Nucleosynthesis

Everything is made of tiny bits.

Long ago, the universe began. Tiny bits joined to make the first things. Most of the universe is made of two simple things.

These are hydrogen and helium. Later, hot stars made more things. Big explosions also made new bits. These bits make up the world around us. They are everywhere in space. Even you are made of these bits!

Everything in our world is made of tiny parts. These parts are called nuclei. The way these nuclei form is called nucleosynthesis.

It all started with the Big Bang. A few minutes after the start, the universe was very hot. Protons and neutrons joined together. This made mostly hydrogen and helium. It also made a tiny bit of lithium.

Later, the first stars formed. Inside stars, a way called fusion makes new parts. This happens when parts squeeze together. This makes elements like carbon and oxygen. Big stars can even make iron and nickel.

When huge stars explode, they are called supernovas. These explosions make many heavy elements. We also see new parts when two neutron stars hit each other.

Some parts are made in other ways too. Cosmic rays can hit nuclei and break them into smaller bits. This is called spallation. On Earth, some parts form from the decay of old atoms. This is called radiogenesis. All these steps make the many elements we see today.

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.

Nucleosynthesis is the fundamental process that creates new atomic nuclei from nucleons, which are protons and neutrons. This process explains how the universe transformed from a simple collection of particles into a complex cosmos filled with diverse elements. By studying nucleosynthesis, scientists can understand the origins of everything from the hydrogen in stars to the heavy metals found on Earth.

This field of study allows us to trace the chemical history of the entire universe.

The process begins with Big Bang nucleosynthesis, or BBN. This occurred just a few minutes after the universe began. Around this time, the universe cooled below ten billion Kelvin. As it cooled, protons and neutrons began to combine. This created mostly hydrogen and helium nuclei, along with traces of lithium and deuterium, a hydrogen isotope.

After about 20 minutes, the universe expanded and cooled so much that these collisions ended. This left the universe with a composition that is still mostly hydrogen and helium today.

As the universe aged, stars began to form through a process called stellar nucleosynthesis. These first stars, known as Population III stars, formed a few hundred million years after the Big Bang. They were made almost entirely of the hydrogen and helium produced during the Big Bang. Inside these stars, extreme heat and pressure trigger nuclear fusion. Fusion is a process where light elements are squeezed together to create heavier ones. In massive stars, this chain of reactions includes hydrogen burning, helium burning, carbon burning, neon burning, oxygen burning, and silicon burning. This sequence can create elements up to and including iron and nickel.

Elements heavier than iron require even more energetic environments. One way this happens is through neutron capture reactions. In the s-process, or slow neutron capture, elements are built up steadily. In the r-process, or rapid neutron capture, nuclei absorb many neutrons very quickly. This r-process often occurs during supernova nucleosynthesis, which happens when a massive star explodes. Supernova explosions are largely responsible for creating elements between oxygen and rubidium.

Another major source of heavy elements is neutron star mergers. When two incredibly dense neutron stars collide, they eject significant amounts of matter that quickly forms heavy elements like europium.

There are also unique ways that lighter elements are produced. Cosmic ray spallation is a process where high-energy cosmic rays hit nuclei and fragment them into smaller pieces. This is a significant source of light nuclei like beryllium, boron, and certain isotopes of lithium and helium. These fragments can be created in the interstellar medium, on asteroids, or even in Earth's atmosphere. On our own planet, new nuclei are also produced through radiogenesis. This is the process where unstable, primordial radionuclides like uranium, thorium, and potassium decay into new elements.

The history of this science is a journey of connecting different theories. In 1920, Arthur Stanley Eddington suggested that stars produce energy by fusing hydrogen into helium. Later, Fred Hoyle did groundbreaking work on how heavier elements form in stars. He used the physics of plutonium fission to understand how supernovae could produce elements in the same proportions found on Earth. In 1957, the famous "B2FH" review paper by Burbidge, Burbidge, Fowler, and Hoyle summarized these processes. This paper allowed astronomers to document and track how different nuclei transform within stars.

Understanding element abundances provides a way to measure the history of galaxies. For example, most magnesium in the interstellar medium comes from core-collapse supernovae, which involve the deaths of massive stars. These stars live for only a few million years. In contrast, much of the universe's iron comes from Type Ia supernovae. These occur when a white dwarf in a binary system pulls material from a companion star until it explodes. Because these different events happen at different timescales, the ratio of elements like magnesium to iron can tell astronomers how old a star or a galactic structure is.

This connection links the tiny scale of the atom to the massive scale of cosmic evolution.