Additional Notes on the Life Cycle of Stars

Additional Notes on the Life Cycle of Stars

What happens at the start of the Red Giant phase

Low to medium mass stars enter the red giant phase after their main sequence stage.

When a star has been fusing hydrogen in its core, eventually most of the hydrogen in the core gets used up. The core then contains mainly helium, and hydrogen fusion in the core stops.

Why the core begins to shrink

While hydrogen fusion was happening in the core, it produced energy. This energy created:

Radiation pressure (from photons)
Gas pressure (from very hot particles)

These pressures pushed outwards, balancing the inward pull of gravity.

When fusion stops in the core:

Energy production falls
Radiation pressure decreases
Gas pressure also decreases

Gravity now wins, so the core starts to contract (shrink).

As the core shrinks:

Its volume decreases
The same amount of mass is squeezed into a smaller space
The density increases
The temperature and pressure inside the core increase

This rise in temperature and pressure heats the layer just outside the core, which still contains hydrogen.

What happens next

The high temperature produced by the contracting core heats the surrounding hydrogen layer enough for hydrogen fusion to start again, but this time:

It happens in a shell around the core, not in the core itself.

This is called hydrogen shell fusion.

Summary

Hydrogen in the core runs out.
Fusion in the core stops.
Radiation and gas pressure decrease.
Gravity causes the core to shrink.
Pressure and temperature increase inside the shrinking core.
This heats the surrounding hydrogen layer.
Hydrogen fusion begins in a shell around the core.

What happens during and after hydrogen shell fusion

Once the core has shrunk and become very hot:

Hydrogen fusion starts in a shell around the helium core.
This shell produces a large amount of energy.
The extra energy causes the outer layers of the star to expand.

As the outer layers expand:

The star becomes much larger
The surface becomes cooler
The star appears red, which is why it is called a red giant.

If the star is not massive enough, the core never becomes hot enough for helium fusion.

In these stars:

The core is mainly helium
Helium does not fuse
Eventually the outer layers drift away forming a planetary nebula
The remaining core becomes a white dwarf

If the star is massive enough, the contracting core reaches a much higher temperature.

In these stars:

The helium core continues to contract and heat up.
When the temperature reaches about 100 million K, helium fusion begins.
Helium nuclei fuse to form carbon, and sometimes oxygen.

In more massive stars

More massive stars enter the red supergiant stage after their main sequence stage.

In these stars, the core can become hot enough for further fusion stages. These stages produce heavier elements such as:

Neon
Magnesium
Silicon
eventually Iron

However, fusion cannot produce energy beyond iron, so fusion stops there.

Why fusion stops with iron in the core of a star

Fusion can release energy only for lighter elements.

In stars, fusion releases energy when light nuclei combine to form heavier nuclei.

This happens because the binding energy per nucleon increases as we go from very light elements up to iron. When the binding energy increases, the new nucleus is more stable, and the extra energy is released.

For example in stars:

Hydrogen → Helium (releases energy)
Helium → Carbon
Carbon → Oxygen
Heavier elements → up to Iron

Each step releases energy that helps support the star against gravitational collapse.

Iron is the most stable nucleus

The element Iron (more precisely iron-56) has one of the highest binding energies per nucleon of all nuclei.

This means:

It is extremely stable
Nuclei heavier than iron have lower binding energy per nucleon

Because of this, if you try to fuse iron nuclei:

The new nucleus would actually be less stable
Instead of releasing energy, the process would require energy

What this means inside stars

Fusion in stars can only continue as long as it produces energy.

When the core becomes mostly iron:

Fusion no longer produces energy
The star cannot generate enough pressure to balance gravity
The core collapses rapidly

This collapse can lead to a supernova explosion in very massive stars.

Elements heavier than iron

Elements heavier than iron are formed during supernova explosions through rapid neutron capture, when enormous energies are available.

Supernova – More details

In very massive stars, fusion continues in stages, producing heavier elements:

hydrogen → helium → carbon → oxygen → neon → silicon → Iron

These reactions occur in layers around the core, a bit like an onion.

Eventually:

The core becomes mostly iron.
Iron cannot release energy by fusion.

So the star loses its energy source in the core.

Core collapse

Since no energy is produced:

Outward pressure from fusion decreases
Gravity pulls the core inward

The core then collapses extremely rapidly.

During this collapse:

Electrons and protons combine to form neutrons.
The core becomes extremely dense.
In a fraction of a second the core shrinks dramatically.

Formation of a neutron core

When the collapsing core becomes extremely dense:

The neutrons resist further compression.
The core suddenly stops collapsing.

This forms a very dense object called a Neutron star (if the remaining core is not too massive).

The rebound shock wave

The outer layers of the star are still falling inward at enormous speed.

When they hit the rigid neutron core:

They bounce outward violently
A powerful shock wave travels outward through the star.

This shock wave blows the outer layers of the star into space.

The supernova explosion

The star then explodes as a Supernova.

During this explosion:

Huge amounts of energy are released.
Temperatures become extremely high.
Atomic nuclei collide with enormous energies.

This allows the formation of elements heavier than iron, such as:

gold
uranium
lead

These elements are then scattered into space, where they later become part of new stars and planets.

What remains after the explosion

After the supernova:

If the remaining core mass is moderate → a neutron star forms.
If the core is extremely massive → it collapses further into a Black hole.