Here's why:

* Nuclear Fusion: The 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: The core is under immense pressure due to the weight of all the layers above it. This pressure is essential for nuclear fusion to occur.

* High Temperature: The heat generated by nuclear fusion makes the core the hottest part of the star, reaching millions of degrees Celsius.

While the surface of a star is also incredibly hot, the core always reaches much higher temperatures.