Nuclear physicists rely on a phenomenological formula developed by Bethe and Weizsacker to estimate the binding energy and thus mass of an atomic nucleus consisting of A nucleons with N neutrons and Z protons. This liquid-drop model formula tells us the following: (a) that uncharged symmetric nuclear matter (i.e., infinitely large with N = Z) is bound by about 16 MeV per nucleon, (b) that infinite matter pays an energy penalty for being asymmetric in N and Z, and (c) that finite real nuclei pay additional energy penalties for having a surface and for being charged. While the asymmetry penalty is, in principle, a function of density ρ and temperature T, as well as asymmetry, finite nuclei only inform us about this energy penalty near the saturation density (ρ0 ∼ 0.15 fm−3, or somewhat less due to the surface), zero temperature, and near zero asymmetry [which is characterized by the variable δ = (N− Z)/A]. However, there are places in nature with densities ranging from much less (in objects such as you and I) to much greater than the saturation nuclear density, with asymmetries extending out to nearly 1 (in neutronstars) and temperatures that are significant on the nuclear scale (again in astrophysical sites). In the case of neutron stars, when calculating the total energy density of the matter resisting gravitational collapse, it is the asymmetry contribution that dominates the energy and thus the pressure. Now, Joseph Natowitz and colleagues at institutions in Germany, Italy, Poland, Russia, and the United States report in Physical Review Letters, a new approach [1] to extracting the density dependence of the asymmetry energy (at low density) that agrees with quantum statistical calculations (Fig. 1). This agreement provides some confidence that, for example, the pressure supplied by asymmetric matter (at low density) is understood. The asymmetry energy [2] (which is a coefficient in the Bethe-Weizsacker mass model multiplying a δ2 term, FIG. 1: Asymmetry energy divided by the asymmetry energy at saturation as a function of density as extracted from experiment (dots) and from a quantum statistical model (QS) at 3 different temperatures. The quantum statistical models that include cluster formation agree well with experiment, as reported by Natowitz et al. (Adapted from [1].)