In the proton-proton chain, the mass of four protons is slightly greater than the mass of a helium nucleus. The "lost"" mass a. binds the particles of the nucleus together. b. cancels the positive charge of the two protons that turned into neutrons. c. escapes as light, positrons, or neutrinos. d. is reabsorbed instantly, turning helium back into hydrogen.

Nuclear fusion is the process of combining two light atomic nuclei to form a heavier nucleus. This reaction releases a significant amount of energy. In stars, the most common type of nuclear fusion is the proton-proton chain, where hydrogen nuclei (protons) merge to create helium.

During the proton-proton chain reaction, four protons come together through several steps, eventually forming one helium nucleus. This process powers the sun and other stars, providing the energy that we experience as sunlight.

Nuclear fusion is a highly efficient energy source because of the strong force that binds protons and neutrons in the nucleus. When they combine, the binding energy is released, resulting in the conversion of a small amount of mass into a large amount of energy, as described by Einstein's equation.

• Proton-proton chain: A sequence of reactions transforming hydrogen into helium.
• Energy release: Due to mass conversion during fusion.

Mass-energy equivalence is a fundamental concept in physics introduced by Albert Einstein. It states that mass can be converted into energy and vice versa. The relationship between mass and energy is given by the famous equation:

E = mc^2

In this equation, E stands for energy, m represents mass, and c is the speed of light (~3 x 10^8 meters per second).

This principle explains why the 'lost' mass in a nuclear fusion reaction isn't missing but instead transformed into energy. For instance, in the proton-proton chain, some of the initial mass (the combined mass of protons) is not present in the final helium nucleus. This 'missing' mass has been converted into energy, which we observe as light (photons), positrons, and neutrinos.

• Energy and mass: Interconvertible as per Einstein's theory.
• Lost mass: Manifested as radiant energy and particles in fusion.

Einstein's theory of relativity fundamentally changed our understanding of physics. It consists of two parts: the Special Theory of Relativity and the General Theory of Relativity.

The Special Theory of Relativity, introduced in 1905, deals with objects moving at constant speeds, particularly those close to the speed of light. One of its key components is the mass-energy equivalence principle, encapsulated by the equation E = mc^2. This equation reveals that a small amount of mass can be converted into a large amount of energy, explaining phenomena such as the energy released in nuclear fusion.

The General Theory of Relativity, published in 1915, extends these ideas to include gravity and accelerated motion. It describes gravity not as a force between masses but as a curvature of spacetime caused by mass. This theory is essential for understanding the large-scale structure of the universe and the behavior of cosmic objects.

• Special Relativity: Applies to high-speed objects, introduces E = mc^2.
• General Relativity: Describes gravity through spacetime curvature.