It's not about the individual stars themselves, but rather the mass of the star. The bigger the star, the hotter its core.

Here's why:

* Gravity and Fusion: Massive stars have a much stronger gravitational pull at their core. This pressure forces hydrogen atoms to fuse together, creating helium and releasing immense energy.

* Higher Fusion Rates: The greater pressure in massive stars leads to faster and more intense nuclear fusion reactions, generating significantly more heat.

* Temperature Gradient: The core of a star is where fusion occurs, and the temperature gradually decreases outward towards the surface. This gradient is steeper in massive stars, resulting in a much hotter core.

Therefore, the most massive stars, like blue supergiants, have the hottest cores.

For example, the star Eta Carinae, one of the most massive stars known, has a core temperature estimated to be over 100 million Kelvin! This is significantly hotter than our Sun's core, which is around 15 million Kelvin.