The Sun

The Sun is the primary energy source in the Solar System because almost all the life forms exist on Earth. It is in the outer portion of the Milky Way Galaxy with being brighter than 85% of all the stars present in the galaxy. The heart of our Solar System, the Sun is classified as a G₂V star where G₂ means it is among the second hottest stars of yellow G-class with a surface temperature of about 5800K; and V meaning it is a dwarf star. The Sun has a diameter of 1.39 million kilometres which is 109 times the diameter of Earth. If the total mass of all the planets is multiplied 743 times, we get the mass of the Sun, i.e., 1.98*10³⁰kg. Being so massive, it comprises 99.85% of the total mass of the Solar System. The star will have quite a strong gravitational force; being 3,30,000 times more massive than Earth. If you weigh something 100kg on the Earth's surface, it will weigh 2,800kg on the Sun's surface. But wait, the star's temperature is so high that no solid or liquid can exist, which means the Sun is a glowing ball of gases and does not have any surface. 

The Sun has formed around 4.6 billion years ago from the remnants of a supernova. The star is 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements, including oxygen, carbon, neon, and iron. The helium content present in it has significantly increased from 24% to 60% because of the nuclear reactions happening in its core, and the heavier elements are pulled towards the centre because of the enormous gravity. 

In the nuclear reaction happening inside the core of the Sun, two hydrogen atoms combine together to form a helium atom, and the energy generated in the nuclei fusion is converted into gamma rays or increases the kinetic energy of the produced helium atom. 

The energy in the nuclear fusion reaction is a result of the enormous pressure and density at the centre of the Sun which makes it possible for the nuclei to overcome electrostatic repulsions and bind together. 

The Sun, for convenience of study, is being divided into parts, as discussed below;

1. Core 

The core of the sun is its innermost region. It is 20-25% of the Sun's radius. The temperature and pressure of the gases here are so high, and absolutely sufficient for nuclear fusion to happen.

2. Radiative Zone

The radiative zone of the sun is its middle part, between 20-25% of the radius and 70% of the radius of the Sun. Here, energy from the core to tachocline transfers by the process of radiation.

3. Tachocline

It is the thin layer between the radiative and convective zone of the Sun. It is hypothesized by the scientists that a magnetic dynamo is present in this region which is responsible for the magnetic field of the Sun. 

4. Convective Zone

The convective zone is the region from 70% of the radius of the Sun to closer to the visible surface of the Sun. Here, the Sun is cool as compared to the core and radiative zone. The process of convection is the primary means of outward heat transfer.

5. Photosphere

The deepest part of the Sun which we can directly observe with visible light from the Earth is known as the photosphere. It is thought to be the inward boundary of the Sun.

6. Atmosphere

The atmosphere is the gaseous nimbus surrounding the Sun. During a total solar eclipse, the part of the Sun not covered by the moon is known as the atmosphere. After the photosphere, the temperature of the Sun decreases rapidly for the next 500km and this is the coolest part of the Sun. Further, the atmosphere is divided into many parts; 

1. Chromosphere
The visible colored flash visible to us during the total solar eclipse is known as the Chromosphere. It is about 2000km thick. The temperature in the chromosphere rises with the rise in altitude with the highest temperature recorded as 20, 000K, due to which the upper part of the Sun becomes partially ionized. 
2. Solar Transition Region
The solar transition region is not a defined disc rather an undefined halo to the chromosphere where the temperature rises rapidly from 20,000K in the top of the Chromosphere to 10,00,000K in the Corona. 
3. Corona
The corona is the extended atmosphere of the Sun where the temperature rises from 10,00,000K to 20,00,000K with the density of this region ranging between 10¹⁵ to 10¹⁶ m^-3. The corona is the source of Solar wind in the interplanetary space. 
4. Heliosphere
The outermost part of the Sun filled with the solar wind, forming a Solar magnetic field, is known as the heliosphere. 

The solar wind is the stream of ionized particles; electrons, protons, and alpha particles; released from the corona with kinetic energy ranging from 0.5 to 10keV. The solar wind varies in density, temperature, and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona. The solar magnetic field is carried out with the help of the  Solar wind.

The Sun has a magnetic field that varies with time and location. Since the Sun rotates, the magnetic field spins out into a large rotating spiral, known as the Parker spiral.

The Sun goes through a phase of its own Solar Cycle, a period of 11 years in which the Sun's geographic poles change their magnetic polarity. Due to a change in polarity of the poles. the photosphere, chromosphere, and corona go through several changes, from being quiet and calm to being highly active and violent. The height of these activities is known as Solar Storm, which includes sunspots, solar flares, and coronal mass ejections. 

Sunspots are regions of extremely strong magnetic fields and reduced temperature visible on the photosphere. Sunspots expand and contract as they move along the surface of the Sun but they eventually decay after a few days, weeks, or months.

Solar flares are sudden flashes of increased brightness due to the sudden explosion of energy on the surface of the Sun near the sunspot. These are caused by the tangling, crossing, or re-organizing of the solar magnetic fields. 

Coronal mass ejection is the significant release of plasma, accompanied by the solar magnetic fields from the Corona. It is due to the changes in the distribution of magnetic fields.

But the point to think about is, that the hydrogen atoms are fusing to make helium. So, one day, eventually, all the hydrogen present within the core of the Sun will end up and the Sun will reach the death of its life. It is expected, that approximately 5 billion years later, the Sun will exhaust all its hydrogen and the core will contract, with the expansion of the other parts of the Sun. The Sun will eventually envelop the inner planets, till Mars, in itself, and become a red giant star. It is expected that the Sun will burn as a red giant star for years and then as the whole hydrogen will deplete in its outer core, the Sun will shrink as a white dwarf star. All the outer material of the Sun will dissipate leaving the planetary nebulae, and that will be the end of the Sun and our Solar System. 

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