Question

Which of the following is the best answer to the question, "Why does the Sun shine?" As the Sun was forming, gravitational contraction increased the Sun's temperature until the core become hot enough for nuclear fusion, which ever since has generated the heat that makes the Sun shine.

Answer 1

The Sun shines due to nuclear fusion in its core, where hydrogen is converted into helium at high temperatures, releasing energy that balances gravitational forces and prevents collapse.

Step-by-step explanation:

Why Does the Sun Shine?

The Sun remains luminous and does not collapse under its own gravity due to a delicate balance between the gravitational forces and the pressures caused by high-temperature gases. The gravitational pull of the Sun's mass tends to contract it inward, while the thermal pressure from nuclear reactions inside the core pushes outward. At the core, temperatures reach around 15 million K, creating the perfect condition for protons to undergo fusion, converting hydrogen into helium through the proton-proton chain. This fusion process, which adheres to the mass-energy equivalence formulated as E = mc², releases an immense amount of energy, making the Sun shine and preventing it from gravitational collapse.

Historically, it was proposed that the Sun's luminosity could originate from gravitational contraction, a theory posited by Lord Kelvin and Hermann von Helmholtz. However, this alone could not account for the immense energy output of the Sun over billions of years. Thus, nuclear fusion is now understood to be the dominant process that powers the Sun, utilizing the abundant hydrogen as 'fuel' and reaching equilibrium on the brink of fusion, preventing any collapse.