IB Physics Sub-topic E5 Notes

Fusion

Whilst fission is an important source of energy production in the world, due to its significant disadvantages there is much research focusing on fusion instead. This is a process that naturally occurs in the core of stars, causing their large energy emissions. The reaction that stars undergo is:

$\mathrm{4}_{1}^{1}H \to \mathrm{}_{2}^{4}He + \mathrm{2}_{1}^{0}e^{+} + 2 \nu$

As part of fusion, mass is lost to produce energy at a high rate, about 4 x 10⁹ kg s^-1 in the Sun. Eventually, this energy makes its way through the different layers of the star out to the surface. Here, the outward radiation generates massive amounts of energy every second to shine.

One of the important components to understand is how stars remain stable despite their high radiation output from the core. This is due the hydrostatic equilibrium generated by the gases:

1. The radiation generated by fusion in the gaseous core produces a massive outward radiation pressure on the surface to push gases to the surface.
2. On the other hand, the high density of gases in the star generate an equally massive inward gravitational force that pulls the gases inward.
3. As a result, the outward pressure and inward pull balance each other out to form a fluid equilibrium, called hydrostatic equilibrium.

Star formation

However, you are expected to understand how a star develops to reach this point of hydrostatic equilibrium. The process occurs as follows:

1. Star formation begins with large interstellar clouds composed of Hydrogen, Helium, and other materials.
2. In these, the cloud’s temperature influences particular kinetic energy and expansion of the cloud while its density influences gravitational potential energy. Overall, if the gravitational potential is greater than the kinetic energy, the cloud remains bound together in equilibrium.
3. These clouds can exist in equilibrium for years until an external event causes a change in the cloud’s mass.
4. If the cloud reaches a mass greater than the critical mass (M_J) as according to the Jeans criterion, it starts to collapse. Typically, high cloud density and low cloud temperature are associated with a higher likelihood of collapse.
5. Collapse of the cloud increases the kinetic energy and temperature of the particles, forming a proto-star.
6. If the temperature is sufficient to trigger nucleosynthesis, the process is called ignition.
7. The fusion reactions in the core slowly start to produce as much energy as is being radiated, maintain the temperature.
8. At this point, hydrostatic equilibrium is established, forming the beginnings of a stable main sequence star.