Astro2110 – The Solar System

How exactly do stars live?

Above is an image of the sun given by SOHO, which is an extraordinarily massive object, which due to that possesses a very large gravitational pull, not just on others but also on itself. Everything that has mass possesses this trait, however these bodies are either two light, or are rigidity enough to counteract this force. However the sun and stars as whole are different, on top of being massive, they are composed of plasma, which is susceptible to deformation when interacted with. So how exactly does these stellar objects not fall into themselves? The reason why is because these bodies are in a constant state of hydrostatic equilibrium.

In the prior paragraph the fluidity of stars was a key issue when involving gravitational force, however this property actually allows the formation of pressure gradient. Imagining the sun as shells of plasma, when gravity forces these shells pushed onto each other, the layers exert a force onto each other as well, one upwards and one downward. As the star contracts these layers are further pushed against one another increasing this inertial force until the upward force is equal to gravity. Repeat this process for as many layers the sun has and a stable sphere is made.

A byproduct of this equilibrium is that since plasma is like a gas, pressure possesses a direct relationship to temperature, which also has direct relationship to energy. So the deeper into the sun you go, the greater the temperature and thus the greater the energy, and at high energy the particles in the sun (Hydrogen and Helium), begin to collide with each other forming bonds. This process is known as nuclear fusion, and is the cause of a stars brightness. The proton-proton chain is a fairly common example of fusion, but what if a star were to be really heavy, or low in hydrogen? This leads to two alternative types of fusion, otherwise known as the CNO cycle and Triple Alpha process.

During the proton proton chain, a star slowly depletes its entire supply of hydrogen. With nothing left to burn and keep up the pressure the core collapses onto itself, heating up the upper layers causing them to expand. Overtime this upper layer starts cooling, so only the now helium predominate (By mass) core is able to undergo fusion. The name of this type of fusion is the Triple Alpha process. In this reaction He-4’s collide with each other forming Be-8 and some gamma rays, this intermediate very quickly decays into C-12 and some He-4. This reaction is far less efficient than the p-p chain producing 1/4 of the amount of energy per cycle, which further cools the star to a red color. This is the type of fusion a red giant undergoes.

Now if a star still has an abundance of heated hydrogen ions a new pathway emerges. First, the more massive the star, the greater the total pressure gradient must be; thus the core temperature and energy must be greater. When a particular star is at least 50% heavier than the sun it is now able to fuse C-12 with H-1, which initiates a chain which starts at N-13, and through fusion and decaying goes through Carbon, Nitrogen and Oxygen. As seen below, initially the CNO cycle may not be as efficient as the p-p chain, however the former possesses a 17th power temperature dependence enabling enormous energy production at high temperatures.

Eventually all of these processes end, and without any energy to keep up the gradient, the star collapses onto itself leading to a wide variety of outcomes. Though that will not happen any time soon, and for now our sun and by extension all stars will keep themselves in equilibrium.

• Noah Herrero