The source of energy in stars is: 

Concept:

The Sun is Earth’s major source of energy, yet the planet only receives a small portion of its energy and the Sun is just an ordinary star. Many stars produce much more energy than the Sun. The energy source for all stars is nuclear fusion.

Stars are made mostly of hydrogen and helium, which are packed so densely in a star that in the star’s center the pressure is great enough to initiate nuclear fusion reactions. In a nuclear fusion reaction, the nuclei of two atoms combine to create a new atom.

Most commonly, in the core of a star, two hydrogen atoms fuse to become a helium atom. Although nuclear fusion reactions require a lot of energy to get started, once they are going they produce enormous amounts of energy.

In a star, the energy from fusion reactions in the core pushes outward to balance the inward pull of gravity. This energy moves outward through the layers of the star until it finally reaches the star’s outer surface.

• \(H^1+H^1\to He^2+\ Energy\)
• The source of energy of stars is due to Conversion of hydrogen to helium.
• The energy released from the collapse of the gas into a star causes the centre of the star to become extremely hot.
• This results in nuclear fusion.
• When two hydrogen atoms combine to form a helium atom, then that process is known as nuclear fusion reaction.

Hence, the correct answer is Option-4-nuclear fusion reaction.