  Neutrino fluxes from proto-neutron stars with and without quarks are studied.
Observable differences become apparent after 10--20 s of evolution.
Sufficiently massive stars containing negatively-charged, strongly interacting,
particles collapse to black holes during the first minute of evolution. Since
the neutrino flux vanishes when a black hole forms, this is the most obvious
signal that quarks (or other types of strange matter) have appeared. The
metastability timescales for stars with quarks are intermediate between those
containing hyperons and kaon condensates.
