Structure of the Sun: Solar Interior

    Before we go further into knowing how sunspots are formed, we need to know what are the structure of the Sun. One thing to note is that the structure of the Sun is a model proposed by scientists. Therefore it remains largely a postulation. By understanding the structure of the Sun, we would then be able to appreciate how Sunspots and other solar activities are caused.


    The core is the inner most region of the Sun. Nuclear fusion takes place in this region to produce energy and release light that eventually escapes from the Sun's surface. We are not able to explore the interior of the Sun in detail... what we have known so far arises from the postulations by scientist and Helioseismologists. In particular, the field of Helioseismology provides much of what is proposed for the structure of the Sun.

Fact Files......

- temperature (@ the very center of the Sun) : 15,000,000 oC
- density: 150 g/cm3 (about 10 times the density of gold or lead)

Both the temperature and the density decreases as one moves outward from the center of the Sun.

Nuclear Fusion

Step 1: 2 Hydrogen atom collides with high speed under the intense temperature and pressure in the Sun's Core. A Deuterium atom is formed and a positron (positively charged electron) and neutrino is released.
Step 2: A Hydrogen atom collides with the Deuterium atom to form a Helium-3 atom and releasing gamma ray in the process.
Step 3: 2 Helium atoms collide with each other to form a normal Helium-4 atom, releasing 2 protons in the process.

View Animation

Radioactive Zone

    This zone is spans from the edge of the core to the interface layer. It is characterised by the radiative mode of transferring energy from the Sun's core to the interface layer. The energy produced by the nuclear fusion in the Sun's core is transported by the photons. If the photons emitted by the nuclear fusion were allowed to escape freely, they will travel at the speed of light into space. However, this is not the case. The photons are hindered by the gas that produced it. A photon interacts strongly with atoms and electrons of the gas. The photon, on its way out from the core, collides with the atom of the gas which absorbs it. One of the electrons of the atom then get 'excited' by the energy carried by the photon and fly off with some of the energy carried by the photon. However, the electron itself will soon be captured by an atom and will go back into orbit about the nucleus. This in turn releases a new photon. The photon can be visualised as a runner in a relay race. It passes energy ,like a baton, to the atom that absorbs it. In turn, the electrons that fly off from the atom that has absorbed the photon will then be captured by another atom and this atom will in turn release a photon. By this 'drunkard' walking of the photons, the energy from the core escapes into space. In fact, the energy that we see today was released from the Sun's core 50 million years ago!

Fact Files......

- temperature falls from 7,000,000 oC to about 2,000,000 oC over the same distance

- the density drops from 20 g/cm3 (about the density of gold) down to only 0.2 g/cm3 (less than the density of water) from the bottom to the top of the radiative zone

Interchange Layer: Tacholine

    The zone is the region in between the radiative zone and convection zone. The fluid motions found in the convection zone slowly disappear from the top of this layer to its bottom where the conditions match those of the calm radiative zone. It's believed that the Sun's magnetic field is generated in this region. Change in fluid flow velocity across the layer is able to stretch magnetic field lines and concentrated them. The name Tacholine arises from this change in fluid velocity.

Convection Zone

    This region is the outermost of the Sun's interior. The temperature of this zone is relatively lower than the other zones at the bottom of the zone. This allows the heavier ions (such as carbon, nitrogen, oxygen, calcium, and iron) to hold onto some of their electrons. This makes the material more opaque so that it is harder for radiation to go through. Heat is trapped in this situation and this leads to the 'boiling' of the fluid in the region and then they start to convect. The density of the fluid at the bottom of the zone is less [because it is heated up and hot gases expand (Density = Mass/ Volume, where mass does not change] than that of the gases at the top of the zone. The difference in density causes the less dense gases at the bottom to rise to the top of the zone, carrying heat energy along. This gases then  loses its heat energy at the top of the zone and the gases cools and condenses. Its density then increases and it will then sink down to the bottom of the layer. This movement of fluid sets up the Convection Current.

Resource book: "Universe (5th Edition)"   

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