Stellar Nucleosynthesis and Its Impact on Cosmic Chemistry

Science Holic
May 31
4 min read

Author: Kayla Otoo

Editors: Justin Tai, Miriam Heikal

Artist: Alicia Chen

Years ago, physicist Arthur Eddington expanded on renowned American astronomer Carl Sagan’s slogan “We are made of star-stuff” by saying “every atom in your body was once inside a star that exploded...The atoms in your left hand probably came from a different star those in your right hand, because 200 million stars have exploded to make up the atoms in your body.” Every one of the countless elements inside our bodies exists because of these explosions, often called supernovae. At the end of a star's life, it will explode and leave behind only a gas, a black hole, or a nebula. However, such explosions are not limited to stars. The world’s current state results from an even bigger explosion that occurred in our universe roughly 13.7 billion years ago, a widely accepted theory called the Big Bang Theory. It states that the cosmos began with an infinitely small and hot piece of dense matter that has been expanding for over 13.7 billion years, even until today. However, the elements that resulted from this event are often not discussed. Not only is the Big Bang Theory an event that potentially contributed to our universe today, but it is also an instance of nucleosynthesis. Nucleosynthesis is the process where elements are forged through nuclear reactions within stars. Thus, the formation of our universe can be viewed at the molecular level, where key components are seen with purpose. Nucleosynthesis can be broken down into three different stellar stages: main sequence, Red-Giant Phase, and Supernova Phase, which each impacts cosmic chemistry.

The formation of elements from nucleosynthesis can be thought of as a spectrum. One side of the spectrum contains elements from hydrogen to iron, which are produced through fusion in stars. Conversely, elements heavier than iron result from the vast majority of stars that end their life with an explosion known as a supernova, through the neutron capture process including the s and r phases. In the s-process, neutrons are captured slowly, whereas in the r-process, neutrons are captured rapidly, oftentimes in an explosive environment such as a supernova. These two phases contribute to the formation of heavy elements beyond iron. However, before discussing the formation of heavy elements, we must first explore the beginning of this process.

The first stage of nucleosynthesis is known as its main sequence. In this sequence, hydrogen within dense stellar nurseries in the cosmos will fuse into helium. This occurs through a proton-proton chain, where two proton atoms converge, producing an isotope of hydrogen: deuterium. Consequently, an additional proton will walk in, fuse with the deuterium, and eventually fuse with another proton, creating an isotope of helium known as helium-3. This progression will continue until Helium 4 is created. This will then release a gamma ray photon with immense energy, illuminating our cosmos. Hydrogen fusion is often considered a cosmic paradox because although hydrogen atoms are depleted during fission, the result is a star’s beautiful luminosity and brilliance in our cosmos.

This process, however, does not end with hydrogen. Afterwards is the Red Giant phase, which uses a process known as the triple-alpha process, where helium nuclei fuse, forming carbon-nuclei. This process, conversely, relies on the size of a star. Three helium nuclei, each arranged with its own intricacy, will merge to form a carbon nucleus; additionally, helium may fuse into oxygen.

With time, helium burning will lead to tremendous rises in stars' internal temperatures, pressure, and mass loss, causing elements heavier than iron to form. This final phase is known as the supernovae phase, where a star reaches its end. This releases an astronomical amount of energy into the universe, which may lead to the creation of other elements that surpass iron, such as gold, uranium, and plutonium. During a supernova, the outer layers of a star will be launched into space at extreme velocities, and the core will collapse under the influence of gravity. Ultimately, all that is left is either a neutron star or a black hole. Nucleosynthesis occurs after this event, taking place in the supernova's core, where neutron stars will continue to host this extraordinary process.

Nucleosynthesis is an important area of study in astrophysics, teaching both the life cycle of stars and the formation of different elements. In addition, nucleosynthesis is a topic included in many advanced studies involving complex nuclear reaction networks as well as differential equations to capture specific reaction rates and processes of decay. However, a common misconception is that the elements produced from stellar nucleosynthesis are not separate processes; in reality, they are interwoven and interconnected. The star's core has a metamorphic property that gives rise to the formation of other elements such as neon, oxygen, and magnesium, all through an alpha process when burned by helium. Thus, a stellar core with substantial elemental diversity is formed. After all, we are all made of stardust.

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