How do scientists determine the chemical composition and energy output of the Sun?

By using spectroscopy to analyze solar light

Since direct travel to the Sun's core is impossible, scientists utilize spectroscopy to understand the solar interior. By analyzing the light spectrum emitted by the Sun, researchers can identify absorption lines that reveal the chemical composition of the solar atmosphere. By combining this data with the known temperature of the photosphere, they can calculate the internal pressure and the specific fusion rate required to maintain the Sun's luminosity. This observational evidence has confirmed that the Sun is composed primarily of hydrogen and helium, and it helps prove that nuclear fusion is the only mechanism capable of sustaining the Sun's energy output over billions of years.