Final answer:
The Sun loses energy through nuclear fusion, converting roughly 4 million tons of matter into energy every second, according to Einstein's mass-energy equivalence principle.
Step-by-step explanation:
The Sun loses energy primarily through nuclear fusion in its core. During this process, matter is converted into energy according to Einstein's equation E = mc². This results in the Sun losing approximately 4 million tons of matter every second, which is converted into energy and released as light and heat. While this seems like a large amount, the Sun has enough mass to continue this process for billions of years. To put this in perspective, about 600 million tons of hydrogen are converted into helium each second, a fraction of which directly contributes to the energy released.