  We give a physical picture of the low-energy sector of the spin-$\frac12$
kagome Heisenberg antiferromagnets (KAFs). It is shown that kagome lattice can
be represented as a set of blocks containing 12 spins, having form of stars and
arranged in a triangular lattice. Each of these stars has two degenerate
singlet ground states which can be considered in terms of pseudospin-$\frac12$.
It is shown using symmetry consideration that the KAF lower singlet band is
made by inter-star interaction from these degenerate states. We demonstrate
that this band is described by effective Hamiltonian of a magnet in the
external magnetic field. The general form of this Hamiltonian is established.
The Hamiltonian parameters are calculated up to the third order of perturbation
theory. The ground state energy calculated in the considered model is lower
than that evaluated numerically in the previous finite clusters studies. A way
of experimental verification of this picture using neutron scattering is
discussed. It is shown that the approach presented can not be expanded directly
on KAFs with larger spin values.
