The part of the Sun that reaches 15,000,000 degrees Celsius is its core.

Here's why:

* Nuclear Fusion: The Sun's core is where nuclear fusion takes place. This is the process where hydrogen atoms fuse together to form helium, releasing immense amounts of energy in the form of light and heat.

* Extreme Pressure and Density: The core is under immense pressure from the weight of the surrounding layers of the Sun. This pressure, combined with the high density of the core, creates the extreme temperatures necessary for fusion.

While the surface of the Sun is about 5,500 degrees Celsius, the core's temperature is significantly higher due to the intense nuclear reactions happening there.