May 10, 2022

Understanding the death of a star with nuclear fusion

Before we talk about the moment of a star’s death and what it produced, let’s try to understand what happened before and more specifically inside the star.

To put it simply, the life of a star is due to a thermal equilibrium, that is to say, from the effects between temperature and pressure, the hydrogen atoms then found in the core of the star will fuse to form helium, which is stellar nucleosynthesis. This fusion produces a colossal energy that is needed to prevent the star from collapsing on its own weight.

This nuclear fusion, which will last for the entire life of the star, is represented by the equation of 4 atoms of hydrogen to finally obtain 1 atom of helium 4 and this energy of about 24 MeV.

Equation of nuclear fusion from the heart of the star

To give a few figures by way of example, 1kg of hydrogen provides 993. 15g of helium during a fusion obtained with a difference of about 7g which is entirely transformed into energy. This equates to 6. 2*10^12 J, which will be divided into mass energy (positrons and neutrinos), kinetic energy (heat) and radiation (photons). This is equivalent to 20,000 tons of coal consumed.

Creation of different energy

As you may have understood, this energy creation causes a mass loss of the star. Knowing that the more massive a star is, the less long it will live. In general, stars at the end of their life will have their core/core which will collapse once all the hydrogen is consumed, the thermal equilibrium is broken because the radiation pressure due to the fusion of the hydrogen is over. As a result, this will cause an increase in pressure and temperature in its center, to initiate the fusion of helium. This will increase the radius of the star and lower the surface temperature. So on to iron 56 which is the most stable atom for supergiant (10* mass of the Sun).

Different cases can be identified for the end of life of stars. Let us first take the example of the red giants. They are formed by a star whose mass is less than ten times that of the Sun. The red giants stop at the fusion of helium to leave a core of carbon and oxygen. The repulsion of the electrons prevents the heart from collapsing any further. The result of this is the creation of a white dwarf.

Diagram of hypothesis for death star

Now let’s take the case of the red supergiants, unlike the red giants, they continue their fusion until iron56, where the fusion stops, the heart collapses in a few seconds, a cataclysmic shock wave is generated and the outer layer is expelled. It’s the supernova. When the heart collapses, protons merge with electrons to produce neutrons to form a neutron star.

In the case of too strong a mass, the neutrons will fuse in their turn and no more will stop the collapse and we will witness the creation of BLACK HOLE.