When a star uses nuclear fusion as its primary source of energy, it can maintain this energy for quite some time. However, once all of the fuel is used up, the star will experience death by collapsing in on itself or by exploding.
During its life, a star will go through two processes that generate energy to help it maintain its internal thermal pressure. These two processes are primordial nucleosynthesis and stellar nucleosynthesis.
Primordial nucleosynthesis is the process that created most of the chemical elements in the early stages of the universe. Stellar nucleosynthesis is when stars, such as our sun, create new elements through nuclear fusion. These two processes combine to create all of the elements that we see today.
This article will discuss these two processes and how they contribute to maintaining a star’s internal thermal pressure.
Photon diffusion
When a star runs out of fuel, it can no longer generate energy through nuclear fusion. As a result, the star begins to collapse due to its own gravitational force.
During this process, the star will go through what is called a post-reduction Beta phase. At this point, the star will be made up of denser atoms, including heavier elements like carbon and nitrogen.
These elements are crucial in helping maintain internal thermal pressure after the end of nuclear fusion. When these atoms are mixed together in the stellar interior, they resist compression. This resistance to compression is what helps keep the star from collapsing in on itself.
Photons are a type of particle that makes up light. As this radiation is diffused, it works to slow down the process of gravitational collapse. The photons bounce around within the interior of the star, preventing it from compressing any further.
Convection
Convection is the transfer of energy via molecules moving through space. In the case of stars, convection occurs when internal thermal pressure causes the star’s plasma to internally flow, or convect.
Convection in a star depends on the size of the star and its temperature. Larger stars have higher temperatures, which allow their plasma to convect more easily. Smaller stars have lower temperatures, making it harder for them to convect.
The difference between these two is how long it takes for convection to occur. A smaller star may take longer to heat up and cool down, thus taking longer for internal convection to occur. Larger stars may have shorter periods between internal convection due to their higher temperature.
Convective cells occur in two forms: linear and spherical. Linear cells occur when internal thermal pressure pushes fluid up and down, creating a line of circulation. Spherical cells occur when internal thermal pressure pushes fluid outward, creating a sphere of circulation.
Conduction
Conduction is the transfer of energy via collisions. In the case of stars, conduction occurs when particles in a star collide with other particles.
If a star had no internal pressure, these collisions would cause the star to dissipate. However, because there is some internal thermal pressure in the star, these collisions can be slowed down.
When a particle in the star moves, it exerts force on other particles nearby it. This force is what causes the other particle to move as well. Since there are more stationary particles in the interior of a star than in its outer layers, conduction can facilitate this internal thermal pressure maintenance.
Convection occurs when a fluid (like liquid plasma) moves under or over itself due to some force. If there is enough internal thermal pressure in a star, convection will occur due to heat being expelled from the core and moving toward the surface. This returns cool plasma to the core where it can recirculate.
Radiative heat transfer
When a star uses fusion to generate energy, that energy has to go somewhere. If there were no mechanisms to transfer that energy, it would all stay within the star itself.
There are two primary ways that thermal energy can be transferred from the star to outer space. The first is radiative heat transfer.
When particles collide with one another, some of their kinetic energy can be transferred to other particles via electromagnetic radiation. This is what causes heat in our environment, and the same principle applies in space.
When a plasma particle in the star’s core collides with a particle of opposite charge, they can both lose some of their speed as electromagnetic radiation. This radiation then travels outward through the surrounding plasma, cooling it down in the process. It then enters into inter-particle collisions with other plasma particles, cooling them down as well.
Conductive heat transfer
In this process, thermal energy in a star is transferred to another body through direct contact. This can occur through gravitational attraction between a star and a planet, or a star and its surrounding dust.
In the case of a planet orbiting a star, the heat is transferred from the star to the planet through its atmosphere. The heat is then dissipated into space via wind or radiation. In both cases, the cool bodies absorb thermal energy from the other body.
Conductive heat transfer plays an important role in maintaining internal stellar pressure. This is because planets and dust in a star’s gravitational pull may contain minerals that carry thermal energy. As these bodies come into close contact with each other, some of this thermal energy is transferred between them.
Aside from planets, interstellar dust can also play a significant role in this process. When there is an abundance of interstellar dust around a star, some of its thermal energy may be transferred to the gravitational field of the star through conductive heat transfer.
Chemical reactions
In the next two sections, we’ll discuss the first and second laws of thermodynamics in more detail. We’ll also explain how these laws apply to stars, including our sun.
The first law of thermodynamics states that energy can be converted from one form into another, but it cannot be created or destroyed. This is true for all systems, including our universe as a whole.
All observable phenomena in our universe depend on energy. For instance, a temperature difference between two places in the cosmos translates into motion between those places.
This motion can be described as gravitational force, which is one type of energy. Other types of energy include electricity and chemical bonds within molecules. All of these are linked, which we’ll explain later.
Nuclear reactions
In addition to nuclear fusion, a star can also generate energy through nuclear fission. Nuclear fission is when a large atomic nucleus is split into smaller nuclei due to physical force.
A star can have both nuclear fusion and nuclear fission, depending on its mass. Low-mass stars do not have enough mass to sustain a nuclear fission process. Only nuclear fusion is present within the star’s core.
Nuclear fission is more common in Earth’s history, as radioactive atoms are present. These atoms are split through physical force, which produces energy. This is what causes radiation sickness and cancer in humans.
A star that has reached the end of its life will not have the ability to maintain nuclear fusion or nuclear fission within its core. Thus, the star will cease to generate internal thermal pressure and collapse in on itself.
