Stars with a core temperature in excess of 20 million K probably have hydrogen fusion occurring in their core.

Here's why:

* Hydrogen fusion is the primary energy source for most stars, including our Sun. It involves the fusion of four hydrogen nuclei (protons) into one helium nucleus. This process releases a tremendous amount of energy.

* The threshold temperature for hydrogen fusion is around 10 million K. This means that stars with core temperatures below 10 million K are not hot enough to sustain hydrogen fusion.

* Stars with core temperatures above 20 million K are definitely hot enough for hydrogen fusion to occur rapidly. They are also likely to have higher pressures in their core, which further enhances fusion rates.

While other fusion processes might be occurring in these stars, hydrogen fusion remains the dominant energy source.