Here's why:

* Extreme Pressure: The core is under immense pressure from the weight of all the layers above it.

* Nuclear Fusion: This pressure forces hydrogen atoms to fuse into helium, releasing enormous amounts of energy.

* High Density: The core's extreme temperature and pressure create a density of about 150 times that of water.

For reference, the density of the core is about 150 times that of water, while the density of the surface is only about 1/10,000th the density of water.