The sun is hot due to the nuclear fusion process taking place within it, which is called stellar nucleosynthesis. The enormous amount of energy released by this process, nearly 383 billion megawatts multiplied by one billion, produces the sun's incredible heat.
According to Planet Facts, the temperatures of the sun’s core and its surface are 27 million degrees Fahrenheit and 10,000 degrees Fahrenheit, respectively. The next layer, the chromosphere, has a temperature of nearly, 7,800 degrees Fahrenheit, while the corona has a temperature of 3.5 million degrees Fahrenheit. According to the National Center for Atmospheric Research, the Type II spicules, or radices, were responsible for the large difference in temperature between the solar corona and the sun's surface.