Solar Structure

The theory of stellar evolution is highly developed today, that we can predict the internal structure of a star like the Sun, knowing its initial mass only as a parameter. The layers of the Sun display a thermal gradient with temperature highest at the core and progressively decreasing as we move out towards the exterior. The entire Sun’s mass is gaseous and mostly composed of hydrogen and helium.

Solar Core

The solar core is the innermost layer of the Sun, with a temperature of around 15 million Kelvin! The density of the core is about 160 gm / cm³, which is ten times that of a block of lead, which is supposed to be one of the densest objects on Earth. Not surprisingly, 40% of the Sun’s mass is contained within the core, which occupies only 10% of its volume!

Radiative Envelope

The next layer, immediately outside the core is the radiative layer or the convection zone. Temperature here is about 4 million Kelvin, which means fusion reactions cannot occur here. The density drops here significantly and the rest of the 60 % percent of solar mass is contained in the 90% volume of the solar outer envelope. Comparatively more opaque, the solar envelope transfers heat slowly towards the outer layers of the Sun through convection, due to the temperature gradient across it. Huge cylinders of circulating gas move out in convection currents stretching for hundreds of kilometers. That is why this envelope is called the convection zone. Convection cells of circulating hot gas are produced and they increase in number towards the outer layers of the Sun.

Photosphere

At the end of the convection zone lies the photosphere, which is the surface of the Sun. Only a few hundred kilometers in thickness, this layer is the outermost and only visible layer of the Sun. Compared to the innermost layers of the Sun, this is also the coolest. This is the layer from which low energy photons that constitute sun rays are emitted. Its composition can be understood by analyzing the emission spectrum of the Sun. The photosphere also has some low temperature or cooler spots that are called ‘Sunspots’. They are generated as a result of the dynamics of the solar magnetic field. They intensify periodically in a 11 year cycle.

Chromosphere

Outside the photosphere, the Chromosphere is one of the thinnest layers of the Sun. It is primarily composed of hydrogen that gives it a red color, which is observed during solar eclipse. Its temperature at 7000 Kelvin is higher than that of the photosphere.

Corona

Outermost region of the solar atmosphere, above the Chromosphere and Transition Region. The corona becomes visible as a white halo surrounding the Sun at a Total Solar Eclipse, and can be observed at other times using a special instrument called a Coronagraph. Coronal material is heated to temperatures of millions of Kelvin, and consequently emits energy at extreme-ultraviolet and X-ray wavelengths. Observations at these wavelengths show the corona over the whole of the Sun’s face, with the cooler, underlying Photosphere appearing dark.

Solar Wind

The Solar Wind consists of solar material flowing into interplanetary space. The Sun’s atmosphere is expanding radially outwards in all directions at supersonic speeds of hundreds of kilometers per second, filling interplanetary space with charged particles and Magnetic fields. This solar wind is made up of an equal number of electrons and protons, with lesser amounts of heavier ions. It carries solar material out into interstellar space at a rate of almost a million tonnes each second, flowing past the planets, which essentially orbit within the Sun’s outer atmosphere. The solar wind carves out a huge bubble in space with the Sun at its centre, known as the Heliosphere, extending out to about 100 AU from the Sun.

Questions to Ponder

• How do we know that sunspots are associated with the Sun's magnetic field?
• What elements make up the Sun?
• Why will fusion wait until hydrogen is gone before starting to fuse heavier atoms?