The major constituents of the universe are hydrogen and helium.

Explanation:
1. Hydrogen:
- Hydrogen is the most abundant element in the universe, making up about 75% of its elemental mass.
- It is a colorless, odorless, and highly flammable gas. It is the lightest and simplest element, consisting of only one proton and one electron.
- Hydrogen is thought to be the primary building block of the universe, forming the first atoms after the Big Bang.
- It is the fuel for nuclear fusion in stars, including our Sun, where hydrogen nuclei combine to form helium, releasing a tremendous amount of energy.

2. Helium:
- Helium is the second most abundant element in the universe, accounting for about 24% of its elemental mass.
- It is a colorless, odorless, and inert gas. It is lighter than air and can be used for filling balloons and airships.
- Helium is formed through nuclear fusion in stars, primarily during the later stages of their evolution. It is produced when helium nuclei combine to form heavier elements like carbon and oxygen.
- It is also present in small amounts on Earth, often trapped in natural gas deposits. However, it is difficult to extract and is considered a non-renewable resource.

3. Other Elements:
- While hydrogen and helium are the major constituents of the universe, other elements, including nitrogen and oxygen, are also present but in much smaller quantities.
- Nitrogen and oxygen are essential elements for life as we know it and are found in the Earth's atmosphere. However, their abundance in the universe is relatively low compared to hydrogen and helium.

In conclusion, hydrogen and helium are the primary constituents of the universe, with hydrogen being the most abundant element and helium being the second most abundant. These elements play crucial roles in the formation of stars, galaxies, and the overall structure of the universe.

It is the constituent of sun