In the proton-proton chain, the mass of four protons is slightly greater than the mass of a helium nucleus. Explain what happens to this "lost" mass.

Nuclear reactions are processes where atomic nuclei are combined (fusion), split (fission), or transformed. These reactions involve enormous amounts of energy compared to chemical reactions. In stars, like our Sun, nuclear fusion is the dominant process.
Nuclear fusion occurs when two light atomic nuclei combine to form a heavier nucleus. This process releases a vast amount of energy. The proton-proton chain is a type of nuclear fusion that happens in stars.
The proton-proton chain converts hydrogen into helium through a series of steps:

• Two protons fuse to form a deuterium nucleus, releasing a positron and a neutrino.
• The deuterium nucleus then fuses with another proton to create a helium-3 nucleus and a gamma-ray photon.
• Two helium-3 nuclei combine to form a helium-4 nucleus, releasing two protons.

The energy released in these steps powers the star. It provides the heat and light we observe from the Sun.

Mass-energy equivalence is a fundamental principle of physics, introduced by Albert Einstein's famous equation \( E=mc^2 \). This equation states that mass \( m \) can be converted into energy \( E \), and vice versa, with the speed of light \( c \) squared acting as the conversion factor.
In the context of the proton-proton chain, the mass difference between the initial protons and the final helium-4 nucleus represents 'lost' mass. This \( \text{missing} \) mass is not actually gone but converted into energy. During each step of the chain, some mass is lost through the emission of particles and energy.
This energy is manifest in several forms:

• Photons (light and heat)
• Neutrinos
• Kinetic energy of the resulting particles

Understanding mass-energy equivalence is crucial to grasping why stars shine. It explains how a small amount of mass can produce a colossal amount of energy, sustaining the star over billions of years.

Star luminosity is the total amount of energy a star emits per unit time. It directly arises from the energy produced by nuclear reactions in the star's core. In our Sun, and many other stars, this energy is primarily generated by the proton-proton chain.
The energy released in the proton-proton chain reactions travels from the core to the star's surface. This process involves various mechanisms like radiation and convection.
Once at the surface, this energy is emitted into space as electromagnetic radiation. We perceive this as the star's light and warmth. Luminosity is a key property of stars, helping us understand their size, age, and distance.
Luminosity depends on two main factors:

• Temperature: Hotter stars are more luminous.
• Size: Larger stars emit more light.

Luminosity helps us measure and compare the energy output of different stars. It gives vital insights into stellar life cycles and the mechanisms powering them.