  Every supernova hitherto observed has been considered to be the terminal
explosion of a star. Moreover, all supernovae with absorption lines in their
spectra show those lines decreasing in velocity over time, as the ejecta expand
and thin, revealing slower moving material that was previously hidden. In
addition, every supernova that exhibits the absorption lines of hydrogen has
one main light-curve peak, or a plateau in luminosity, lasting approximately
100 days before declining. Here we report observations of iPTF14hls, an event
that has spectra identical to a hydrogen-rich core-collapse supernova, but
characteristics that differ extensively from those of known supernovae. The
light curve has at least five peaks and remains bright for more than 600 days;
the absorption lines show little to no decrease in velocity; and the radius of
the line-forming region is more than an order of magnitude bigger than the
radius of the photosphere derived from the continuum emission. These
characteristics are consistent with a shell of several tens of solar masses
ejected by the star at supernova-level energies a few hundred days before a
terminal explosion. Another possible eruption was recorded at the same position
in 1954. Multiple energetic pre-supernova eruptions are expected to occur in
stars of 95-130 solar masses, which experience the pulsational pair
instability. That model, however, does not account for the continued presence
of hydrogen, or the energetics observed here. Another mechanism for the violent
ejection of mass in massive stars may be required.
