Question: How hot is the middle of the sun?

Susan Cartwright answered on 14 Jun 2015:

About 15.6 million degrees (Celsius or Kelvin: the extra 273 doesn’t really matter on this scale).

This is calculated by complex computer programs which input known properties of the Sun – the amount of energy radiated per second, its mass, its radius, its surface temperature, its surface chemical composition – and the details of known physical processes such as the rates of different nuclear reactions, the way that different elements emit and absorb light, the energy required to ionise different elements, and so on. The program then tries to solve for the structure of the Sun, in such a way that the known properties are reproduced. This is called the Standard Solar Model, and is due mainly to an American physicist called John Bahcall, who really should have got a Nobel prize for it but didn’t. The output parameters of the Standard Solar Model include the temperature as a function of depth within the Sun (that’s where I got the 15.6 million degrees), as well as the density, pressure, rate at which energy is being generated by different nuclear reactions, number of neutrinos produced and so on. Because the output parameters of thre Standard Solar Model are in good agreement with what we observe, and the input physical processes are rather well understood, we believe that the central temperature it calculates is likely to be right.