Question

Why is the Sun’s upper layer – the photosphere, where spectral lines are created – considered “cool” despite its average temperature of 5800 K? What do atoms in the photosphere have that atoms deeper inside of the Sun may not have?

Answer 1

The photosphere is much cooler than the sun's core, which has a temperature well above 10 million degrees. That's why relatively it's considered cooler.

the sun's corona is heated by tiny explosions called nanoflares. These are impulsive heating bursts that individually reach incredibly hot temperatures of some 10 million Kelvins – even greater than the average temperature of the corona - and provide heat to the atmosphere. This super hot solar material is called plasma.

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