nucleosynthesis

Hydrogen makes up approximately 74% of all ordinary matter in the universe by mass — about 92% of all atoms. Helium accounts for roughly 24% of ordinary mass. Everything else — every carbon atom in every living cell, every silicon atom in every rock, every iron atom in every star's core — adds up to approximately 2% of the universe's total atomic mass. The dominance of hydrogen and helium is not a coincidence or a geological quirk. It is a direct record of the conditions that existed in the first three minutes after the Big Bang. Understanding why the cosmos is almost entirely these two lightest elements tells you how the universe was born and how it will eventually end.

Most of the carbon in your body was not made in a supernova. It was made in the bloated outer layers of a dying star on the asymptotic giant branch — a phase of stellar evolution so chemically productive that a single AGB star can enrich the surrounding interstellar medium with more carbon than a thousand supernovae. These stars pulse, dredge up nuclear-processed material from their interiors, and blow it into space in slow, dense winds that eventually become planetary nebulae.

Cassiopeia A is the expanding debris field of a star that exploded around 1680 CE — yet almost no one on Earth recorded it. Today, X-ray and infrared telescopes reveal a forensic masterpiece: shock waves, newly minted heavy elements still glowing from the blast, and a neutron star cooling at the center. Cas A is not just a beautiful object. It is the best laboratory we have for understanding what supernovae actually do to the universe.

Nucleosynthesis is the process by which atomic nuclei are built from simpler components. Hydrogen and helium were made in the first three minutes after the Big Bang. Every heavier element was forged inside stars or in their violent deaths. Much of the gold and platinum is made in neutron star mergers — cosmic events detected as gravitational waves.

Stars don't burn out quietly. Depending on their mass, they end their lives as cooling embers, supernova explosions, neutron stars, or black holes. Here's what actually happens.

A supernova outshines an entire galaxy for weeks. It also forges every element heavier than iron — the calcium in your bones, the iodine in your thyroid, the gold in jewellery. Here's how they work.