In the universe, stars are not only the source of light, but also the mysterious furnace of elemental formation. From the simplest of hydrogen to the complex of iron, nuclear fusion reactions inside stars are constantly playing out a cosmic version of alchemy.
In the core of the star, in a high-temperature and high-pressure environment, hydrogen fuses to form helium and releases a huge amount of energy. These energies not only maintain the brilliance of the stars, but also provide the impetus for the further evolution of the elements. Subsequently, helium continues to fuse under extreme conditions, producing heavier elements such as carbon and oxygen. This series of nuclear reactions is like a chain of chemical reactions that are constantly going on in the universe, with one link closely linked to the next.
Not to be outdone, carbon is also an element whose fusion process can produce a variety of elements such as neon, sodium, magnesium and aluminum. Among these elements, the fusion of silicon is particularly critical because it ultimately produces iron. The formation of iron marks a milestone in nuclear fusion, as iron has the highest specific binding energy, making it the end point of fusion reactions within stars.
However, iron is not the end of the element's evolution. How are elements heavier than iron in the universe formed? This involves more complex and rare astrophysical processes, such as supernova explosions and binary neutron star mergers, which make it possible to produce superheavy elements.
Iron: The cornerstone of the stability of the universe
After learning about the formation of elements inside stars, we can't help but ask, why is iron so special and the terminator of fusion reactions? This is thanks to an important physical concept – specific binding energy.
Specific binding energy is a key measure of the stability of atomic nuclei. It represents the energy provided to completely separate the nucleons (protons and neutrons) in the nucleus. The magnitude of the specific binding energy is directly related to the stability of the nucleus, and the greater the specific binding energy, the more stable the nucleus. In the periodic table, the specific binding energy of iron-56 is the largest, which means that iron-56 is the most stable of all atoms. Thus, iron plays the role of a terminator in nuclear fusion reactions inside stars.
It is because of the high stability of iron-56 that the fusion process stops when the nuclear fusion reaction inside the star progresses to the element iron. This is because to further fuse iron into heavier elements, the energy required will exceed the energy released by the reaction, which is not possible under the law of conservation of energy. Therefore, iron and its subsequent superheavy elements cannot be produced in the interior of stars through ordinary nuclear fusion reactions, and their formation must involve more exotic astrophysical processes.
Stellar Evolution: The cosmic crucible of the elements
Stars of different masses have very different fates in their life processes and element formations. For low-mass stars, such as red dwarfs with a mass of less than 0.8 suns, their internal temperature and pressure are not enough to ignite the fusion reaction of helium. Therefore, the nuclear fusion reaction of such stars in their lifetime mainly stays at the stage of hydrogen fusion into helium.
Mesomass stars such as the Sun can cause the fusion of helium through gravitational contraction after hydrogen is depleted, resulting in the formation of heavier elements such as carbon and oxygen. However, the sun's fusion reaction can only proceed to carbon and oxygen, and cannot further produce iron. Only massive stars with a mass of at least 10 times the mass of the Sun can continue their nuclear fusion reactions until iron is formed.
At the end of the evolution of these massive stars, the large amount of iron accumulated in the interior and the presence of a high density of neutron streams provided the conditions for the generation of superheavy elements. Through the neutron capture process, iron can continuously absorb neutrons and transform into heavier elements. This process can occur both in the late stages of a star's life and in dramatic events such as supernova explosions or binary neutron star mergers, which sow heavy elements in the universe.
Interstellar Forge: The birth of the super-heavy element
The formation of superheavy elements is a miracle in the physics of the universe. Inside the star, since iron-56 has the highest specific binding energy, any attempt to fuse it further will run into an energy barrier. However, at the end of the evolution of massive stars, as well as during events such as supernova explosions and binary neutron star mergers, the neutron trapping process provides the possibility for the formation of superheavy elements.
The neutron capture process is divided into slow neutron capture process (S process) and fast neutron capture process (R process).
In the slow neutron capture process, the rate of the capture reaction is slower due to the lower neutron density, which limits the amount of superheavy elements produced. On the contrary, in the process of fast neutron capture, due to the high-density neutron flow generated by supernova explosions or binary neutron star merger events, the neutron capture reaction proceeds rapidly, and a large number of superheavy elements are generated.
Specifically, when a massive star runs out of nuclear fuel in its core, its interior collapses to form a dense neutron star or black hole, and in the process, the material outside the star explodes violently, which is called a supernova explosion. In such an explosion, the high temperature and high density conditions allow iron to quickly trap neutrons, forming a series of elements heavier than iron. A similar effect can be achieved in binary neutron star merging events, where neutron star matter quickly captures neutrons after merging, resulting in the formation of superheavy elements.
These superheavy elements, formed through the neutron capture process, play a crucial role in the chemical evolution of the universe. They are released into space after the death of stars and become the raw materials that make up a new generation of star and planetary systems.
Cosmic Elemental Atlas: The Legacy of Stars
The abundance of elements in the universe is an important clue to understand the history of chemical evolution in the universe. After the Big Bang, there were mainly two elements in the universe, hydrogen and helium, which accounted for the vast majority of the total mass of the universe. Over time, stars begin to form and undergo nuclear fusion reactions, gradually producing heavier elements such as carbon, oxygen, silicon, and iron.
Stellar nucleosynthesis mainly contributes to the formation of light elements in the universe and plays a decisive role in events such as the generation of heavy elements, supernova explosions, and binary neutron star mergers. The heavy elements formed during these events are released into interstellar space and become the material basis for the construction of a new generation of star and planetary systems. Therefore, the abundance of elements that we observe on Earth today is actually the result of a combination of astrophysical processes in the history of the universe.
The Earth Element: A Witness to Interstellar Evolution
The planet is an astonishing diversity of elements, from the lightest hydrogen to the heaviest uranium, a total of 92 elements that make up the material world as we know it. However, the heaviest element found in abundance in nature is uranium, and transuranic elements that surpass uranium are extremely rare in nature, and most of them can only be synthesized artificially in the laboratory.
The synthesis of these transuranic elements is actually a legacy of extreme celestial events in the universe. In supernova explosions and binary neutron star mergers, large amounts of superheavy elements are produced due to extreme physical conditions. These elements wander through the universe and eventually become part of the elements on planets such as Earth. This allows each of us to be directly or indirectly connected to the events of stars and black holes in the distant universe.
The Human Element: The Microscopic Imprint of the Universe
The human body contains a variety of elements, some of which are essential for sustaining vital activities. For example, copper is found in muscles and bones, arsenic is found in hair and skin, and selenium is found in heart muscle and skeletal muscle. Although these elements are present in very small quantities in the human body, their presence is a necessary condition for life activities.
The astronomical origins of these elements can be traced back billions of years to supernova explosions and binary neutron star merger events. In those cosmic explosions, these elements were created, and over time, they were sucked into the solar system and became part of the Earth, which in turn formed the basis of the human body. Therefore, every atom in our body is a witness to the evolutionary history of the universe, and carries the memories and stories of the universe. This is undoubtedly the most romantic and poetic aspect of science, making us deeply feel our own close connection to the universe.