iron element can still undergo fusion, but the process of fusion of to no longer releases energy, but absorbs energy. So we usually say "the star nuclear fusion stops when it reaches iron element". After all, we usually think that nuclear fusion should release energy.
Why do elements more important than iron absorb energy instead of releasing energy when they fuse?
Because iron element is the most stable, the professional term is: iron element has the highest specific binding energy.
What is the binding energy of to ? First understand the concept of combined with .
The nucleus is composed of neutrons protons and other nucleons . The strong nuclear force binds the nucleons together, and to separate the nucleons requires strong energy. This energy is the binding energy.
The more nucleons the nucleus has, the higher the binding energy. The specific binding energy is actually the "average binding energy". The total binding energy divided by the number of nucleons is the specific binding energy. The higher the specific binding energy, the more stable the element is. The specific binding energy of iron is the highest, so iron is the most stable.
It is precisely because the iron element is the most stable, so it will be very difficult to stuff more nuclei into iron atoms, and it will consume more energy.
Let’s talk about why star nuclear fusion cannot continue when fusion absorbs energy?
The reason why stars can carry out stable nuclear fusion is due to the balance of two forces: the external thrust generated by nuclear fusion, and the inward gravity brought by its own mass. Only when the two forces are balanced can stars continue to be stable.
starts with the fusion of hydrogen element (which requires the high temperature and high pressure of the star core to provide energy), and becomes the heavier helium element . This process will release energy, and the released energy will provide energy for the subsequent fusion to continue nuclear fusion. The cycle continues like this.
However, when nuclear fusion reaches the iron element, not only will it no longer release energy, but it will absorb energy. As a result, nuclear fusion cannot continue. The result is that the balance of the two forces is broken, gravity completely takes over, and the star The material begins to collapse sharply inward, and the massive star will trigger the supernova explosion .
A supernova explosion instantly releases super energy, second only to the energy of the Big Bang. In just a few seconds, it is hundreds of times more than the total energy released by the sun in 10 billion years!
The huge energy generated by the supernova explosion is like a super furnace, enough to continue the fusion of iron elements.
We should be grateful for supernova explosions, because the gold and silver jewelry and other heavy metals we see today are basically the products of supernova explosions.
