Nucleosynthesis

I

Introduction

Nucleosynthesis, process that builds chemical elements from protons and neutrons, the particles that form the nucleus of an atom. All the atoms that exist in the modern universe started from different forms of nucleosynthesis. The term nucleosynthesis comes from Greek words meaning “putting together a nucleus.”

Most nucleosynthesis can only occur under conditions of extreme temperature and pressure that overcome the mutual repulsion of atomic nuclei. The number of protons in the nucleus of an atom—its atomic number—determines which element it is. Neutrons help stabilize many nuclei and can occur in different numbers in atoms of the same element. These nuclei with differing numbers of neutrons are called isotopes.

The earliest nucleosynthesis took place for only a few minutes following the big bang that began the universe, creating nuclei of hydrogen (each hydrogen nucleus has 1 proton) and of helium (2 protons) and a tiny amount of lithium (3 protons). When the first stars formed millions of years later, a new type of nucleosynthesis began that builds up heavier elements by fusing lighter elements together in the cores of the stars. The process starts by fusing hydrogen into helium and can continue until reaching iron (26 protons). Special types of stars that explode, called supernovas, create conditions that can form elements heavier than iron up to uranium (92 protons).

Other processes in nature can also transform atomic nuclei of one chemical element into another by adding or removing protons. These processes include impacts from cosmic rays and emissions of particles in radioactive decay. Artificial elements heavier than uranium can be created in nuclear reactors and in high-energy laboratory experiments.

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II

Big Bang Nucleosynthesis

According to the big bang theory, the universe began from a tiny, extremely dense and hot energy state that expanded explosively. Some of the energy condensed into quarks that then combined to form protons and neutrons as the universe continued to cool. After one second, the protons and neutrons existed in a ratio of about 6 to 1 and conditions allowed the particles to combine as nuclei, beginning the era of big bang nucleosynthesis (BBN).

For a period of about 1,000 seconds, nuclei of the lightest elements could form everywhere in the universe. As the universe continued to rapidly cool and expand, the density dropped to a point where protons and neutrons no longer combined into nuclei. Big bang nucleosynthesis stopped.

At the end of this first period of nucleosynthesis, hydrogen (1 proton) made up close to 75 percent of the ordinary matter in the universe by mass, deuterium (hydrogen isotope with 1 proton and 1 neutron in the nucleus) about 0.001 percent, isotopes of helium (2 protons) close to 25 percent, and the isotopes of lithium (3 protons) a tiny trace. A very small amount of a beryllium (4 protons) isotope may have formed that later decayed to lithium.

Each of these nuclei had a positive charge. They could not become atoms until the universe had cooled enough that negatively charged electrons could stably combine with nuclei. Hydrogen began to form atoms about 300,000 years after the era of nucleosynthesis. The formation of the first atoms was complete about one million years after the big bang. Because electrons in atoms can absorb photons that only have particular energy levels, huge numbers of free photons in the early universe suddenly became visible as electromagnetic radiation, an event now detectable as the microwave background radiation.

III

Stellar Nucleosynthesis

The first stars formed about 100 million years after the big bang and began a new type of nucleosynthesis. The extreme temperatures and pressure in the core of stars overcame the mutual repulsion of atomic nuclei and allowed nuclei to fuse together into heavier elements.

The basic fusion process in stars combines hydrogen nuclei to form helium nuclei. In stars about the mass of the Sun or smaller, a process called the proton-proton chain combines 4 protons into 2 nuclei of deuterium (1 proton + 1 neutron). Two deuterium nuclei then combine to form an isotope of helium with 2 protons and 1 neutron, releasing energy as a gamma ray photon and leaving a free neutron.

A more complex form of hydrogen-to-helium fusion called the carbon cycle occurs in more massive stars. The carbon cycle fuses hydrogen into helium by a series of steps that require interactions with nuclei of the heavier elements carbon, nitrogen, and oxygen. However, the carbon cycle could not have begun in the earliest giant stars, which were pure hydrogen and helium. Astronomers are still uncertain about how the earliest giant stars began burning hydrogen in their cores. These stars may also have been more massive than any giant stars in the modern universe.

The fusion cycles can continue if the star is massive enough and can create enough pressure and high temperatures in its core. Helium nuclei can fuse into carbon, carbon and helium into oxygen. Other types of helium fusion processes form nitrogen and neon. Carbon fusion processes can form nuclei of sodium and magnesium. Cycles of oxygen fusion can form silicon, phosphorus, and sulfur. The most massive stars fuse elements in their cores until they reach iron (26 protons). Fusing iron nuclei takes up energy instead of releasing it. When their cores no longer release energy, the stars collapse, usually exploding as supernovas that leave behind a neutron star or a black hole.

When the first stars shed their outer layers or exploded as supernovas, they created enriched gas and dust from which the next generations of stars could form. This second generation of stars contained additional heavier elements such as carbon, nitrogen, and oxygen created by the first stars. All elements beyond hydrogen and helium are called “metals” by astronomers.

IV

Supernova Nucleosynthesis

Supernovas are stars that explode, releasing huge amounts of energy in the form of light and radiation, and spreading debris into space. There are two main types of supernova explosions. In one form, a massive star with 10 or more times the mass of the Sun reaches a stage where its core begins to form iron. Fusing iron nuclei together takes up energy instead of releasing it. The core collapses from gravitation, often forming a neutron star or a black hole. The outer layers of the star are no longer pushed back by energy from the core and start to fall onto the core. At some point the density of the in-falling gas, probably pushed by the massive outflow of subatomic particles called neutrinos from the core, causes the outer layers of the star to explode.

The other type of supernova is thought to occur when a white dwarf detonates in a thermonuclear explosion. A white dwarf is the compressed core of a star with a mass less than 1.4 times that of the Sun and that has gone through its life cycle of fusion. A white dwarf is about the size of Earth but is mainly made of carbon and oxygen atoms and is extremely dense. It is stable because the mutual repulsion of densely packed electrons (in a state called electron degeneracy) balances the inward pull of gravitation.

If the white dwarf orbits in a binary pair with another star that still is active and has a large atmosphere, extra matter from the other star can accumulate on the white dwarf. If the extra matter pushes the mass of the white dwarf over 1.4 times the mass of the Sun, gravitation overwhelms the degenerate electron state. The inward collapse causes elements in the white dwarf to fuse in a thermonuclear event. The white dwarf explodes as a supernova.

Special processes that create new elements can occur in supernovas: (1) explosive nucleosynthesis, (2) neutron-capture (slow and fast processes), and (3) neutrino processes. These processes can combine or modify the nuclei of elements that formed in the star before its catastrophic explosion.