  In the far future long after star formation has ceased the universe will be
populated by sparse degenerate remnants, mostly white dwarfs, though their
ultimate fate is an open question. These white dwarfs will cool and freeze
solid into black dwarfs while pycnonuclear fusion will slowly process their
composition to iron-56. However, due to the declining electron fraction the
Chandrasekhar limit of these stars will be decreasing and will eventually be
below that of the most massive black dwarfs. As such, isolated dwarf stars with
masses greater than $\sim 1.2 M_\odot$ will collapse in the far future due to
the slow accumulation of iron-56 in their cores. If proton decay does not occur
then this is the ultimate fate of about $10^{21}$ stars, approximately one
percent of all stars in the observable universe. We present calculations of the
internal structure of black dwarfs with iron cores as a model for progenitors.
From pycnonuclear fusion rates we estimate their lifetime and thus delay time
to be $10^{1100}$ years. We speculate that high mass black dwarf supernovae
resemble accretion induced collapse of O/Ne/Mg white dwarfs while later low
mass transients will be similar to stripped-envelope core-collapse supernova,
and may be the last interesting astrophysical transients to occur prior to heat
death.
