The Contribution of Faint, Failed and Defunct Stars to the "Stellar" Masses of Galaxies

A substantial fraction the stellar mass attributed to galaxies is invisible: stars close to the hydrogen burning limit, brown dwarfs, white dwarfs, neutron stars and black holes. These constituents do, however, gravitationally micro-lens background quasars, thereby permitting measurement of the total stellar contribution to the mass surface density along the line of sight. We report the results of such a measurement using a sample of ten quadruply lensed quasars. We discuss the prospects for improving upon this measurement with a larger sample and describe efforts to find new quadruple lenses. If we invert our argument and take the stellar mass to be known, we derive a value for the fraction of the dark halo in MaCHOs (including 20 solar mass primordial black holes) of somthing less than 10%, confirming the widely ignored result of Mediavilla et al (2009).