This blog post explores how elements are created within the evolution of the universe and stars, and what impact they have on life and planetary formation.
All things in the universe are composed of elements, with approximately 100 known types to date. These elements serve as the fundamental building blocks of all cosmic matter, playing diverse roles from the birth of life to the formation of planets and stars. While it’s commonly assumed these elements existed since the universe’s inception, their origins actually vary, providing crucial clues to understanding cosmic history. Elements can be created during the birth of the universe, during the evolution of stars, or when stars explode. Understanding the origins of elements thus becomes the key to unraveling the complex process of cosmic evolution.
The Big Bang, the massive explosion that occurred when the universe was created, produced hydrogen and some helium. The universe immediately after the Big Bang was extremely hot and dense, allowing atomic nuclei to form rapidly. Hydrogen and helium were the first elements created during the universe’s initial formation phase, serving as fundamental building blocks for its basic structure. These elements provided the foundation for the development of more complex elements as the universe expanded, temperatures dropped, matter condensed, and eventually the first stars formed.
The remaining helium and other elements were created during the evolution of stars. Stars use hydrogen as their primary fuel to initiate nuclear fusion reactions, a process that gives birth to various elements. In the early stages of forming stars more massive than the Sun, helium is created from hydrogen, requiring extremely high temperatures exceeding ten million degrees (10⁷ K). The reaction where atomic nuclei fuse into larger nuclei at high temperatures is called nuclear fusion. After the stage where hydrogen undergoes fusion to form helium ends, when the core temperature of the star rises to around 100 million degrees (10⁸ K), elements heavier than helium begin to form. In this process, three helium nuclei fuse to form carbon, and adding one more helium nucleus to carbon creates oxygen. These elements form the basis of the diverse materials found in the universe and act as essential components for life to exist.
When the core temperature exceeds one billion degrees (10⁹K), carbon and oxygen break down into smaller elements and recombine to form heavier elements like magnesium, silicon, and sulfur. These newly created elements are ejected into the star’s outer layers, eventually dispersing throughout the universe and contributing to the birth of new stars and planets. However, elements heavier than iron cannot be created through simple nuclear fusion alone.
None of the currently existing elements heavier than iron are formed solely by nuclear fusion. Even if elements heavier than iron are temporarily created under the conditions where iron is formed, they quickly decay back into stable iron. This is because iron has the highest nucleon binding energy among all elements. Nucleons are protons or neutrons, and their binding energy refers to the energy required to remove a nucleon from the nucleus. For elements lighter than iron, nucleon binding energy increases with mass. However, for elements heavier than iron, nucleon binding energy decreases with mass. Consequently, iron occupies the most energetically stable point in the atomic nucleus, requiring special conditions for heavier elements to form within stars.
Elements heavier than iron are explained as being created when stars explode. As more iron is produced through nuclear fusion, the star contracts. As this contraction towards the star’s core progresses, the temperature rises until it reaches a critical point, causing the star to explode. This explosion is called a supernova, and during this process, the star’s material is ejected into space. Elements heavier than iron are created instantaneously when high-density protons and neutrons, generated during the star’s explosion, combine with previously formed elements. Elements like radium and uranium are created this way. Supernova explosions are crucial events driving the chemical evolution of the universe, continuously supplying new elements throughout space.
Ultimately, all elements except hydrogen and some helium can be considered remnants of stars that existed long ago. Every element present in the universe is a trace left by past stars, and also a seed enabling the birth of new stars and planets. These elements continuously recycle throughout cosmic history, forming the foundation of diverse matter, including life itself. Furthermore, elements can be seen as having a birth order within the universe. This suggests that the universe’s evolutionary process is based on complex yet systematic principles. Understanding this allows humanity to explore the origins of the universe and ourselves more deeply.
