the CNO cycle in the Sun. This is the first
experimental evidence of this process obtained with the unprecedentedly
radio-pure large-volume liquid-scintillator Borexino detector located at the
underground Laboratori Nazionali del Gran Sasso in Italy. The main difficulty
of this experimental effort is to identify the excess of the few counts per day
per 100 tonnes of target due to CNO neutrino interactions above the
backgrounds. A novel method to constrain the rate of \bi contaminating the
scintillator relies on the thermal stabilisation of the detector achieved over
the past 5 years. In the CNO cycle, the hydrogen fusion is catalyzed by the
carbon (C) - nitrogen (N) - oxygen (O) and thus its rate, as well as the flux
of emitted CNO neutrinos, directly depends on the abundance of these elements
in solar core. Therefore, this result paves the way to a direct measurement of
the solar metallicity by CNO neutrinos. While this result quantifies the
relative contribution of the CNO fusion in the Sun to be of the order of 1\%,
this process is dominant in the energy production of massive stars. The
occurrence of the primary mechanism for the stellar conversion of hydrogen into
helium in the Universe has been proven.
