There is just one more topic we need to study before understanding stellar evolution and that is nuclear reactions in stars. Stellar Astrophysics involves a lot of nuclear physics, thermodynamics, particle physics, electrodynamics, statistical mechanics and gravitational physics. It is one of the most active branches of research in Astrophysics. In this article, we will learn about the key reactions in stellar nucleosynthesis. This is a very important article of this series. So let us begin…
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 up of hydrogen and helium.
In Astrophysics, in contrast to conventions in chemistry, every element except hydrogen and helium is termed as metal. So in Astrophysics, 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 just study the reactions without caring much about the evolution of stars. The next article will give a detailed account of the same.
Hydrogen fusion is the fundamental nuclear reaction in stars. In the article of Hertzsprung Russell Diagram, we learnt 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.
The PP Chain
PP Chain stands for Proton-Proton chain. In this reaction, 4 hydrogen nuclei combine to form 1 helium nucleus as shown below.
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 behind the existence of every life form on Earth. 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.
The 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:
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.
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.
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 begins at a whooping 500 million K. The common products of this reaction are neon, oxygen, sodium and magnesium. Stars below 8 solar masses 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.
Neon burning begins at temperature of around 1.2 billion K. During neon burning, oxygen and magnesium accumulate in the central core while neon is consumed. After a few years the star consumes all its neon and the core ceases producing fusion energy and contracts.
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 sulfur core. This reaction takes place in a few years and the amount of energy released is tremendous.
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 reaction sequence stops at Ni-56. The next element in the chain is Zn-60 but conversion from Ni to Zn is thermodynamically unfavorable. This is 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 a black hole.
All our cards are on the table now. In principle, my task to introduce the basic concepts of Astrophysics is over. From here on, we’ll be playing with these concepts to understand the life of stars. We started with the basic question: What is Astrophysics? We covered the EM spectrum, Stefan’s law, concept of magnitude, classification of stars, Saha’s equation, the structure of Sun and most importantly, the Hertzsprung Russell diagram. Juggling with these concepts, we are now ready to study stellar evolution in the next articles. Stay tuned!