  Neutrinos are elementary particles which are known since many years as
fundamental messengers from the interior of the Sun. The Standard Solar Model,
which gives a theoretical description of all nuclear processes which happen in
our star, predicts that roughly 99% of the energy produced is coming from a
series of processes known as the "pp chain". Such processes have been studied
in detail over the last years by means of neutrinos, thanks also to the
important measurements provided by the Borexino experiment. The remaining 1% is
instead predicted to come from a separate loop-process, known as the "CNO
cycle". This sub-dominant process is theoretically well understood, but has so
far escaped any direct observation. Another fundamental aspect is that the CNO
cycle is indeed the main nuclear engine in stars more massive than the Sun. In
2020, thanks to the unprecedented radio-purity and temperature control achieved
by the Borexino detector over recent years, the first ever detection of
neutrinos from the CNO cycle has been finally announced. The milestone result
confirms the existence of this nuclear fusion process in our Universe. Here,
the details of the detector stabilization and the analysis techniques adopted
are reported.
