Nuclear astrophysics strives for a comprehensive picture of the nuclear
reactions responsible for synthesizing the chemical elements and for powering
the stellar evolution engine. Deep underground in the Gran Sasso laboratory the
cross sections of the key reactions of the proton-proton chain and of the
Carbon-Nitrogen-Oxygen (CNO) cycle have been measured right down to the
energies of astrophysical interest. The salient features of underground nuclear
astrophysics are summarized here. The main results obtained by LUNA in the last
twenty years are reviewed, and their influence on the comprehension of the
properties of the neutrino, of the Sun and of the Universe itself are
discussed. Future directions of underground nuclear astrophysics towards the
study of helium and carbon burning and of stellar neutron sources in stars are
pointed out.Comment: Invited review, submitted to Annu. Rev. Nucl. Part. Scienc
