When the Sun runs out of fuel in its core, the core will be a. empty. b. filled with hydrogen. c. filled with helium. d. filled with carbon.

Hydrogen fusion is the process by which hydrogen atoms combine to form helium, releasing a tremendous amount of energy. This process occurs in the cores of stars, including our Sun.

During hydrogen fusion, four hydrogen nuclei (protons) come together to form a single helium nucleus. This process releases energy because the mass of the helium nucleus is slightly less than the total mass of the four protons; the mass difference is converted into energy according to Einstein's equation, \(E = mc^2\).

This energy produced is what powers the star and provides the heat and light we receive on Earth. Essentially, hydrogen fusion is the engine driving a star's vibrant journey through its early and middle years. Without hydrogen fusion, stars would not have the necessary energy to combat the forces of gravity and would collapse. Thus, hydrogen fusion is a crucial phase in a star's lifecycle.

The main sequence phase is the longest and most stable period in a star's life. During this phase, the star continually fuses hydrogen into helium in its core.

For a star like the Sun, the main sequence phase lasts for billions of years. The energy generated from hydrogen fusion creates an outward pressure that counteracts the inward pull of gravity. This balance keeps the star in a stable state.

Characteristics of stars in the main sequence include:

• Steady luminosity
• Stable size and temperature
• Balance between gravitational forces and fusion energy

Eventually, as the star depletes its hydrogen fuel, it transitions out of the main sequence phase and evolves into later stages, like the red giant phase.

As a star exhausts its hydrogen fuel, the core no longer supports hydrogen fusion. The fusion by-product, helium, starts to accumulate in the core.

During the transition out of the main sequence phase, helium accumulation becomes significant. The core, once dominated by hydrogen, becomes rich in helium. This transformation leads to:

• Changes in pressure and temperature within the star
• The core contracting and heating up while the outer layers expand
• The star eventually evolving into a red giant

The star's journey continues with helium fusion in later stages if the core temperature becomes sufficiently high. But in the case of the Sun, it will ultimately lead to a state where the core is filled primarily with helium.