Origins of Elements: Big Bang, Stars, and Supernovae

An element is defined by the number of protons in its nucleus; this number determines the element's identity.
Examples:
- $1$ proton → Hydrogen
- $8$ protons → Oxygen
- $29$ protons → Copper
Change the number of protons, you change the element and its properties (e.g., Hydrogen vs Oxygen vs Gold vs Carbon).
An element is a type of atom with a specific proton count.

Hydrogen is the simplest atom: $1$ proton and $1$ electron.
Two hydrogen atoms can come near each other and share electrons to form $ext{H}_2$ (a covalent bond).
This sharing creates molecular hydrogen, not yet a new element.

In the early universe (~ $13.77\times 10^9$ ) years ago, the cosmos was mostly hydrogen and helium; today it’s about $93\%$ hydrogen and $7\%$ helium, with trace amounts of lithium/beryllium.
Big Bang nucleosynthesis (BBN) produced the first synthesis of light elements: $ext{H} + ext{H} \rightarrow \text{He}$ (simplified description from the lecture).
No heavier elements (like carbon, oxygen) were formed yet in the initial era.

After the Big Bang, hydrogen and helium formed diffuse gas clouds (interstellar nebulae) held together by gravity.
Gravity pulls gas together: bigger gas clouds gain more mass, which strengthens gravity, attracting even more gas.
Gravity is the key driver of structure formation in the universe and helps pull material into stars.

When gas clouds grow large enough, their cores reach temperatures and pressures that enable nuclear fusion.
A star balances two opposing forces: gravity pulling inward and outward pressure from fusion pushing outward (hydrostatic equilibrium).
Inside stars, hydrogen fuses to helium, releasing energy.
Stars build heavier elements through successive fusion: He → C, O, and so on, up to Fe (iron, $Z=26$ protons).
This stellar nucleosynthesis gradually creates heavier elements inside stars.

Fusion in stars continues up to iron ( $Z=26$ ). Beyond iron, fusion no longer yields net energy.
When a massive star exhausts its fuel and iron builds up, the star can no longer support itself against gravity, leading toward catastrophic ends.

A supernova occurs when a massive star collapses and explodes, releasing enormous energy.
During a supernova, elements heavier than iron are formed and scattered into the surrounding nebula.
The ejected material enriches the interstellar medium, seeding future generations of stars and planets with heavy elements (e.g., gold, platinum).

Post-supernova nebulae become the birthplaces of new stars and planets.
Much of the matter in Earth and life is formed in stars or during supernovae; we are effectively "stardust".

Big Bang nucleosynthesis: H + H → He in the early universe.
Stellar nucleosynthesis: fusion inside stars builds elements up to iron.
Supernova nucleosynthesis: elements heavier than iron formed in supernovae and dispersed into space.

Gravity is the attraction between mass/energy; in simple terms, it brings matter together to form clouds, stars, and planets.
In more advanced terms, gravity can be described as the bending of spacetime by mass, influencing motion.