Question

deep in the core of the sun, 620 million metric tons of hydrogen nuclei are converted into 616 million metric tons of helium nuclei every second. what happens to the \missing mass\ in the suns core?

the \missing mass\ is transformed into energy in a fusion reaction.

the \missing mass\ is transformed into energy in a fission reaction.

the \missing mass\ is converted to heat during an exothermic chemical reaction that produces a new chemical species.

the \missing mass\ is converted to heat during an endothermic chemical reaction that produces a new chemical species.

Explanation:

In the Sun's core, hydrogen nuclei undergo fusion. According to Einstein's mass - energy equivalence ($E = mc^{2}$), the mass difference (missing mass) between the reactants (hydrogen nuclei) and products (helium nuclei) is converted into energy. Fission is a different nuclear process (splitting of heavy nuclei). Chemical reactions (exothermic or endothermic) involve electrons and do not account for such large - scale mass - energy conversions as seen in nuclear fusion in the Sun.

Answer:

A. The "missing mass" is transformed into energy in a fusion reaction.