Final answer:
The Sun is primarily composed of hydrogen, which accounts for 73% of its mass, followed by helium at 25%. These elements are essential for the fusion reactions that power the Sun and are common in other stars as well.
Step-by-step explanation:
The Sun is primarily composed of hydrogen. About 73% of the Sun's mass consists of hydrogen, making it the most abundant element in the solar atmosphere. It is followed by helium, which makes up another 25%. These two elements together form about 98% of the Sun's mass, and this is similar for other stars as well. The remaining mass is comprised of other elements like carbon, oxygen, and nitrogen, but they only constitute a small fraction.
This composition can be determined from a star's absorption line spectrum and reflects the vital role of fusion reactions in stars. In the case of our Sun, hydrogen nuclei fuse to form helium, releasing a tremendous amount of energy in the process. This fusion reaction is central to the Sun's ability to emit light and heat.