Why is the average molar mass of the Sun’s matter (0.61 * 10-3 kg / mol) less than the molar mass of hydrogen (1 * 10-3 kg / mol) and helium (4 * 10-3 kg / mol), of which it is composed?
The matter of the Sun is in the form of a plasma and therefore, when calculating the average molar mass, it is necessary to take into account not only protons and nuclei of helium atoms, but also electrons, which have a very small mass. For 91 protons there are 9 helium nuclei and 109 electrons detached from them.
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