Summary
Highlights
Gravity caused hydrogen particles to coalesce, increasing their energy and temperature, eventually forming the first stars as hydrogen became plasma. Inside these stars, nuclear fusion began, converting hydrogen into helium. This process releases energy, balancing gravitational forces and allowing the star to maintain its size for billions of years.
Once a star's core depletes its hydrogen, fusion stops, and gravity causes the star to contract, increasing temperature further. This allows helium to fuse, creating heavier elements. Depending on the star's size and heat, fusion can produce elements up to iron (26 protons). Once iron is formed, the fusion process ceases.
For smaller stars, after fusion stops, gravitational collapse is halted by the Pauli Exclusion Principle, which prevents electrons from occupying the same energy state. These stars become white dwarfs, glowing for billions of years before cooling down and fading.
In bigger stars, gravity overcomes the Pauli Exclusion Principle, leading to a catastrophic supernova explosion. During a supernova, elements up to uranium are manufactured. The star's core continues to collapse, crushing atoms, and forcing protons and electrons to form neutrons, resulting in a super-dense neutron star, which is incredibly small and dense.
If a star is massive enough, gravity can even overcome the neutron Pauli Exclusion Principle, causing the star to contract indefinitely into a black hole—a singularity with no spatial dimensions. The remnants of supernovae, however, are dispersed into space, where gravity eventually draws them together to form new stars, continuing the cycle.
The universe, created 13.7 billion years ago by the Big Bang, initially consisted almost entirely of hydrogen and a small amount of helium, the first two elements on the periodic table.