The Sun’s Structure – Space Weather Explorers Week

The Sun's Structure

The Sun is a star, which is a giant ball of ionized gas, called plasma. The Sun is mostly hydrogen (74%) and helium (24%), with all the other elements of the periodic table making up only a total of 2% of the Sun’s matter.

The Sun has layers, like an onion. The layer that our eyes can see is called the photosphere. It is the layer of the Sun that gives off visible light. We call the photosphere the Sun’s surface. However, it is not a solid surface that one could stand on. The density of gas there is much less than the density of air in Earth’s atmosphere.

The layers beneath the photosphere are not visible. We determine their properties by carefully studying waves on the surface of the Sun that pass through its interior. Just as geologists infer the structure of the interior of Earth by studying how seismic waves produced by earthquakes travel and reflect throughout it, so, too, do solar scientists study how similar waves travel through the Sun’s interior. The study is called Helioseismology. Using this technique, along with physical theory about how gas behaves under certain temperatures and pressure, solar scientists construct models of the Sun’s interior.

The Core

The core is extremely hot and dense. There, the temperature is 15 million Kelvins and the densities are so high that nuclear fusion reactions can occur. Nuclear fusion happens when protons are able to get close enough to one another that the strong nuclear force can bind them together more strongly than the repulsion they feel from the electromagnetic force. This fuses hydrogen into helium. More energy is produced by this reaction than goes into it. This provides the Sun with enough heat to produce outward pressure that resists the inward crush of gravity. The two forces are balanced and the Sun is in a state of equilibrium.

Radiative Zone

Above the core, densities and temperatures drop and nuclear fusion reactions are not possible. Energetic light interacts with the matter here and heats it up, continuing to carry outward the energy produced from the core. This region is called the radiative zone, because electromagnetic radiation transports the energy outward.

Convective Zone

Above the radiative zone, temperatures and densities are lower still. In this next layer, large parcels of gas carry energy outward through convection. Pockets of hot gas rise from low in this zone and cool as they rise, then fall back down, heating up as they go. Convection like this can easily be seen in lava lamps or miso soup when it is served hot. This region is known as the convective zone.

Photosphere

The top of the convective zone is the photosphere. High-resolution observations of the photosphere reveal the surface to be churning, due to the convective motions of the gas beneath it. The photosphere often appears splotchy-the technical term is granulation. This is caused by the hotter and cooler convection cells at the surface. The average temperature of the photosphere is 5,800 Kelvin.

Chromosphere

Above the photosphere, the density of gas continues to drop, but its temperature actually begins to rise. The thin layer just above the photosphere is called the chromosphere. Here the temperature rises to about 10,000 degrees Kelvin. Unlike most hot things, it actually starts getting hotter as you move away from the surface. This layer produces most of the ultraviolet light that leaves the Sun. The chromosphere is visible to special telescopes that can make images using ultraviolet light. It is also visible in a specific red color of visible light produced by hydrogen; scientists often refer to this color as H-alpha (wavelength = 656.3 nanometers—1 nanometer is a billionth of a meter). Some visible light telescopes are made with special filters that block all light except this red color, allowing the observer to see the chromosphere.

Corona

Above the chromosphere is the Sun’s outer atmosphere. It is called the corona, and can be seen in visible light during total eclipses of the Sun. It extends far out into space. The density of the corona is very low (around 1 proton per cubic centimeter), but the temperature is very high, around 1 million Kelvin. The corona is so hot that it emits extremely energetic ultraviolet light as well as X-rays.

Temperature of the corona

Even though the corona has a very high temperature, it has very little thermal energy. This is because its density is very low. Temperature measures the average energy of particles in a substance, but thermal energy measures the total energy of all the particles. Even when all the particles individually have a lot of energy, if there are very few of them, then the total energy is low. Think about the difference between an oven and a boiling pot of water. The temperatures used for cooking in an oven are often much hotter than the temperature of boiling water (373 K). Yet one regularly can stick their hand into a hot oven to remove the contents without getting burned, whereas one would not even think about sticking their hand in a pot of boiling water to remove the contents. The water has a lower temperature, but its density is so much greater. So the thermal energy in the water is actually greater than the thermal energy in the oven.

Solar Wind

As the distance from the Sun increases the corona becomes less bound to the Sun. The particles in the Sun’s outer atmosphere have high temperature and feel a pressure from sunlight that pushes them away into a flow that becomes the solar wind. The solar wind is a stream of charged particles emanating from the Sun and blowing throughout the Solar System. The solar wind blows a bubble in space about 200 AU in diameter (1 AU is the average distance between the Sun and Earth). The space inside this bubble is called the Heliosphere.

Heliosphere

The heliosphere takes on a comet-like shape because of the motion of the Sun through the stuff between the stars (a.k.a. the interstellar medium). The bubble is compressed in the direction of the Sun’s motion and drawn out into a tail in the direction opposite the Sun’s motion.