The Sun is the reason of our existence. It is the head of our planetary family: the solar system. However, every object in the Universe has a definite lifetime and our Sun is no exception. The question is: When and how will our star end its life? What is the science behind the death of Sun? Let us find out.
The Current Situation
Before learning about the end of the Sun, let us figure out the current position of Sun on its timeline. Sun is 5 billion years old. In its core, the temperature is about 15 million Kelvin. At such a high temperature, the hydrogen is fusing into helium: a nuclear reaction known as the p-p chain (proton-proton chain), illustrated below.
A total of 4 hydrogen atoms are used to produce 1 helium atom. Each reaction is releasing nearly 26 MeV of energy. A large number of such reactions are taking place in the core and the combined energy, in the form of heat and light, is what powers our world and supports life on it. So from a wider perspective, a proton proton collision is what drives life on Earth!
Currently, hydrogen is converting into helium in the core. Mathematics tells us that Sun has enough hydrogen in the core to sustain this nuclear reaction for another 5 billion years. Also, when the core is active, the star is in a perfect hydrostatic equilibrium. This means that the inward gravitational collapse of the massive star is perfectly balanced by the outward gas pressure due to the nuclear reaction. If you did not understand this, just imagine two forces: one acting inwards on a sphere, from outside, and other acting outwards from the center. In hydrostatic equilibrium, the two opposite forces have same magnitude and hence a dynamic equilibrium is maintained within a star. This phase of life is known as Main Sequence. A star spends around 90% of its life on main sequence.
5 Billion Years Later
Roughly 5 billion years later, the Sun will run out of hydrogen in its core. The core is now composed of helium. Here's the problem: The current temperature of Sun is around 15 million K, just enough for the fusion of hydrogen to helium. Now, the core is composed of helium. The next nuclear reaction is helium to carbon, that requires a temperature of about 100 million K. In the absence of the required temperature, the core shuts down and becomes inert. Due to the lack of an outward gas pressure (the outward force), gravity gains the upper hand and starts collapsing the star.
Becoming A Red Giant
With an inert core, the star is in trouble. This is when a thick shell of hydrogen around the core starts burning into helium. This is known as the hydrogen shell burning. The star expands and cools, becoming a red giant. Remember, redder a star, lesser is its temperature. The ash from the hydrogen shell burning deposits on the core, increasing its mass. When the mass and temperature of the core become sufficiently high, the next nuclear reaction, helium to carbon, starts with a bang! This event is known as the Helium Flash. It is so explosive that immediately, 6% of the core turns into carbon.
The Final Reaction
All the helium in the core converts to carbon in a few million years and the star faces a similar crises again. The core is now made up of carbon and the next reaction requires a temperature of about 500 million K. Remember the shell that was burning hydrogen into helium? Well that shell now starts burning helium into carbon and a new shell, above this one, starts burning hydrogen into helium as shown below.
Sun like mid sized stars do not have the potential to host a full scale carbon fusion. So the nuclear reaction from helium to carbon was the last one. The core now shuts down forever. Sun will then expel most of its outer material, creating a planetary nebula, until only the carbon–oxygen core is left. This process is responsible for the carbon–oxygen white dwarfs which form the vast majority of observed white dwarfs.
So the Sun will ultimately end its life as a white dwarf, forming a planetary nebula. Such a nebula is nothing but a how glow of expanding gases ejected from the dying red giant star.
Why Doesn't The White Dwarf Collapse?
You may ask that if there is no core reaction going on in the white dwarf, why doesn't gravity collapse the star? Well, this is where the electrons come to the rescue. Electrons are Fermions that obey Pauli's exclusion principle. Thus, no two electrons can be in the same quantum state. In layman's terms, electrons hate being crushed. Thus, when the inward gravitational force tries to crush the white dwarf, electrons exert an outward pressure, known as the electron degeneracy pressure and halt the collapse. This prevents the star from collapsing into a singularity and becoming a black hole. So the reason why the Sun will never become a black hole is its low mass. Stars that are more massive than Sun end up becoming neutron stars and black holes.