This research is an attempt to find the limiting energy density for compact stars using a variational ansatz for the unknown equation of state of ultra-dense nuclear matter. The Harrison-Wheeler/Negele-Vautherine equation of state is used for the outer/inner crust regime of compact stars up to nuclear density, and is assumed to be the valid equation of state for this region. Beyond nuclear density, the equation of state is almost completely unknown and is thus created using a variation ansatz. The equations of state generated from this ansatz are constrained by causality (in the high pressure regime), and microscopic stability (Le Chatelierâ?Ts principle). We also assume that Einsteinâ?Ts theory of General Relativity is the correct theory of gravity. The equations of state generated from the variational ansatz are used to generate stellar sequences of compact stars in order to investigate energy density limits for stars of given masses. From this data, conclusions can be drawn about phase transition regions within compact stars at certain densities.Also, knowing the mass limit for compact stars is of key importance for estimating the number of low-mass black holes in Galaxies.