According to $E=mc^2$, what happens to the mass when four protons form a helium nucleus?

Answer

The resulting helium nucleus is measurably lighter

When four hydrogen nuclei fuse into one helium nucleus, the resulting helium nucleus has a mass deficit ($\Delta m$), which is the mass precisely converted into radiant energy.

According to $E=mc^2$, what happens to the mass when four protons form a helium nucleus?
hydrogennuclear fusionstarsastrophysicshelium