Final answer:
Mass in the Sun's core is converted to energy through nuclear fusion, where 600 million tons of hydrogen are fused into helium every second, converting about 4 million tons of mass into energy. The Sun has sufficient fuel to continue this process for another 5 billion years.
Step-by-step explanation:
In the Sun's core, mass is converted into energy predominantly through nuclear fusion, specifically the proton-proton chain reaction. Hydrogen nuclei, which are protons, fuse to form helium, a process that takes place at temperatures of around 15 million Kelvin. According to Einstein's equation of E = mc², this fusion converts some of the mass of hydrogen into energy. Roughly 600 million tons of hydrogen fuse every second, converting about 4 million tons of mass into energy.
The Sun has been shining for approximately 4.6 billion years, and it has enough nuclear fuel to last for about another 5 billion years. When the central core of the Sun eventually becomes depleted of hydrogen, it is estimated that the mass of the Sun would have decreased by a relatively small percentage, due to the vast amount of matter it contains.