The Sun is the sources of the overwhelming majority of light, heat, and energy on Earth’s surface, and is powered by nuclear fusion. But less than half of that, surprisingly, is the fusion of hydrogen into helium. Public domain image.

The Sun’s Energy Doesn’t Come From Fusing Hydrogen Into Helium (Mostly)

It does undergo nuclear fusion, but there are more reactions and more energy released from reactions other than H → He.

“The sun is a miasma
Of incandescent plasma
The sun’s not simply made out of gas
No, no, no
The sun is a quagmire
It’s not made of fire
Forget what you’ve been told in the past” -They Might Be Giants

If you start with a mass of hydrogen gas and bring it together under its own gravity, it will eventually contract once it radiates enough heat away. Bring a few million (or more) Earth masses’ worth of hydrogen together, and your molecular cloud will eventually contract so severely that you’ll begin to form stars inside. When you pass the critical threshold of about 8% our Sun’s mass, you’ll ignite nuclear fusion, and form the seeds of a new star. While it’s true that stars convert hydrogen into helium, that’s neither the greatest number of reactions nor the cause of the greatest energy release from stars. It really is nuclear fusion that powers the stars, but not the fusion of hydrogen into helium.