  A precise test of the theory of stellar evolution can be performed by
measuring the difference in average energy between the neutrino line produced
by ${\rm ^7Be}$ electron capture in the solar interior and the corresponding
neutrino line produced in a terrestrial laboratory. The high temperatures in
the center of the sun broaden the line asymmetrically, FWHM = 1.6~keV, and
cause an average energy shift of 1.3~keV. The width of the $^7$Be neutrino line
should be taken into account in calculations of vacuum neutrino oscillations.
