 So far, we have discussed the energy contained in nuclei in terms of their masses. An equivalent and commonly used concept is that of nuclear binding energy. We can understand binding energy by considering a nucleus with Z protons and N neutrons and imagining breaking it up into its constituents, that is Z separated protons and N separated neutrons. If we measured the combined mass of all of these separate nucleons, we would find that this summed mass is greater than the mass of the bound nuclear system. The difference between these values, when described in terms of its energy equivalent using E equals MC squared, is called the nuclear binding energy. The figure on the lower left is a graph of all of the known nuclear binding energies obtained from the experimentally measured masses of all of these nuclei. They are plotted as a function of the total number of nucleons, A, this being the sum of the numbers of protons and neutrons in each nucleus, that is A equals N plus Z. It is clear that the binding energy has an approximately linear dependence on A. This leads us towards considering the quantity of binding energy per nucleon, i.e. dividing the measured binding energies by A. This gives us the graph on the lower right side, where we again display the plot as a function of the total nucleon number. It is amazing that when you plot over 2,500 different nuclear binding energies in this way, the data follows a very regular and distinctive pattern. The binding energy per nucleon peaks at around A equals 56, and this phenomenon is sometimes described in simple terms, such as iron 56 is the most stable nucleus known. While this is not a completely accurate statement, it is approximately true. In fact, because nuclei around A equals 56 are the most stable, this is one of the reasons that a star, which starts as an almost pure mixture of mostly hydrogen with a bit of helium, eventually end up having a central core that is mostly iron. As you move to heavier systems, the binding energy per nucleon decreases slowly, whereas the lighter systems have a much steeper slope. Looking at the average across all nuclei, the binding energy per nucleon is approximately constant at around 8 MeV per nucleon.