The major constituent of the Sun is Helium and Hydrogen.

Explanation:
The Sun is a star located at the center of our solar system. It is made up of several layers, each with its own composition and characteristics. The major constituent of the Sun is its core, where nuclear fusion occurs.

The core of the Sun is extremely hot and dense, with temperatures reaching about 15 million degrees Celsius. In this region, hydrogen atoms combine to form helium through a process called nuclear fusion. This fusion reaction releases an enormous amount of energy in the form of light and heat.

The core of the Sun is composed primarily of hydrogen, which makes up about 74% of its mass. Helium is the second most abundant element in the Sun's core, making up about 24% of its mass. These two elements, hydrogen and helium, are the main constituents of the Sun.

Hydrogen:
- Hydrogen is the lightest and most abundant element in the universe. It is composed of a single proton and a single electron.
- In the core of the Sun, hydrogen atoms are squeezed together under extreme pressure and temperature, causing them to collide and fuse.
- The fusion of hydrogen atoms releases a tremendous amount of energy in the form of light and heat, which powers the Sun and allows it to shine.

Helium:
- Helium is the second lightest element and is composed of two protons and two neutrons in its nucleus, surrounded by two electrons.
- As hydrogen atoms fuse in the core of the Sun, they combine to form helium atoms.
- The fusion of hydrogen to helium releases energy and produces an enormous amount of heat, which maintains the high temperatures and pressures necessary for nuclear fusion to continue.

In conclusion, the major constituent of the Sun is helium and hydrogen. These elements undergo nuclear fusion in the Sun's core, releasing tremendous amounts of energy and powering the Sun's radiant heat and light.