The Sun, which is the closest star to Earth, is primarily composed of hydrogen and helium. These two elements make up the majority of the Sun's mass and are responsible for its energy production through a process called nuclear fusion.

- Hydrogen and Helium:
The Sun is made up of about 74% hydrogen and 24% helium by mass. These two elements are the lightest and most abundant in the universe. Hydrogen atoms are the building blocks of the Sun and exist in its core, where they undergo fusion to form helium. This fusion process releases an enormous amount of energy in the form of light and heat. The Sun's immense gravitational force keeps these elements confined within its core.

- Nuclear Fusion:
Nuclear fusion is the process by which hydrogen atoms combine to form helium. In the Sun's core, where temperatures reach millions of degrees Celsius, hydrogen atoms collide with each other, creating extreme pressure and heat. Under these conditions, hydrogen nuclei fuse together to form helium nuclei, releasing a tremendous amount of energy in the process. This energy is what makes the Sun shine and provides heat and light to the entire solar system.

- Other Elements:
Although hydrogen and helium are the primary constituents of the Sun, there are trace amounts of other elements present as well. These include elements like carbon, nitrogen, oxygen, and small amounts of heavier elements like iron and nickel. However, their concentrations are relatively low compared to hydrogen and helium.

In conclusion, the Sun is primarily composed of hydrogen and helium. These elements undergo nuclear fusion in the Sun's core, releasing vast amounts of energy in the form of light and heat. The Sun's fusion reactions sustain its energy output, making it the main source of light and heat for the solar system.