What is the temperature at the visible surface of the Sun, in degrees Celsius?
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Answer
About 5,500 °C
Oddly, the Sun's thin outer atmosphere, the corona, is hundreds of times hotter than its surface, and physicists are still working out why.
How to estimate it
Hot objects glow with a colour set by their temperature. Wien's law says peak wavelength times temperature is about 2,900,000 nm·K. Sunlight peaks near 500 nm, in the green part of the spectrum, so 2,900,000 / 500 = 5,800 kelvin. Subtract 273 to convert and you get roughly 5,500 °C, which matches the measured value of about 5,500 °C.
Source: NASA
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