Fusion in Stars

Star Stability

The Earth orbits the Sun because of the gravitational force of the Sun on the Earth.
The Sun is many times larger than Earth and has a much greater gravitational force.
- However, if the gravitational force of the Sun is so great, why does the Sun not collapse in on itself?

The Sun is not a solid but a plasma with extremely high temperatures.
It does not collapse in on itself because there is not a force imbalance.
- Otherwise, a force imbalance would have caused the Sun to contract over time as the gravitational force pulled the outer layers of plasma inwards.
The force counteracting the gravitational force is produced by nuclear fusion reactions in the core of the Sun.
This situation is called stellar equilibrium.
- Stellar equilibrium occurs in a star when the outward radiation (thermal) pressure is balanced by inwards gravitational forces.

In the center of a stable star, hydrogen atoms undergo nuclear fusion to form helium.
- A huge amount of energy is released in the reaction.
- This provides a radiation pressure that prevents the star from collapsing under its gravity.

The total mass of the helium nucleus is less than the total mass of the individual nucleons.
- Hence the reaction releases energy, which provides fuel for the star to continue the burning.

For nuclear fusion to occur, both nuclei must have sufficiently high kinetic energy to overcome the electrostatic repulsion between protons.
The conditions required to achieve this are:
- Very high temperatures.
  - On the scale of 100 million Kelvin.
- Very high pressure and density.
Main sequence stars are stable stars which are fusing hydrogen atoms into helium in their cores.
- Our Sun is a main sequence star, for example.

The lifetime of a main sequence star is the duration for which a star is fusing hydrogen into helium, emitting radiation and maintaining stellar equilibrium.
- The rate of fusion in more massive (hotter) stars is so much greater that they have significantly shorter lifetimes.

The Hertzsprung Russel (HR) diagram plots the luminosity of stars onto their temperature.
Certain regions of the graph are classified into specific types of stars where they form clusters.
- Most stars are clustered in a band called the main sequence.

We can use the HR diagram to determine the radius of a star.
- This is done by combining the Stefan-Boltzmann equation for luminosity with the area of a sphere.

Sources