Astronomy Practice Exam 2026 – Complete Preparation Guide

During the main sequence phase, stars primarily fuse hydrogen into helium in their cores. This process is known as nuclear fusion, and it releases an enormous amount of energy, which provides the outward pressure necessary to counteract the gravitational forces trying to collapse the star.

Hydrogen is the simplest and most abundant element in the universe, making it the main fuel for this stage of stellar evolution. As stars like the Sun spend the majority of their lifetimes in the main sequence phase, they rely heavily on hydrogen fusion to maintain their stability and luminosity.

In contrast, while carbon, oxygen, and helium are important elements in other stages of a star's lifecycle (such as during the later fusion processes that occur in more massive stars or in red giants), they do not play the primary role in the main sequence stage. Thus, the correct answer highlights the essential role of hydrogen as the fundamental fuel source during this critical phase of stellar development.