In this section, we will focus on sunlike star evolution. A star spends most of its life on the main sequence, fusing hydrogen into helium. What happens in the later stages of the life of a star with a mass similar to that of the sun? We will address this question in detail, looking at the evolutionary sequence of a sunlike star.

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Recall that in a sunlike star, fusion only happens in the very core of the star, where the temperature is high enough for PP chain fusion to take place. Energy is transported outward through the radiation zone via the random walk of the photons, and then out through the convection zone via the motion of the plasma.

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When a star is in hydrostatic equilibrium, the inward pull of gravity is balanced by the outward push of pressure. When a star heats up, the pressure increases. If the pressure is greater than the gravitational force, the star expands. The expansion cools the star, which means that the particles are moving slower, decreasing the pressure. The star comes back into hydrostatic equilibrium. Besides the gas pressure exerted by the particles, there is also radiation pressure, exerted by the photons.

Fusion in the early universe converted some of the hydrogen to helium, about 92% hydrogen and 8% helium, by number of atoms. Since a helium atom is more massive, the relative mass of hydrogen is about 25%, and 25% helium.

When a star is born, its matter consists of this primordial ratio of hydrogen and helium, with a small amount of heavier elements, mixed throughout the star. As the star evolves, more and more of the hydrogen is fused into helium. Since the helium is more massive, it tends to fall to the core. Over time, the core fills up with helium.

Our sun is now about five billion years old. A star like our sun lives on the main sequence for about ten billion years, fusing hydrogen into helium. Over time, the core gets gradually hotter, and the star expands and gets more luminous.

 

Helium atoms contain two protons, so they have twice the charge of hydrogen atoms. This means they must be moving much faster to overcome the electromagnetic repulsion and fuse.

 

As the core fills with helium, there is less fusion, producing less heat. There is less thermal pressure, as well as less radiation pressure.  The center of the core fills with helium ash, and fusion moves to the outer core. While the pressure decreases, the gravitational force does not decrease, so the core contracts.

When the core fills with helium ash, the fusion in the core halts. With no fusion in the core, the pressure greatly diminishes and the core contracts under the pull of gravity. As the core contracts, it heats up.

Hydrogen shell burning stage

 

When the core contracts, it causes the shell of material around the core to fall inward. The hydrogen atoms speed up as they fall, changing gravitational potential energy to kinetic energy. This heats up the material, until hydrogen begins to fuse into helium in a shell surrounding the core. This fusion produces a great deal of pressure, causing the star to expand into a red subgiant.

 

The core continues to contract, and the shell fusion increases. In the shell fusion stage, the rate of fusion is higher than it was in the core fusion stage. This causes an increase in luminosity.

 

Much of the material in the star is opaque to radiation, and energy is transported outward via convection, keeping the surface temperature relatively constant.

 

The increase in luminosity, together with the constant surface temperature, causes the star to move vertically upward in the HR diagram into the red giant region. Its radius is about a hundred times bigger than it was as a main-sequence star.  The core is still not hot enough for helium to fuse. Since helium nuclei have two protons, the electromagnetic repulsion is much stronger. Much higher temperature is required to fuse helium than is required for hydrogen fusion.

Electron degeneracy pressure

 

In the red giant phase, the strong gravitational force in the core brings about another kind of pressure, due to quantum mechanical effects. Electron degeneracy pressure arises because electrons are fermions, indistinguishable particles that obey the Pauli exclusion principle. Two electrons cannot occupy the same state. This means that they strongly resist being compressed too tightly together.

 

Electron degeneracy pressure is independent of temperature. As the temperature in the core rises, the gas pressure increases, but the electron degeneracy pressure does not. As the core compresses, the electron degeneracy pressure dominates the gas pressure. This allows the temperature to rise to the point where helium fusion is possible.

Helium nuclei, produced by fusion in the shell, sink into the core, increasing its mass. This increases the gravitational pull in the core, causing it to contract.

 

The core continues to shrink, and shell fusion increases until the core finally reaches the temperature where helium can take place. Helium fusion happens explosively in the core. Shock waves pass through the star with such force that up to about one-third of the outer region of the star is blown off.

Helium fuses into carbon in a two step process. Two helium nuclei fuse to form beryllium, which is highly unstable, and then another helium nucleus joins to form carbon. Helium nuclei are also called alpha particles. Often the fusion of helium into carbon is called the triple alpha process, since it involves three alpha particles.

With the outer portion of the star gone, the luminosity of the star lowers. The high temperature from helium fusion in the core causes the core to expand. The expansion causes it to cool somewhat, and the fusion rate in the core lowers. The core expansion also pushes the shell farther out, reducing its temperature and fusion rate. The reduction of the fusion rate reduces the luminosity of the star.

 

The helium core fusion and hydrogen shell fusion continue, and strong stellar winds carry matter off the surface of the star.

 

Similarly to what happened to the main sequence star, when the helium filled up the core, now the helium fusion in the core causes it to fill up with carbon. This will again halt the fusion in the core. The core will again contract and heat up.

When the temperature in the contracting inert core gets high enough, helium fusion begins in a shell around the core. A second, outer shell where hydrogen fuses into helium also contributes to the heat and pressure in the star.

The double shell fusion increases the pressure to an enormous level, which causes the star to expand greatly. It rises back up to the red giant region, larger than ever. The fusion becomes unstable, and a series of explosive helium flashes take place in the helium fusion shell. The outer layers of the star pulsate and become unstable, being ejected off into space.

This image of a planetary nebula shows outer layers of the star expanding out into space. The core of the star lies intact in the center. The white-hot inert carbon core is now a white dwarf star.

Main sequence star

Hydrogen fusion core

Thermal pressure supports against gravity in the core.

Hydrogen fusion shell

Inert helium core

Enormous pressure

Star leaves the main sequence

Helium core fusion

Red giant

Star greatly expands

Inert helium core

Red subgiant

Helium flash

Hydrogen shell fusion

Helium fusion explodes

Hydrogen shell fusion

Helium core fusion

Hydrogen shell fusion

Inert carbon core

Hydrogen shell fusion

Inert carbon core

Star spends most of it's life in this phase, fusing hydrogen into helium

Helium fusion sets off huge explosion, blows away up to 1/3 of the star

Helium shell fusion

Helium shell fusion