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We have mathematical models describing just about every behavior of stars based on their mass and composition. The sun's surface temperature is a good indicator, however. Temperature, or rather total energy, should increase proportionally with smaller volume as you go deeper into the star. At some point, the pressure and temperature is enough for fusion to occur. We know exactly what conditions are necessary having made a hydrogen bomb ourselves.
Anonymous
The math involved is based on the surface temperature and the heat radiating away, then doing some crazy math that I can barely comprehend.
Harley Drive
pure guesswork it could be anywhere from 12 million degrees to 40 million degrees we have no way of estimating or measuring either temperature