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>How do we measure the temperature of the sun, If it is so hot that no tool of temperature measurement can get the most accurate number?
You've no doubt heard of something being "red hot" or seen a stove element glowing red or orange. When things are heated they emit what's called "black body radiation". The hotter the object the higher the average frequency of the light produced. The filament in a light buld is much hotter than a red stove element, so it produces light which is (on average) of a higher frequency than that produced from a red hot stove element.
We can measure the frequency distribution of light from the sun's surface and determine the temperature it must be to produce that spectrum of light. Part of that spectrum is divided up for use to see in rainbows.
