08 The Sun

1. Properties of the Sun

A star is a glowing ball of gas held together by its own gravity and powered by nuclear fusion at its center

Our star is a typical one. However, knowledge gathered from the Sun can be applied to other stars easily.

Key Properties:

Period: It is observed from the spots on the surface. However different periods are measured from different locations.

➤ This shows differential rotation like Jupiter and Saturn.

Temperature: Observing the solar spectrum and using radiation laws an effective surface temperature is calculated

➤ Blackbody @ 5800 K

Surface: It has a surface but not a solid one; it is a gas ball. Therefore, surface is taken as the layer where radiation escapes to space

➤ The layer is photosphere which is ~ 500 km thick


It is the total energy radiated by the Sun. It can be calculated from the fraction of that energy that reaches Earth.

2. Solar Interiors

Using this good data and using the standard solar model Solar density and temperature can be calculated.

3. Energy Transport Mechanism

In or near the core (having high T), the gas is completely ionized.

So what happens to the energy?

Since we see the Sun therefore energy reaches to the surface. How?

➤ Convection

➤ Convection cells

Therefore energy transported to upper layers by physical motion of solar gas.

There is a hierarchy in the convective zone

Then, radiation takes over the transport.

4. Solar Atmosphere

In atoms or ions:

In normal conditions:

they will create absorbtion lines. This is modified for the Sun and we observe:

Spectral analysis can tell us what elements are present. This spectrum has lines from 67 different elements. The most common ones:

Element __ % of Number __ % of Mass

The lines we see on Earth tend to come from higher, cooler levels in the solar atmosphere.

So, except the core, the spectral lines might represent the entire Sun.


Transition Zone


Solar Wind

5. Solar Magnetism


Sunspots ⟺ Solar Rotation

Sunspot Cycle (or Solar Cycle)

6. Active Sun

Solar prominence

Solar Flare

Coronal Mass Ejection (CME)

7. Heart of the Sun

Nuclear fusion 

nucleus 1 + nucleus 2 → nucleus 3 + energy

E = mc2

a small amount of mass is the equivalent of a large amount of energy.

So, as a summary   :  4(1H) → 4He + energy + 2 neutrinos 

Age of the Sun