  A protoneutron star is formed immediately after the gravitational collapse of
the core of a massive star. At birth, the hot and high density matter in such a
star contains a large number of neutrinos trapped during collapse. Trapped
neutrinos generally inhibit the presence of exotic matter -- hyperons, a kaon
condensate, or quarks. However, as the neutrinos diffuse out in about 10-15 s,
the threshold for the appearance of strangeness is reduced; hence, the
composition and the structure of the star can change significantly. The effect
of exotic, negatively-charged, strangeness-bearing components is always to
soften the equation of state, and the possibility exists that the star
collapses to a black hole at this time. This could explain why no neutron star
has yet been seen in the remnant of supernova SN1987A, even though one
certainly existed when neutrinos were detected on Feb. 23, 1987. With new
generation neutrino detectors it is feasible to test different theoretical
scenarios observationally.
