  $\eta$ Carinae is an extraordinary massive star famous for its 19th century
Great Eruption and the surrounding Homunculus nebula ejected in that event. The
cause of this eruption has been the centre of a long-standing mystery. Recent
observations, including light-echo spectra of the eruption, suggest that it
most likely resulted from a stellar merger in an unstable triple system. Here
we present a detailed set of theoretical calculations for this scenario; from
the dynamics of unstable triple systems and the mass ejection from close binary
encounters, to the mass outflow from the eruption caused by the stellar merger
and the post-merger wind phase. In our model the bipolar post-merger wind is
the primary agent for creating the Homunculus, as it sweeps up external
eruption ejecta into a thin shell. Our simulations reproduce many of the key
aspects of the shape and kinematics of both the Homunculus nebula and its
complex surrounding structure, providing strong support for the
merger-in-a-triple scenario.
