The Nuclear Reactions In Stars

Stellar Astrophysics is one of the most active branch of Astronomy. It involves a lot of Physics. Subjects like nuclear physics, statistical mechanics, thermodynamics, electrodynamics and particle physics are crucial to understand stellar evolution deeply. Today, let us learn about the nuclear reactions in stars. I have tried to put this article in the simplest words and I really want you to understand this important branch of Astrophysics that isn't taught by pop science.

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Nuclear Reactions In Stars

Before we begin studying the nuclear reactions in stars, let us understand the basic elementary structure of the Universe. The Universe is made up of two major elements: hydrogen and helium. Stars form when huge clouds of dust and gas collapse under their own gravity. These clouds are also made of hydrogen and helium. In Astrophysics, in contrast to the chemical convention, every element except hydrogen and helium is termed as metal. So in Astronomy, non metals such as carbon, nitrogen, oxygen etc are all called metals. This is just a convention due to the relative abundance of the first two elements. Now stars begin their life with fusion of hydrogen. In this article we will be just studying the reactions without caring much about the evolution of stars. The next article will give a detailed account of the same.

How are the elements formed?

Hydrogen Fusion

Hydrogen fusion is the fundamental nuclear reaction in stars. From Hertzsprung Russell Diagram, we know that any star that is fusing hydrogen in its core is known as a main sequence star. Our Sun is a main sequence star. The two most prominent reactions that fuse hydrogen into helium are: PP Chain and CNO Cycle.

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PP Chain

PP Chain stands for Proton-Proton chain. In this reaction, 4 hydrogen nuclei combine to form 1 helium nucleus as shown below.

Nuclear Reactions In Stars - PP Chain
The PP Chain

Two protons come together and form a deuterium nucleus (one proton and one neutron). This is a two step process. First two protons combine to form a diproton. Then one of the two protons changes into a neutron by releasing a positron and a neutrino (beta plus decay). Now, on this deuterium, another proton attacks and forms helium-3 as shown above. This helium-3 combines with another helium-3 produced parallel to it and forms a helium-4 thereby releasing 2 hydrogen atoms as shown. Note that the total mass number (number of nucleons) is always conserved.

This nuclear reaction is the reason why we are alive. This is how the Sun is producing its energy. A single reaction produces 26.4 MeV of energy. In a single second, the Sun produces more energy than produced by the mankind so far. The PP chain initiates at about 15 million K. So, when the temperature of the collapsing cloud of gas reaches this mark, stars are formed. This reaction is slow. For a Sun like star, it will take 10 billion years to convert hydrogen into helium in its core. If you did not understand the reaction, it's okay. Understanding its importance is enough.

CNO Cycle

CNO stands for Carbon-Nitrogen-Oxygen. The CNO cycle is yet another nuclear reaction by which stars produce helium from hydrogen using carbon, nitrogen and oxygen as catalysts. The CNO cycle is a dominant source of energy for stars that are about 1.3 times more massive than the Sun. This reaction becomes dominant at about 17 million K. The core temperature of Sun is 15 million K and thus PP chain is the dominant reaction. The reaction mechanism is shown below:

Nuclear Reactions in Stars - CNO Cycle
The CNO Cycle

Helium Fusion

Triple Alpha Process

Once all the hydrogen has been converted into helium in the core, it is time for the next nuclear reaction. After helium, carbon forms via the triple alpha process. This reaction is simple. Two helium-4 nuclei come together and form beryllium-8. This beryllium-8 nuclei is further attacked by a helium-4 and forms a stable carbon-12 as shown below. The net release of energy is about 7.275 MeV and the reaction requires a temperature of 100 million K.

Nuclear Reactions In Stars - Triple Alpha Process
The Triple Alpha Process

One important thing to note in this reaction is the temperature dependence. The energy released in previous PP chain reaction is proportional to the 4th power of temperature while that in triple alpha process is proportional to a whooping 17th power of temperature. Thus the energy released is enormous. Once a star starts burning helium to carbon, end of the star is near.

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Production of Heavier Elements

The reaction sequence does not stop at carbon. However, it should be noted that only massive stars can host full scale nuclear reactions beyond this point. Let us glance over some key nuclear reactions in stars beyond helium

Carbon Fusion

Carbon fusion begins at a whooping 500 million K. The common products of this reactions are neon, oxygen, sodium and magnesium. Stars below 8 solar masses 8 cannot host a carbon fusion. Stars between 8-11 solar masses begin carbon fusion with a flash but this disrupts the star. The ones with mass above 11 solar masses go on to fuse even heavier elements.

Nuclear Reactions In Stars
The Carbon Fusion

Oxygen Burning

The oxygen core that forms due to previous nuclear reactions requires very high temperatures to fuse further elements. At about 2 billion K, oxygen core transforms into a silicon, phosphorus and sulphur core. This reaction takes place in a few years and the amount of energy released is tremendous.

Alpha Ladder

Once silicon forms in the core, a ladder of reaction begins. Silicon has a mass number of 28. Beyond silicon, heavier alpha elements form. This means the elements that have mass number of multiples of 4 beyond silicon as shown below.

The Alpha Ladder

The reaction sequence stops at Ni-56. The next element in the chain is Zn-60 but conversion from Ni to Zn isn't thermodynamically unfavorable. This because the reaction is endothermic (absorbs energy). Silicon fusion begins at about 3 billion K. The intensity of this reaction can be realized from the fact that while PP chain took 10 billion years to finish, silicon burning ends in a single day. So Nickel and Iron are the last major fusion products in the core. The star then collapses and forms either a neutron star or black hole.

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