  Neutrino produced in a chain of nuclear reactions in the Sun starting from
the fusion of two protons, for the first time has been detected in a real-time
detector in spectrometric mode. The unique properties of the Borexino detector
provided an oppurtunity to disentangle pp-neutrino spectrum from the background
components. A comparison of the total neutrino flux from the Sun with Solar
luminosity in photons provides a test of the stability of the Sun on the
10$^{5}$ years time scale, and sets a strong limit on the power production in
the unknown energy sources in the Sun of no more than 4\% of the total energy
production at 90\% C.L.
