Composition of the Sun: The Sun is primarily composed of hydrogen (approximately 71%) and helium (approximately 27%). A small percentage (around 2%) consists of heavier elements like oxygen, carbon, nitrogen, and iron.

Electromagnetic Radiation Emitted: The Sun radiates energy across a broad spectrum of electromagnetic radiation. The main regions of emission are:

• Infrared (IR) Radiation: Longer wavelengths, associated with heat. Examples include infrared lamps and heat vision goggles.
• Visible Light: The portion of the electromagnetic spectrum that the human eye can detect. This is what allows us to see. Different wavelengths within visible light correspond to different colours (red, orange, yellow, green, blue, indigo, violet).
• Ultraviolet (UV) Radiation: Shorter wavelengths than visible light. UV radiation can cause sunburn and is associated with vitamin D production. UV-A, UV-B, and UV-C are different types with varying levels of energy.

The Sun emits all these types of radiation, but the relative intensity of each varies. The Sun's surface temperature determines the peak wavelength of the emitted radiation. The Sun's surface temperature is approximately 5800 Kelvin, which corresponds to a peak wavelength in the visible light spectrum (around 500nm, which is blue-green).

As a star ages and exhausts the hydrogen fuel in its core, the rate of nuclear fusion in the core significantly decreases. This is because the fusion process requires a certain temperature and density to occur efficiently. With the hydrogen fuel depleted, the core temperature and density decrease, slowing down the fusion reaction.

The decrease in fusion rate leads to a reduction in the outward pressure supporting the core. This causes the core to contract under gravity. The contraction increases the temperature of the core, but not enough to reignite hydrogen fusion. As the core contracts, the outer layers of the star expand and become cooler, forming a red giant.

The eventual fate of the star depends on its initial mass. For stars with relatively low mass, the core will eventually become hot enough to initiate helium fusion. However, these stars do not have enough mass to fuse helium into heavier elements. They will eventually shed their outer layers, forming a planetary nebula, leaving behind a white dwarf – a dense, hot remnant core supported by electron degeneracy pressure. For more massive stars, the core will continue to fuse heavier elements, eventually leading to a supernova explosion and the formation of either a neutron star or a black hole.

Stars are in a state of equilibrium because they exist in a dynamic balance between two opposing forces: gravity and the outward pressure generated by nuclear fusion.

Gravity acts inwards, attempting to collapse the star under its own weight. This force is proportional to the mass of the star – the more massive the star, the stronger the gravitational pull.

Nuclear fusion, occurring in the star's core, generates a tremendous amount of energy. This energy is released as heat and light, creating an outward pressure that counteracts gravity. This outward pressure is proportional to the temperature and density of the core.

The nuclear fusion process is crucial for maintaining this equilibrium. The energy released by fusion provides the necessary pressure to resist the inward pull of gravity. If fusion were to stop, gravity would cause the star to collapse. Therefore, the ongoing nuclear fusion is essential for the star's stability and long-term existence.