The temperature, ionization and line emission of the ejecta of SN 1987A has
been modelled between 200-2000 days in its nebular phase, using a
time-dependent model. The modelling of the evolution of the temperature
and ionization of the various abundance zones shows that the metal-rich
core undergoes a thermal instability, often referred to as the
IR-catastrophe, at 600 - 1000 days. The hydrogen-rich zones, on the
other hand, evolve adiabatically after 500 - 800 days. It is found that
freeze-out of the recombination is important in the hydrogen and helium
zones. The temperature and ionization of the supernova ejecta determine
the evolution of the observed emission lines. The IR-catastrophe is seen
in the metal lines as a transition from thermal to non-thermal excitation,
most clearly in the [O I] 
6300, 6364 lines, and from the
evolution of the [Fe II] lines. The distribution of hydrogen, helium, and
oxygen is determined from the line profiles. It is also shown that most
lines have contributions from more than one composition region. For
example, the [Fe II] lines have a large contribution from primordial iron
residing in the hydrogen-rich regions. A main reason for this modelling is
to study the nucleosynthesis that took place in the progenitor, as well as
in the supernova explosion. By comparing the model calculations with
observations masses for hydrogen, helium, nitrogen, oxygen, neon, magnesium
and stable nickel is estimated for the supernova ejecta.