5.3. Nuclear Fusion & Stars (E5)

Nuclear fusion in stars combines light nuclei (like hydrogen) into heavier elements (like helium, carbon, oxygen), releasing enormous energy.

In massive stars, fusion continues up to iron, while elements heavier than iron are formed during supernova explosions.

Stellar nucleosynthesis explains the abundance of elements found in the universe today.

Gravity pulls matter inward, while radiation pressure from fusion reactions pushes outward, maintaining hydrostatic equilibrium.

As nuclear fuel is exhausted, stars move across the Hertzsprung-Russell (HR) diagram, changing temperature, size, and luminosity.

A star’s mass determines its fate: low-mass stars become white dwarfs, high-mass stars can end as neutron stars or black holes.

Observations of stellar evolution, galaxy movement, and cosmic background radiation suggest the universe is expanding.

Models predict possible futures: continued expansion, heat death (maximum entropy), or big crunch depending on the universe’s density and dark energy content.

Studying current stellar life cycles and cosmic structures helps infer the universe’s long-term evolution.