The nature of the nuclear sources in M82 and NGC 253

New infrared photometry, spectroscopy, and mapping of M82 and spectroscopy of NGC 253 are used with previously published results to constrain star-burst models of the energetic nuclear sources in these galaxies. Bursts of star formation can account quantitatively for the properties of these sources, including the high luminosities, strong nonthermal radio emission, correspondence of radio and infrared sources, large populations of red giants and supergiants, large ultraviolet fluxes, and X-ray luminosities. However, the initial mass function (IMF) in these regions must be biased against the formation of solar-mass stars, compared with the IMF in the solar neighborhood, and the conversion of gas into stars must be very efficient.