Science made simple: the anatomy of our sun

Anatomy of our Sun. Credit: ESA


This is where the Sun generates its energy. The temperature in the core is around 15 million degrees Celsius. This, combined with the enormous pressure and density of the plasma force the hydrogen nuclei to fuse together, creating helium and releasing large amounts of energy in the process. Each second, the Sun converts four million tons of matter into energy, which begins a slow journey towards the surface.

Radiative zone

This is the layer above the core. Even though it is not as dense as the heart, the plasma is still so dense in the radiative area that convection cannot take place. Instead, the energy created in the nucleus slowly diffuses through the plasma. It takes around 170,000 years for photons to pass through the radiative zone: photons travel at the speed of light, but can only travel a few millimeters at a time before being absorbed by a atom then reissued in any direction. At the top of the area, the temperature is around two million degrees Celsius. At the base, next to the solar core, the temperature is around seven million degrees Celsius.

Convective zone

This lies between the deeper radiative zone and the photosphere. The convective zone is 200,000 km deep. While the top layer has the same temperature as the photosphere (between 4500 and 6000 degrees Celsius), the base of the convective zone reaches two million degrees Celsius. The plasma at the base of the area is heated quickly. This makes it float and so it rises rapidly, creating a turbulent convection pattern, much like a pot of boiling water – only 200,000 km deep and surrounding all of the Sun.


It is the limit between the convective zone and the radiative zone. Below the tachocline, the Sun turns like a solid body. Above, the Sun rotates at different speeds depending on its latitude. The change in rotational speed through the tachocline is very rapid and this results in shear forces which are believed to be important in creating the magnetic fields which lead to sunspots.


It is the visible “surface” of the Sun. Almost all of the solar radiation is emitted by this thin layer, several hundred kilometers thick, which lies at the upper limit of the convection zone. This is where the energy generated in the nucleus can finally move freely in space. The temperature of the photosphere varies from place to place but is between 4500 and 6000 degrees Celsius.


This is the layer above the photosphere, where the density of the plasma drops dramatically. Typically, the chromosphere is about 1,000 to 2,000 kilometers thick, with a temperature that increases from about 4,000 to about 25,000 degrees Celsius. Arrows of chromospheric gas, called spicules, can reach heights of 10,000 km.

Transition region

This is a thin, irregular layer that separates the relatively cool chromosphere from the much warmer corona. Through the transition zone, the temperature of the solar plasma rises to nearly a million degrees Celsius. While the convection zone and partly the solar photosphere are dominated by fluxes capable of moving regions of strong magnetic flux, the transition region and corona are dominated by the magnetic field which forces the plasma to move mainly along field lines.


It is the outer atmosphere of the Sun and stretches millions of kilometers in space. It is more easily visible during a total solar eclipse. The corona plasma is extremely hot at over a million degrees Celsius, yet it is very rarefied. Its density is usually only one trillionth of the density of the photosphere. The solar wind has its source in the corona.


They are large structures, often thousands of kilometers in extent. They are made up of entangled magnetic field lines that keep dense concentrations of solar plasma suspended above the surface of the Sun and often take the form of loops that arch from the chromosphere. They can persist for several weeks or even several months.

Solar eruption

It is a sudden release of energy. An eruption is usually created when the magnetic field lines that create the sunspots quickly change into more stable patterns. It’s kind of like a stretched rubber band that snaps and releases all of its stored energy when it snaps back into place. The energy released by solar flares strongly influences the behavior of the solar wind.

Sun spots

These are temporary features on the photosphere. They look like dark spots against the brightest region of the photosphere because they are around 1000 degrees cooler, so they don’t emit as much light. They are caused by magnetic fields passing through the Sun’s photosphere and cooling the gas in it. Sunspots can range from a few tens of kilometers to over 150,000 km.


These are convection patterns that occur in the photosphere. Each pellet is approximately 1000 km wide and consists of hot plasma rising in its center. As it releases its energy into space, the plasma cools, causing it to sink down the sides of the pellet and back down into the photosphere. Individual granules persist for about 20 minutes, after new ones grow in slightly different places.

Coronal mass ejections

These are vast eruptions of billions of tons of plasma and magnetic fields from the solar corona. They move away from the Sun at speeds of hundreds to thousands of kilometers per second and, if sent down the Earth’s path, can create geomagnetic storms.

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