Final answer:
The outermost layer of the Sun's atmosphere is called the corona, which has temperatures reaching a few million degrees Celsius.
Step-by-step explanation:
The outermost part of the Sun's atmosphere is called the corona. This layer was first observed during total solar eclipses and has been known for centuries as detailed by historical figures like Plutarch and Kepler. The corona is significant because despite being the outer layer, it features some of the hottest temperatures in the Sun's atmosphere, with temperatures reaching a few million degrees Kelvin (K).
To answer the question directly: the outermost part of the sun is called the corona, and it is millions of degrees Celsius. It's important to note that when converting from Kelvin to Celsius for the temperature of the corona, we subtract 273.15 from the Kelvin temperature, so when we say the corona is a few million degrees Kelvin, this roughly translates to the same in degrees Celsius as well.