At temperature, $\mathrm{T}=10^{7} \mathrm{~K}$, we have

Average thermal energy will be $=(3 / 2) \mathrm{kT}$

$=(3 / 2) \times 1.38 \times 10^{-23} \times 10^{7}$

We get,

$=2.07 \times 10^{-16} \mathrm{~J}$

As a result, the average thermal energy of a helium atom at the core of a star is $2.07 \times 10^{-16} \mathrm{~J}$