A partial solar eclipse rises over the Capitol Building in Washington DC. Image Credits: Bill Ingalls, NASA, 2021-06-10
Composition
The Sun is the star around which the Earth and the other planets in our solar system orbit. Its diameter is around 864,000 miles or 1,392,000 km, making it over 100 times bigger than our planet! The Sun is made up of mostly hydrogen and helium; these elements make up 71% and 27% of its total mass, respectively. However, over 60 other elements from the periodic table have been detected in the Sun.
Structure
The Sun is made up of different layers. Its interior (up to the Sun’s surface) consists of the core, the radiative zone, and the convection zone. Above the Sun’s surface are mixtures of gases that form the solar atmosphere. The layers of the solar atmosphere consist of the photosphere, chromosphere, and corona.
The table below gives a thorough introduction to a breakdown of the structure of the Sun:
| Layer of the Sun | Description | |
| Solar Interior | Core | * The very centre of the Sun; consists of extremely dense plasma and is also the hottest part of the Sun. * Temperatures here can reach around 15 million Kelvin. * Can be thought of as the Sun's power generator. * Nuclear fusion reactions take place here, turning hydrogen nuclei into helium nuclei. This generates energy, which we can eventually see as light from the Sun. |
| Radiative Zone | * Also known as the ‘radiation zone’; this layer consists of tightly packed plasma. * This is where energy is transferred to the outer layers of the Sun through processes called radiation and conduction. * Radiation transfers energy in the form of photons (little packets of light), which bounce off particles throughout this layer. |
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| Convection Zone | * The unstable layer of the Sun, where energy is transferred to the outer layers through a process called convection. * The inner plasma (closest to the radiative zone) gets heated up rapidly, causing it to rise through the layer and quickly transferring heat to the Sun’s surface. * This is similar to boiling water in a pot – the water near the pot’s bottom gets heated to a gas and floats to the surface as bubbles. |
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| Solar Atmosphere | Photosphere | * This bright layer is the Sun’s visible surface, where most of the Sun’s visible light that we can see from Earth gets emitted. * Temperatures here are usually around 4,500 to 6,000 Kelvin. |
| Chromosphere | * This is the irregular level of the Sun’s atmosphere, where there is a large drop in the density of plasma. * The chromosphere has a red/pinkish colour that we can normally only see during the start and end of a solar eclipse. This colour comes from the emission of light at the wavelength of Hydrogen-alpha (around 656 nanometres). |
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| Corona | * The outermost layer of plasma, which has a white colour that we can normally only see during a total solar eclipse. This is because the corona is a lot less dense than the photosphere, making it roughly a million times dimmer than the light from the Sun’s surface. * It also has an irregular shape and extends over a million kilometres out from the Sun’s surface and into space. * Peculiarly, the corona is extremely hot compared to the rest of the solar atmosphere – it can reach temperatures of around 2 million Kelvin. |
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Scientists who are interested in studying the Sun by observing solar eclipses are typically interested in the layers of the Sun’s atmosphere – especially the corona. You can learn more about the solar corona by visiting this page.
Glossary
| Term | Definition |
|---|---|
| Conduction | The transfer of heat energy through materials by the collision of neighbouring particles. This is most seen in solids because of their low intramolecular space. |
| Convection | The transfer of heat energy in which material moves from hotter to cooler regions. This most commonly occurs in fluids, where fluid ‘blobs’ can move more freely compared to solids. |
| Kelvin | A unit of measurement for temperature – like Fahrenheit (degrees F) and Celsius (degrees C). Kelvin is represented as just ‘K’ (so written without the phrase ‘degrees’) because it is an absolute temperature scale. The zero value (0 Kelvin) of this temperature scale is the lowest theoretical temperature, and the point at which particles have minimised kinetic energy. To convert from Kelvins to Celsius, add ‘273’ to the temperature. |
| Nuclear fusion | The formation of a larger, heavier nucleus from smaller, lighter nuclei combining. This results in an absorption or release in energy due to the difference in mass between the reactants and the products. |
| Plasma | Gas that is so hot that its atoms are split up into ions and electrons (charged particles). It is regarded as the fourth state of matter, after solid, liquid and gas. |
| Radiation | The transfer of energy in space or a material in the form of particles or (electromagnetic) waves. |
| Total solar eclipse | A solar eclipse where the Moon completely blocks out the Sun’s light from view, from the Moon fully lining up with the Sun (as the Moon passes between the Earth and Sun). |
Further reading
Department of Physics, The Chinese University of Hong Kong (2017) Structure of the Sun, Solar eclipse. Available at: (Accessed: 19 June 2023).
Dunbar, B. (2017) The Sun, NASA, (Accessed: 19 June 2023).
Hathaway, D.H. (2022) The Solar Interior , Solar Physics, Marshall Space Flight Center, (Accessed: 19 June 2023).
Stoller-Conrad, J. (2021) What is the Sun’s Corona?, NASA, (Accessed: 19 June 2023).
Zirin, H. and Lang, K. (2023) Sun, Encyclopædia Britannica, (Accessed: 19 June 2023).