Editorial Feature

Stellar Evolution and Nucleosynthesis: Unraveling the Life Cycles of Stars

Stars serve as laboratories for the nuclear reactions that control their fundamental behavior within the vastness of the cosmos. A vital astronomical process called nucleosynthesis shapes the development of chemical elements within stars during their life cycle, dramatically affecting the universe's composition.

Stellar Evolution and Nucleosynthesis, Stellar Evolution, Life Cycles of Stars

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Nuclear processes take place in the enthralling environment of stars, which are characterized by extremely high temperatures and pressures. Massive amounts of energy are released during these high-energy fusion processes, which give stars their brilliance and preserve their delicate hydrostatic equilibrium. The fundamental astrophysical phenomenon of nucleosynthesis emerges as a crucial mechanism sustaining cosmic evolution and the diversity of elements in the universe formed during the life cycle of stars.

Any star's life cycle, including all of the stages from birth to death, will span billions of years.

A star's mass, or how much gas gathered and collapsed to form the star, determines the life cycle of the star's trajectory primarily because that gas will act as the star's fuel.

When nuclei collide with enough energy to overcome electromagnetic repulsion,  the powerful nuclear force fuses with a fraction of their mass, converting it into enormous volumes of energy, governed by the famous formula E = mc2.

Therefore, only by fusing their nuclei together at its extremely hot core can a star release sufficient energy to counteract the consequences of gravity's constant pull radially inward.

This implies that the amount of fuel depends on the amount of mass that makes up the star. The life cycle of a star and its ultimate fate is dependent on its mass.

Small-Mass Stars

Small mass stars are stars with the least amount of matter that can initiate nuclear fusion sufficiently to be considered a star. Any star will start as a cloud of gas spread across at least a few light-years

When stars first began to develop, this substance was entirely hydrogen and helium, as after the seventeen minutes of nucleosynthesis following the Big Bang, this was what was left.

Due to gravity, this mass gathers, contracting and pushing inward. Over a few million years, conditions became so hot that nuclear fusion eventually started, establishing an equilibrium resulting in a yellow or red star that glows due to the energy released as a result of internal collisions.

Two protons fuse at the beginning of the fusion process. This is followed by beta decay to produce a deuteron, a heavy hydrogen atom nucleus, from a proton and a neutron. Then helium, which has two protons and two neutrons, is created via reactions involving deuterons.

For billions of years, such a star will continue in this fashion, gradually fusing until most of its core converts the hydrogen into helium while keeping its size, luminosity, and temperature constant.

At this phase, the leftover hydrogen will burn even more intensely when the star's core contracts and warms up. All the additional energy being produced will radiate outward and accelerate, pushing the outer layer away from the core.

The star grows brighter and redder as the outer layers expand and consequently cool. For another billion years, the star ascends into a red giant star. The core, however, gets smaller and hotter after nearly all of the hydrogen has been consumed.

At this point, with the temperature soaring, a phase known as helium flash occurs. During helium flash, larger nuclei like carbon and, subsequently, oxygen are created by fusing heavier helium nuclei. This process is called the triple-alpha process, which signifies that the star has a new source of fuel from all the helium it has produced over the course of billions of years.

As the star exhausts its remaining energy, it starts to pulse and shrink in size while heating up further and turning bluer until a considerable portion of the helium has become heavier nuclei through fusion.

Once the core is primarily composed of carbon and oxygen with only a thin helium shell surrounding it with another shell of hydrogen around that, the star has little material left to burn. Subsequently, the core collapses. At this time, the star grows quickly and transforms back into a big star, ejecting the outer layer with the last bursts of energy, forcing it back into the interstellar medium and away from the core. Leaving behind nothing but a small, extremely hot, naked core that is roughly the size of Earth.

As there is no fuel left to burn, this will progressively cool enough and will not have the means for fusion. It will continue to contract until it becomes a white dwarf star, which will contain carbon or oxygen nuclei.

The ejected shell is referred to as a planetary nebula.

A planetary nebula's substance will thereafter combine with new gas particles to create a new life cycle for a new star.

Stars with High Mass

The life cycle is very different for high-mass stars, like those substantially more massive than the sun.

A gas cloud gathers under the pull of gravity to begin the formation. This cloud will be significantly larger than those that give rise to low-mass stars. It will, therefore, have a much larger mass. Gravity increases with mass; therefore, the force pulling downward is considerably stronger. The star becomes much hotter. Faster fusion occurs at higher temperatures, which increases the external pressure to offset gravity's stronger pull inward.

This produces a hot, large, bright, and blue star. Here, things start to diverge from what happens in low-mass stars.

High-mass stars burn up their fuel far faster than low-mass stars, which require billions of years. They exhaust all of the hydrogen in their cores in about 10 to 100 million years depending on size.

As the fuel begins to deplete, the star's core shrinks and heats up, generating additional energy. Similar to low-mass stars.

However, as a high-mass star continues to compress, its core becomes much hotter than the surface, developing the ability to fuse helium nuclei to create carbon and oxygen but also silicon and neon. Each nucleus forms a concentric shell from the core radially outwards from the heaviest nuclei.

Iron, the heaviest element that can fuse within a star, is located right in the center. Once all the fuel is exhausted, fusion energy production ceases. The star now collapses in less than a second under the force of gravity. The outer layers hit the core and bounce back, causing an explosion spewing all the heavy nuclei the star has produced out into space. This occurrence, one of the universe's most violent and explosive events, is known as a supernova.

In this brief instant, a supernova produces such an incredible surge of energy that numerous elements that are heavier than iron can also be created. The death of a massive star thus triggers nuclear synthesis that produces all the heavier elements found in the periodic table.

Outlook

The nucleosynthesis process has had a significant impact on the life cycle of stars and the dynamics of the cosmos, as evidenced by the complex symphony of nuclear reactions found inside star interiors. Each stage, from the initial fusion of hydrogen to the breathtaking nucleosynthesis beyond supernovae, has a profound impact on the development of stars and the origin of the atoms that make up the cosmos. In order to understand the cosmos and where mankind fits into it, nucleosynthesis must be further explored. Nucleosynthesis continues to be a key driver of astrophysical phenomena.

More from AZoQuantum: James Webb Telescope Makes Exciting Discovery in K2-18b Atmosphere

References and Further Reading

Hall, I. (31 July 2023) Nuclear Reactions in Stellar Interiors and Nucleosynthesis.[Online] TheAverageScientist. Available at: https://theaveragescientist.co.uk/2023/07/31/nuclear-reactions-in-stellar-interiors-and-the-complexity-of-nucleosynthesis/

Ekström S (2021) Massive Star Modeling and Nucleosynthesis. Front. Astron. Space Sci. 8:617765. doi: 10.3389/fspas.2021.617765

ASTRONOMY BASICS. (25 April 2023) A Cosmic Journey: Exploring the Life Cycle of Stars.[Online] Universe Unriddled. Available at: https://universeunriddled.com/post/the-life-cycle-of-stars

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Written by

Ilamaran Sivarajah

Ilamaran Sivarajah is an experimental atomic/molecular/optical physicist by training who works at the interface of quantum technology and business development.

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