Question

The Sun's energy comes primarily from fusion of light elements in its core. It is estimated that a very small fraction of mass of the Sun (~10−1210−12 times the abundance of hydrogen) is uranium (both235 and 238 isotopes). But given the huge mass of the sun (~2∗10302∗1030kg), the mass of uranium in sun will come to around 2∗10182∗1018kg, which is again a significant quantity. Since uranium is heavier compared to other elements in the Sun, you reckon that most of it will sink to core or at least be concentrated in inner regions of the Sun. This uranium would undergo spontaneous fission, but further, nuclear fusion taking place in the core releases a large amount of neutrons every second (which can cause fission of uranium nearby, similar to what happens in a multistage thermonuclear weapon). Is there fission of uranium taking place in sun as well? What percent of Sun's energy output can be attributed to fission?

Answer 1

The Sun's energy primarily comes from nuclear fusion, not from the fission of elements like uranium. The fusion of hydrogen into helium in the Sun's core produces significantly more energy than nuclear fission, making any energy contribution from fission negligible.

Step-by-step explanation:

While the Sun does contain uranium and this uranium can undergo spontaneous fission, the primary source of the Sun's energy is nuclear fusion. In the Sun's core, where temperatures reach approximately 14 million kelvin, hydrogen nuclei (protons) fuse to form helium nuclei, releasing a tremendous amount of energy in the process. This energy is far greater than that released by nuclear fission. The reaction can be summarized as:

4 1H → He + 201n

The mass difference between the hydrogen and helium nuclei is converted into energy, specifically about 3.6 × 1011 kJ of energy per mole of He produced. This energy is what powers the Sun and is vastly more than the energy produced by the fission of uranium. While fission might occur, its contribution to the Sun's total energy output is negligible. Therefore, we attribute virtually none of the Sun's energy output to fission.