  We propose a model based on ionization shadows to explain the formation of
the long and narrow strings of Eta Carinae. Five strings are known, all located
along the symmetry axis outside the Homunculus. The model assumes that each
string is formed in a shadow behind a dense clump near the symmetry axis. The
surrounding gas is ionized first, becomes much hotter, and compresses the gas
in the shadow. This leads to the formation of a radial, dense, long, and narrow
region, i.e., a string. Later the neutral material in the strings is ionized,
and becomes brighter. Still later it re-expands, and we predict that in about
200 years the strings will fade. The condition for the model to work is that
the ionization front, due to the diffuse ionizing recombination radiation of
the surrounding gas, proceeds into the shadow at a velocity slower than the
compression speed. From that we get a condition on the mass loss rate of the
mass loss episode that formed the strings, which should be less than 10^{-4}
Mo/year. The model can also explain the strings in the planetary nebula NGC
6543.
