Question

The source of energy in the Sun is:

• A. Conversion of hydrogen into helium in nuclear fusion reaction.
• B. Uranium fission.
• C. Beta decay of uranium.
• D. Reaction of hydrogen with uranium.

Hint:

Nuclear fusion, where hydrogen is converted into helium, is the primary energy source in stars, including the Sun.

Answer 1

The Correct Option is A

The primary source of energy in the Sun is the conversion of hydrogen into helium in a nuclear fusion reaction. This process occurs in the Sun's core and is known as the proton-proton chain reaction. Here is a step-by-step explanation of how this energy is generated:

1. Nuclear Fusion: At extremely high temperatures and pressures in the Sun's core, hydrogen nuclei (protons) overcome their mutual electrostatic repulsion and fuse together. This is due to the intense gravitational pressure compressing the core.
2. Fusion Process: The fusion process primarily involves several steps where hydrogen nuclei are converted into helium. The primary reactions in the proton-proton chain are:
    
3. Step 1: Two protons collide, and one converts into a neutron via beta-plus decay, forming a deuteron (a nucleus of deuterium, i.e., heavy hydrogen). This reaction releases a positron and a neutrino.
4. Step 2: The deuteron then fuses with another proton, forming helium-3 and releasing gamma radiation.
5. Step 3: Two helium-3 nuclei collide to form helium-4, releasing two protons.
   The net effect of these reactions is that four hydrogen nuclei (protons) combine to form one helium nucleus, and this process releases a significant amount of energy.
6. Energy Release: The energy released during each fusion reaction propagates outward from the core. This energy travels through the radiative and convective layers of the Sun and eventually reaches the photosphere, where it is emitted as sunlight.

This mechanism of hydrogen fusion into helium is the process that powers the Sun and other stars, making it the correct answer.

Answer 2

This question asks what actually powers the Sun. Let's examine each proposed process in turn.

1. Conversion of hydrogen into helium in nuclear fusion: The Sun's core reaches temperatures of about 15 million kelvin and enormous pressure, conditions under which hydrogen nuclei can overcome their mutual repulsion and fuse to form helium, releasing energy at every stage of the process.
2. Uranium fission: The Sun is made almost entirely of hydrogen and helium, with only trace amounts of heavier elements, so there is nowhere near enough uranium present in the Sun to power it through fission.
3. Beta decay of uranium: Beta decay releases a comparatively small amount of energy per event and, again, the Sun does not contain enough uranium for this to be a meaningful energy source, let alone the dominant one.
4. Reaction of hydrogen with uranium: There is no such nuclear reaction that releases energy this way, and even if there were, the near absence of uranium in the Sun rules it out.

Only the fusion of hydrogen into helium matches both the composition of the Sun and the conditions in its core, and it is the process confirmed to power stars like the Sun.

Therefore, the correct answer is conversion of hydrogen into helium in nuclear fusion reaction.