  Isotopes of nickel play a key role during the silicon burning phase up to the
presupernova phase of massive stars. Electron capture rates on these nickel
isotopes are also important during the phase of core contraction. I present
here the microscopic calculation of ground and excited states Gamow-Teller (GT)
strength distributions for key nickel isotopes. The calculation is performed
within the frame-work of pn-QRPA model. A judicious choice of model parameters,
specially of the Gamow-Teller strength parameters and the deformation
parameter, resulted in a much improved calculation of GT strength functions.
The excited state GT distributions are much different from the corresponding
ground-state distributions resulting in a failure of the Brink's hypothesis.
The electron capture and positron decay rates on nickel isotopes are also
calculated within the framework of pn-QRPA model relevant to the presupernova
evolution of massive stars. The electron capture rates on odd-A isotopes of
nickel are shown to have dominant contributions from parent excited states
during as early as silicon burning phases. Comparison is being made with the
large scale shell model calculation. During the silicon burning phases of
massive stars the electron capture rates on $^{57,59}$Ni are around an order of
magnitude bigger than shell model rates and can bear consequences for
core-collapse simulators.
