  We study the runaway mass loss process of major eruptions of luminous blue
variables (LBVs) stars, such as the 1837-1856 Great Eruption of Eta Carinae. We
follow the evolution of a massive star with a spherical stellar evolution
numerical code. After the star exhausted most of the hydrogen in the core and
had developed a large envelope, we remove mass at a rate of 1 Mo/year from the
outer envelope for 20 years. We find that after removing a small amount of mass
at a high rate, the star contracts and releases a huge amount of gravitational
energy. We suggest that this energy can sustain the high mass loss rate. The
triggering of this runaway mass loss process might be a close stellar companion
or internal structural changes. We show that a strong magnetic field region can
be built in the radiative zone above the convective core of the evolved massive
star. When this magnetic energy is released it might trigger a fast removal of
mass, and by that trigger an eruption. Namely, LBV major eruptions might be
triggered by magnetic activity cycles. The prediction is that LBV stars that
experience major eruptions should be found to have a close companion and/or
have signatures of strong magnetic activity during or after the eruption.
