The Structure of Sun - I

The Milky way galaxy comprises around 100 billion stars. The most important star for us is the Sun. It is the magnificent being around which the Earth and other planets revolve. Today, we are going to have a closer look at this effulgent being. In the thirteenth article of our Basics of Astrophysics series, we shall discuss the composition and structure of the Sun.

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Being the nearest star, the Sun has offered a lot of scope for detailed physical studies. It is the only star that has a disc-like appearance. The modern sophisticated instruments and efficient observation techniques enable us to study the true physical characteristics of the Sun. But we can only observe the solar atmosphere and its extreme superficial layers. The scientists combine the laws of physics with the observation from the upper solar layers. In this way, they extend their knowledge to solar interiors. In this article, I shall try to give you a brief overview of the structure, composition and various observed phenomena in the Sun.

Composition of Sun

We have discussed The Hertzsprung Rusell Diagram in our previous article. According to this, the Sun is a G-type main sequence star based on its spectral class. It is informally referred to as a yellow dwarf. It consists of 73% Hydrogen and the rest (around 25%) is mostly Helium. Heavier elements like oxygen, carbon, neon, and iron are present in much smaller quantities inside the Sun.

Various layers of the Sun

There are two main regions of the Sun- an inner and an outer region. The inner region comprises the solar core, followed respectively by the radiation and convective zones. The thermonuclear reactions take place inside the core of the sun. These reactions are the source of Sun's abundant energy. The region outside this inner zone is the solar atmosphere. It comprises of - the photosphere, chromosphere, transition region, and the corona.

Structure of the Sun

The Photosphere

As clear from its name, photosphere comprises the visible part of the Sun. The light from this region emanates to the outer regions of the solar atmosphere. The chromosphere (next layer of the stellar atmosphere) is invisible due to the excessive brightness of the photosphere. The disrupted magnetic field regions called Sunspots also appear in this zone. Temperature ranges from 5770 K-5780 K in this region.

The Chromosphere

The word chrome means color. As the name suggests, this region appears with a pinkish or reddish hue. As the density of chromosphere is very less, i.e, around 10−8 times that of the Earth's atmosphere at sea level, it is visible only during a solar eclipse and not otherwise. It has a temperature of around 20,000 K.

Transition Region

The Chromosphere is followed by the transition region. In this region, the temperature increases abruptly from 20,000 K to 1,000,000 K. The transition region is not easily visible from Earth's surface. However, it is readily observable from space by the instruments sensitive to the extreme ultraviolet portion of the spectrum.

The Corona

The outermost part of the sun is the Corona. It is a smooth extension of the chromosphere but differs greatly from it. The corona can be clearly seen during a solar eclipse. It appears like a halo embracing the solar limb by the inner end. Its outer end extends far beyond the solar disk and gradually merges into interplanetary space. It has a temperature above one million Kelvin. Now this is an anomaly. How can the source of heat be at a lower temperature than the structure it is heating? How can heat travel from colder photosphere at about 5900 K to the corona at one million K? Surely something else must be heating the corona. This is one of the major unsolved problem in Astrophysics: The Solar Corona Heating Problem.

Image of solar corona during a solar eclipse

People knew about corona for centuries but could not guess its real existence and nature. Scientists considered it as an optical illusion. Even an astronomer like Kepler could not identify its actual nature. In 1869, American astronomers- W. Harkness and C. A. Young studied the coronal spectrum for the first time. In 1930, the French physicist- B. Lyot obtained the first photograph of the solar corona using his instrument named the Coronagraph. Structures such as a solar prominence also appear in this region.

Structure of Sun

Image of a Major eruptive prominence (Skylab 1973)

Author's Message

Solar Physics is yet another branch of Astrophysics that we are covering in our series. Being a plasma physicist myself, I have researched on the Alfven waves that might be responsible for the abnormal heating of the solar corona. If you want to study the structure of Sun deeply, you need a strong hold over subjects like Electrodynamics and Plasma Physics. In the next article of this series, we will be studying about the surface features of the Sun: Sunspots, Solar Flares etc. Stay Tuned!

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