The Sun - The Structure

The graphic shows a cross-section of the sun. The individual areas from core (1) to corona (9) are explained in the adjacent text. (Graphics: KIS)

As the only star whose basic processes can be studied both spatially and temporally based on their characteristic scales, the sun is a highly coveted object of research. This is also so, of course, because of the great influence the sun has on our lives. The sun is a giant ball of gas that consists mostly of hydrogen and helium. Its diameter is approximately 109 times as large as the diameter of the earth, and it emits every second an unimaginable quantity of energy of 4 x 1026 joules from its entire surface.

All of this is made possible through processes of nuclear fusion of hydrogen to helium, which are activated in the core (1) and are made possible by a very high temperature (15 million kelvins) and high pressure.

The energy that is freed from this process is then transported by radiation (2: radiation zone) and then convection (3: convection zone) into the photosphere (4). In the photosphere, the energy transport is taken over again by radiation. This thin layer, which is only a few hundred kilometres thick, is for us the visible surface of the sun, in which one can observe the honeycombed structure of the surface – granulation (6) – and sunspots (5). More than 99.9% of all observed solar photons are irradiated in this layer.

The layer which lies just above, the chromosphere (7) with its protuberances (8), can to be seen through the use of filters. The outermost layer of the sun, the corona (9), which continuously expands into the heliosphere, is able to be observed with the naked eye during a solar eclipse as rays of light around the solar sphere.

Graphical representation of the differential rotation of the sun. The equator, which is found in the red-coloured area of the graphic, has a rotational period of 26 days. In the direction of the poles, the colour-coding fades into blue, which represents a rotational period of 38 days. That is, the sun rotates faster at the equator than it does at the poles. (Graphics: KIS)

At the intersection of the radiation zone and the convection zone, there is a strong shear flow, which is noticeable in the differential rotation of the sun. The diagram shows a cross-section of the interior of the sun, where R is the radius of the sun and r is the distance from the centre.  The surface of the sun is found at r/R=1.  The dashed line indicates where the radiation zone and the convection zone meet.  With the colour-coding, it is easy to recognize that a differential rotation is employed here.  Even the surface does not rotate at the same speed everywhere: at the equator, the rotational period is 26 days (red) and in the directions of the poles, the surface rotates more slowly.  At the poles, a rotation takes 38 days (blue).  The differential rotation has far-reaching consequences.  It plays an integral role in the generation of solar magnetic fields, which are the cause of many solar phenomena.  The most well-known phenomenon that can be observed is the solar magnetic field in the form of sunspots.

Today, one assumes that the magnetic field on the bottom edge of the convection zone (dashed line) is coiled and intensified through the shear flow around the sun, before it becomes unstable and ascends to the surface.  On the surface and on the overlying chromosphere and corona, it causes a number of magnetic phenomena.  Through the discharge of magnetic energy, magnetic clouds can be flung into interplanetary space, which are then responsible for, as an example, the flaring up of polar lights on earth.