 The slope of the curve for light nuclei is steeper, and shows considerably more scatter in the values. Although it is difficult to see the exact values on the graph, we list here the binding energies per nucleon for hydrogen 2 and helium 4. Remember that the isotope of hydrogen with mass 2 also goes by the alternative name of deuterium. If we fuse two deuterium nuclei to make a helium 4 nucleus, then we liberate 24 MVV of energy. This energy density is a step above even the fission of uranium. Recall the comparison we made earlier between the energy released by either burning 1000 grams of fossil fuel or fissioning 1000 grams of uranium 235. We found that the fission of uranium was 2 million times more energetic than the burning of fossil fuel. Now looking at the diffusion of deuterium, we see that the energy density is another 10 times higher than that for uranium 235. Furthermore, although deuterium is only a very small fraction of natural hydrogen, there is so much hydrogen on Earth, imagine the oceans, that this could be an almost unlimited source of energy. However, there is a problem. It is easy to fission uranium 235 by heating it with a neutron, since the neutron has no charge and it is easy to get it close to the uranium 235 nucleus. However, deuterium nuclei are positively charged and we need to overcome the electrostatic repulsion between them to initiate fusion. This involves accelerating them to high energies, which is equivalent to heating the deuterium gas to high temperatures, while still containing them close enough so they can fuse. In fact, the optimal energy for deuterium fusion is close to 100 million degrees. The difficulty of achieving this condition means that nobody has yet made a continuous fusion reactor with a net energy output. However, we all know a place where this form of energy production is continuously happening, and that is the Sun, where the high gravity holds the deuterium nuclei close together at very high temperatures up to millions of degrees in the core, enabling fusion. Meanwhile, back on Earth, we are still some decades away from being able to make fusion power plants.