Nucleosynthesis

Everything in our universe is made of tiny building blocks. These are called nuclei, and the way they form is called nucleosynthesis.

This process explains why we have different elements like oxygen, carbon, and iron. Without nucleosynthesis, the universe would be very simple and empty. It is the reason why stars shine and why planets can exist. Understanding this helps us see how the universe grew from nothing into a place full of variety.

Nucleosynthesis happens in several different ways. It began with Big Bang nucleosynthesis just minutes after the universe started. During this time, protons and neutrons joined together to make mostly hydrogen and helium. Later, stars began to form and used a process called stellar nucleosynthesis. Inside a star's core, heat and pressure cause fusion. Fusion is when light elements are squeezed together to make heavier ones.

Massive stars can fuse elements all the way up to iron and nickel. When stars explode as supernovas, they create even heavier elements through explosive nucleosynthesis. Other heavy elements come from neutron star mergers, which happen when two tiny, dense stars collide.

Scientists have spent a long time figuring out how this works. In 1920, Arthur Stanley Eddington suggested that stars make energy by fusing hydrogen into helium. This idea was not accepted right away because the math was hard. Later, Fred Hoyle did important work on how heavier elements form in stars. He even showed how exploding supernovas could create elements like we see on Earth. In 1957, a famous paper called "B2FH" summarized many of these ideas. This paper helped astronomers track how different elements are made in different places.

There are many specific facts about these elements. About 74% of the matter in the universe is hydrogen from the Big Bang. Another 24% is helium. Most other elements are called "metals" by astronomers, even though they are not all solid metals like iron. The first stars were called Population III stars because they were made almost entirely of hydrogen and helium. These stars formed a few hundred million years after the Big Bang. Some elements, like lithium, were made very early on. Other elements like beryllium and boron are often made by cosmic ray spallation. This is when cosmic rays hit nuclei and break them into smaller pieces.

We can see the results of nucleosynthesis all around us. The atoms in your body were once inside a star or part of an explosion. The iron in your blood was likely made in a massive star or a supernova. Even on Earth, we see nucleosynthesis through radiogenesis. This is when old atoms like uranium or thorium decay into new ones. By looking at the amounts of different elements, astronomers can even tell how old a star is. This shows us that we are all connected to the history of the stars and the Big Bang.