The life cycle of a star

• The inward force of gravity (trying to collapse the star) is exactly balanced by the outward radiation pressure (caused by the enormous energy released during nuclear fusion).
• This period of stability lasts for billions of years.
• The energy released in fusion comes from a tiny loss of mass, calculated using mass-energy equivalence: $E = mc^2$.
• The path a star takes after this stable main sequence phase depends entirely on its initial mass.

Life Cycle of Sun-Sized Stars

Stars do not live forever; eventually, they run out of fuel. When a star about the same size as the Sun exhausts the hydrogen in its core, the outward fusion pressure decreases.

• The core contracts under gravity and heats up, allowing helium and other light elements to begin fusing.
• The energy released by this new fusion causes the outer layers of the star to expand and cool, forming a red giant.
• Sun-sized stars will only produce elements up to carbon and oxygen during this phase.
• Once fusion reactions finish completely, the star becomes unstable and ejects its outer layers of gas and dust.
• This leaves behind a hot, dense, solid core called a white dwarf, which is roughly the size of the Earth.
• As the white dwarf cools over time and stops emitting significant heat or light, it becomes a theoretical black dwarf.

Life Cycle Sequence: Nebula $\rightarrow$ Protostar $\rightarrow$ Main Sequence $\rightarrow$ Red Giant $\rightarrow$ White Dwarf $\rightarrow$ Black Dwarf.

Life Cycle of Massive Stars

The bigger the star, the more dramatic its end. Stars much larger than the Sun follow a different, more violent life cycle after the main sequence.

• When hydrogen runs out, massive stars expand and cool to become a red supergiant.
• They fuse heavier elements in distinct cycles (such as neon and magnesium) all the way up to iron.
• When fusion stops at the iron limit, there is no longer outward pressure to balance gravity, causing the star to collapse rapidly and explode in a colossal supernova.
• The remaining core forms an incredibly dense neutron star (about the size of a small city, where a teaspoon weighs millions of tonnes).
• If the initial star was among the most massive stars, the core collapses entirely to form a black hole—an object so dense that its gravity prevents even light from escaping.

Life Cycle Sequence: Nebula $\rightarrow$ Protostar $\rightarrow$ Massive Main Sequence Star $\rightarrow$ Red Supergiant $\rightarrow$ Supernova $\rightarrow$ Neutron Star OR Black Hole.

Nuclear Fusion & The Formation of Elements

Every atom of carbon in your body and oxygen in the air was forged inside the fiery core of a star. Nuclear fusion requires extraordinary conditions (temperatures of 15–100 million $^\circ\text{C}$ and immense pressure) to overcome the electrostatic repulsion between positively charged nuclei.

• Hydrogen fusion: Occurs during the main sequence (fusing 4 hydrogen nuclei into 1 helium nucleus).
• Helium fusion: Occurs in red giants and supergiants (e.g., $3 \times ^4_2\text{He} \rightarrow ^{12}_6\text{C}$).
• The Iron Limit: Fusing elements lighter than iron releases energy (it is exothermic). Fusing iron or elements heavier than iron requires energy (it is endothermic).
• Because fusing iron consumes energy rather than releasing it, radiation pressure fails, leading to the collapse of the red supergiant.

Supernovae & The Distribution of Elements

Where did the gold in a wedding ring or the uranium in a nuclear reactor come from? Elements heavier than iron cannot be produced during a star's stable life; they are created only during the extreme conditions of a supernova explosion.

• The supernova explosion scatters and distributes these heavy elements throughout the universe.
• Gravity pulls these enriched, scattered elements together to form new nebulae, which eventually collapse to form new stars and planetary systems.
• The presence of heavy elements like gold and uranium on Earth is scientific evidence that our solar system was formed from the remnants of previous supernovae, as the Sun is not massive enough to have created them.