Helium (element) is believed to be the second most common element in the universe.

Helium is believed to be the second most common element in the universe. Here's an explanation of why this is the correct answer:

1. Structure of the Universe:
The universe is composed of various celestial bodies like stars, galaxies, and nebulae. These celestial bodies are made up of different elements. By analyzing the spectral lines emitted by these objects, scientists can determine the composition of the universe.

2. Abundance of Hydrogen:
Hydrogen is the most abundant element in the universe, making up about 75% of its elemental mass. It is formed through nuclear fusion in stars and can be found in massive quantities in stars, interstellar gas clouds, and other cosmic structures.

3. Abundance of Helium:
After hydrogen, helium is the most abundant element in the universe, accounting for about 24% of its elemental mass. Helium is formed through nuclear fusion as well, primarily in stars. It is created during the later stages of stellar evolution, particularly during the red giant phase and supernova explosions.

4. Stellar Nucleosynthesis:
During the nuclear fusion process in stars, hydrogen atoms combine to form helium through a series of fusion reactions. This process releases a tremendous amount of energy, which powers the stars. As a result, helium is continuously produced within stars and released into the surrounding space.

5. Cosmic Microwave Background Radiation:
The abundance of helium in the universe is supported by the measurement of cosmic microwave background radiation (CMBR). The CMBR is residual radiation from the Big Bang, and its analysis has revealed the presence of helium in the early universe, further confirming its abundance.

6. Other Elements:
While helium is the second most abundant element, it is followed by heavier elements like oxygen, carbon, and nitrogen, which are found in smaller quantities. These elements are synthesized through nucleosynthesis processes in stars of different masses and during supernova explosions.

In conclusion, helium is believed to be the second most common element in the universe after hydrogen. Its abundance is a result of stellar nucleosynthesis processes and is supported by observations of the cosmic microwave background radiation.