The Sun's core is not defined by a specific thickness in kilometers. Instead, it's defined by the region where nuclear fusion occurs, which extends approximately 150,000 kilometers from the Sun's center.

Here's why it's not a simple thickness:

* Gradual Transition: The Sun's core doesn't have a sharp boundary. The temperature and density decrease gradually as you move outward, making the transition to the radiative zone more gradual.

* Fusion Zone: The core is defined by the region where nuclear fusion is happening, which is a process that gradually diminishes as you move outward.

* Dynamic Nature: The Sun is constantly undergoing changes, and the core is not a static region. The extent of nuclear fusion can fluctuate slightly due to various factors.

Therefore, it's more accurate to say that the Sun's core extends about 150,000 kilometers from its center, rather than having a specific thickness.