The sun is hot due to nuclear fusion reactions occurring in its core. Specifically:

• The sun’s core is extremely dense and hot, with temperatures reaching over 15 million degrees Celsius.
• At these immense temperatures, hydrogen atoms in the core are undergoing nuclear fusion, where their nuclei are fusing together to form helium.
• This nuclear fusion process releases an enormous amount of energy in the form of heat and light, which radiates outwards from the sun’s core.
• The energy produced by fusion in the core takes thousands of years to reach the sun’s surface, where it is then radiated out into space as the sunlight and heat that we experience on Earth.

So in summary, the sun’s high temperature is a result of the nuclear fusion reactions continuously occurring at its very hot and dense core, which is the primary energy source powering the sun.