The Sun

Our Sun is the source of all useful energy on Earth. It is absolutely necessary for life. It’s a gigantic nuclear fusion reactor that uses hydrogen as fuel. It is the center of the Solar System, around which all the other planets rotate. But, how does it really work?

Structure

Most scientists divide the Sun into six layers. The innermost layer is the core, which is the hottest and densest part. It is the source of the fuel for nuclear reactions. It has a temperature of 15 million K and makes up about 20% of the star’s interior.

The layer above the core is the radiative zone. It is named after the mode of energy transfer in this region, which is radiation. The energy is transported very slowly, since the photons do a ‘drunkard’s walk’. Matter is so dense in this region, that a photon does not travel very far before encountering a particle. It is like getting through a giant maze.

The outermost layer of the interior is the convective zone: energy is transported using convection. Convection is the process where hot material rises and cooler material falls. The plasma ‘bubbles’ to the surface, releasing energy to space and then sinking back down.

The first of the outer layers of the sun is the photosphere. It is the layer that appears opaque to us. It is not very dense, as it is only about 10% of Earth’s pressure at sea level. What’s interesting is that the surface exhibits a granulation pattern. They are 700-1000 km in diameter and last just a few minutes. By studying the Doppler shifts in the gas spectra, we can see that the bright granules are rapidly rising material. As the gas cools down, it falls down into the Sun leaving the dark spots between the granules.

A high quality image of the granules taken by the DKIST.

The second layer of the atmosphere is the chromosphere. It is about 2000-3000km thick. Its spectrum consists of strong emission lines in the red region. It gives this layer its reddish color and it is caused by hydrogen. This layer gave way to the discovery of helium in 1868. Scientists discovered. The spectrum had a distinct yellow line in it that didn’t correspond to a known element. They realised that it must be a new element, and they named it helium (after the Sun helios). It took until 1895 for the element to be discovered on Earth.

The outermost region of the Sun is called the corona. It was first observed during solar eclipses. It extends millions of kilometers above the photosphere. It thins out dramatically with altitude and it even extends to Earth’s orbit! Technically, we are living inside the Sun’s atmosphere, but it is so thin that we don’t really feel it. Before we get to the corona, there is a transition region, where temperature rises dramatically. It’s only a few kilometers thick but it corresponds with this tremendous temperature spike.

Graph of temperature in Kelvin vs. the altitude in the Sun’s atmosphere. Pay attention to the rapid temperature spike in the transition region.

Nuclear fusion

Nuclear fusion is the mode of energy production in the Sun. It fuses hydrogen nuclei into helium, releasing tremendous amounts of energy. The basic model for hydrogen fusion is this

Two of the protons change into neutrons. You might be wondering, where the energy comes from. This is because of the mass defect. It is the difference between the mass of the particle and the sum of its components. This mass gets directly converted to energy using Einstein’s famous E = mc² . The speed of light is a huge number, so even with a tiny mass you can still have lots of energy released. Nuclear fusion seems like a great way to generate electricity on Earth, however it requires tremendous temperature and pressure. However, we are getting better at creating these extreme conditions. ITER is the world’s biggest fusion experiment and it is looking very promising. In my opinion, we can expect these reactors to be functional in the 2050s.