Our very own star, the Sun, is the alpha and omega of the solar system. Life on Earth is made possible by the energy derived from it. The interior structure of our star is layered like that of an onion. In this article, I present a tour of the solar interior, beginning from the hot core, which is the source of its phenomenal power towards the corona, ultimately reaching the solar wind.
The theory of stellar evolution is so well-developed today, that we can predict the internal structure of a star like the Sun, knowing only its initial mass as a parameter. The solar layers show a thermal gradient, with the temperature being highest at the core and progressively decreasing, as we move out towards the exterior. Solar mass is entirely gaseous and mostly composed of hydrogen and helium.
The solar core is the innermost of all layers, 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, though it occupies only 10% of its volume.
At such high temperatures and high densities, the hydrogen nuclei don't bounce off when they hit each other but fuse together to form helium. This is the process of nuclear fusion, which is responsible for the phenomenal energy output of the Sun. A fraction of the fused mass is converted into energy. Fusion reactions release very high energy gamma rays and neutrinos that start their journey from the inner core and make their way towards the photosphere. On its way, the high energy gamma ray photon is converted into several low energy photons, through collisions with gas atoms. The neutrinos are unstoppable and they pass through the solar envelope, without reacting with any matter.
The next layer, outside the core, is the radiative layer or the convection zone. Temperature here is around 4 million Kelvin, which means that fusion reactions cannot occur here. The density also drops significantly and the rest of the 60% percent of solar mass is contained in the 90% volume of the outer solar envelope. Relatively more opaque, the solar envelope transfers heat slowly towards the outer layers, through convection, due to the temperature gradient across it. Huge cells 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 created, which increase in number towards the outer layers.
At the end of the convection zone, is the photosphere, which is the surface of the Sun. Only a few hundred kilometers in thickness, this is the outermost and only visible layer of the Sun. Compared to the inner layers, this is also the coolest. This is the layer, from which the low energy photons that constitute sunrays are emitted. Its composition can be studied by analyzing the emission spectrum of the Sun. The photosphere consists of some low temperature or cooler spots, that are called 'Sunspots'. They are created due to the dynamics of the solar magnetic field. They intensify periodically, in an 11 year cycle.
Outside the photosphere, is one of the thinnest layers, called the chromosphere. It is primarily composed of hydrogen, which gives it a red color, that is observed during solar eclipse. Its temperature is higher than the photosphere, at 7000 Kelvin.
After the chromosphere, is the corona. It surrounds the chromosphere and is the rarest of all the layers. Surprisingly, this layer has a temperature ranging from 1 million to 3 million Kelvin. The source of coronal heating is still a mystery. There is a conjecture that the corona is heated by the effect of the magnetic field but it remains to be validated. The corona is witness to high energy outbursts of energy called solar flares, which stretch for distances greater than many Earth radii and last for many hours. Their temperatures have been recorded to be close to 11 million Kelvins. In the image above, you can see a large solar flare, photographed by NASA's SOHO spacecraft.
Other observed phenomena in the corona are masses of ejected gases called prominences. Corona can only be observed clearly during solar eclipses when the photosphere is blocked out by the Moon.
Outside the corona, the solar gas stretches out in the form of solar wind, which are high energy particles like protons and electrons, that are ejected from the Sun and travel to the edge of the Solar system and beyond. On Earth, we are shielded from these high energy particles, traveling at speeds in excess of 500 m/s, by the Earth's magnetic field, which traps these particles. The point at which the reach of solar wind ends is called the heliopause. This point is not exactly known, as it extends far beyond the farthest frontiers of our solar system.
To think of it, despite its gigantic size and overwhelming energy output, Sun is only an average dwarf star, compared to other stars in our galaxy. The single parameter that dictates the energy output and internal structure of a star is its initial mass. Massive stars have short but very eventful lives, while low mass stars like our Sun have a comparatively uneventful life and last longer. They act as centers of planetary systems and sustain life that may eventually develop on blue dots like our Earth.