Fusion in Stars

Basics of Fusion in Stars

Fusion is the process that powers stars, including our Sun. It involves the joining together of small atomic nuclei to form a larger nucleus. In stars, hydrogen nuclei (protons) fuse to create helium nuclei. This fusion process releases a huge amount of energy, which is the source of a star's light and heat.

The main fusion reaction in stars is:

$4\,{}^{1}\mathrm{H} \rightarrow {}^{4}\mathrm{He} + \text{Energy}$

This energy is what makes stars shine brightly for millions or billions of years.

Conditions for Fusion

Fusion requires extremely high temperatures and pressures to happen. This is because hydrogen nuclei are all positively charged and repel each other due to electrostatic forces. To fuse, they must get close enough for the strong nuclear force to overcome this repulsion.

In the core of a star:

• Temperatures reach millions of degrees Celsius (e.g. around 15 million 6C in the Suns core).
• Pressures are incredibly high due to the stars gravity compressing the core.

These conditions provide the energy needed for hydrogen nuclei to collide with enough force to fuse.

Energy from Fusion

During fusion, a small amount of the mass of the hydrogen nuclei is converted into energy. This follows Einsteins famous equation:

$$E = mc^2$$

where:

• $E$ is the energy released (in joules),
• $m$ is the mass lost during fusion (in kilograms),
• $c$ is the speed of light ($3.0 \times 10^8$ metres per second).

This energy is emitted as radiation (light and heat), which sustains the stars brightness and provides the energy that eventually reaches planets like Earth.

For instance, the Sun releases energy continuously through fusion, keeping it stable and bright.

Example: If 0.001 kg of mass is converted into energy in the Sun, the energy released is:

$$E = 0.001 \text{ kg} \times (3.0 \times 10^8 \text{ m/s})^2 = 0.001 \times 9.0 \times 10^{16} = 9.0 \times 10^{13} \text{ joules}$$

This enormous energy output explains why stars shine so powerfully.

Fusion and Star Life Cycle

Fusion is the main energy source during the longest stage of a stars life, called the main sequence. During this time, the star steadily fuses hydrogen into helium in its core.

As the star uses up its hydrogen fuel, the fusion reactions change, leading to different stages in the stars life cycle (covered in the Life Cycle of Stars topic).

The balance between fusion energy pushing outwards and gravity pulling inwards keeps the star stable during the main sequence.