  Here are reviewed the insights from observations at optical and infrared
wavelengths for low mass limits above which stars do not seem to end as
luminous supernovae. These insights are: (1) the absence in archived images of
nearby galaxies of stellar progenitors of core-collapse supernovae above 16-18
solar masses, (2) the identification of luminous-massive stars that quietly
disappear without optically bright supernovae, (3) the absence in the nebular
spectra of supernovae of type II-P of the nucleosynthetic products expected
from progenitors above 20 solar masses, (4) the absence in color magnitude
diagrams of stars in the environment of historic core-collapse supernovae of
stars with >20 solar masses. From the results in these different areas of
observational astrophysics, and the recently confirmed dependence of black hole
formation on metallicity and redshift of progenitors, it is concluded that a
large fraction of massive stellar binaries in the universe end as binary black
holes.
