  The recent supernova (SN) known as SN 2009ip had dramatic precursor eruptions
followed by an even brighter explosion in 2012. Its pre-2012 observations make
it the best documented SN progenitor in history, but have fueled debate about
the nature of its 2012 explosion --- whether it was a true SN or some type of
violent non-terminal event. Both could power shock interaction with
circumstellar material (CSM), but only a core-collapse SN provides a
self-consistent explanation. The persistent broad emission lines in the
spectrum require a relatively large ejecta mass, and a corresponding kinetic
energy of at least 10^51 erg, while the faint 2012a event is consistent with
published models of core-collapse SNe from compact (~60 R_Sun) blue
supergiants. The light curves of SN 2009ip and another Type IIn, SN 2010mc,
were nearly identical; we demonstrate that their spectra match as well, and
that both are standard SNe IIn. Our observations contradict the recent claim
that the late-time spectrum of SN 2009ip is returning to its progenitor's
LBV-like state, and we show that late-time spectra of SN 2009ip closely
resemble spectra of SN 1987A. Moreover, SN 2009ip's changing H-alpha equivalent
width after explosion matches behavior typically seen in core-collapse SNe IIn.
Several key facts about SN 2009ip and SN 2010mc argue strongly in favor of a
core-collapse interpretation, and make a non-terminal 10^50 erg event
implausible. The most straighforward and self-consistent interpretation is that
SN 2009ip was an initially faint core-collapse explosion of a blue supergiant
that produced about half as much 56-Ni as SN 1987A, with most of the peak
luminosity from CSM interaction.
