Solar Effective Temperature

An effective temperature is not a thermometer reading; it is the temperature a perfect blackbody of the same size would need to emit the same total power. Feed the nominal luminosity and nominal radius into the Stefan–Boltzmann law and 5772 K falls out, which is why the IAU could fix it exactly alongside them. The visible photosphere spans roughly 4500 K at the dark centres of sunspots to 6000 K in the surrounding granulation, and the temperature climbs back to a million kelvin in the corona — a reversal still not fully explained.

The number sets the peak of sunlight near 500 nm, in the green, which is squarely inside the range human vision evolved to use. It also anchors the G2V spectral class: everything astronomers say about hotter blue O stars and cooler red M dwarfs is calibrated against this one value.