It is now known by direct observation that much of the light curve (the graph of luminosity as a function of time) after the occurrence of a type II Supernova, such as SN 1987A, is explained by those predicted radioactive decays. Although the luminous emission consists of optical photons, it is the radioactive power absorbed by the ejected gases that keeps the remnant hot enough to radiate light. The radioactive decay of Ni through its daughters Co to Fe produces gamma-ray photons, primarily with energies of 847 keV and 1,238 keV, that are absorbed and dominate the heating and thus the luminosity of the ejecta at intermediate times (several weeks) to late times (several months). Energy for the peak of the light curve of SN1987A was provided by the decay of Ni to Co (half-life 6 days) while energy for the later light curve in particular fit very closely with the 77.3-day half-life of Co decaying to Fe. Later measurements by space gamma-ray telescopes of the small fraction of the Co and Co gamma rays that escaped the SN 1987A remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power sources.