Read this essay to learn about the sun.

Sun is one of the stars, which is revolving along with millions of other stars in one galaxy. The earth is a member of the solar system which is at an average distance of 150 million km from the sun. The sun supplies all the energy received by earth. Sun is the main source of energy for the living organisms and almost all the processes occurring on earth surface.

The sun is a ball of churning gases that are heated by continuous nuclear reactions. Heat energy received from other stars and heavenly bodies is negligible. Most of the sun’s energy is wasted in its way to the earth. Only a small portion of the whole output is received by the earth.

Some of the characteristics of the sun are:

1. The mass of the sun is 99.87 per cent of the total mass of the solar system.

2. The diameter of the sun is 1.39 x 106 km.

3. The mean distance of the sun from earth is 1.5 x 108 km.

4. The temperature at the centre of the sun is 15 million Kelvin.

5. It rotates about its axis in 4 weeks. Rotation of the sun is different from solid body. It takes 27 days at the equator and 30 days at the poles.

6. Surface temperature of the sun is 5762°K.

7. Density of the interior is 80 to 100 times that of water.

The life giving energy of the sun, the source of all the heat and light in our solar system, is generated by nuclear reactions in its interior which raise the temperature deep inside it to about 14 million °C. So tremendous is the radiation rate of the sun’s energy that it loses some 4 million tons in weight every second. Yet despite this rate of loss, it is estimated that sun will survive for about at least another 8000 million years.

Energy at the sun is produced in the core. It is an immense nuclear furnace where hydrogen atoms are fusing through fusion process to form helium atoms. This process takes place in the interior of the sun and energy is transferred from the centre to outer part of the Sun. The temperature is estimated to be around 27 million degrees. The pressure is 200 billion times the pressure at the earth surface.

The energy leaves the core of the sun as Ï’-rays that are absorbed and emitted millions of times by hydrogen and other atoms. Then energy is transformed into x-rays, ultraviolet light and visible light. The wavelength of the radiation increases as the temperature falls from the core towards the surface.

1. Central region of the sun extends from 0 to 0.23 R, where R is radius of the sun. It contains 40 per cent mass and 15 per cent of the volume of sun and generates 90 per cent of the energy of sun.

2. The density of the mass drops to 0.07gcm-3 and temperature drops to 1,30,000°K at a distance of 0.7R from the centre.

1. Convective zone of sun extends from 0.7R in the interior to the outer surface.

2. The temperature in the convective zone is 5000°K and the density is 10-8gcm-3.

Photosphere is the upper layer of the convective zone and is called sphere of light. Photosphere has sharp edges. It is opaque and consists of strongly ionized gases. It has low density. It emits and absorbs radiation. This layer represents the visible disc. The disc is a source of solar radiation. The depth of this layer is 0.0005 solar radius. The temperature ranges from 4500°K at the top to 7300°K at the bottom.

The solar atmosphere lies outside the photosphere. It is several hundred km deep. It is almost transparent and is also referred as Reversing Layer.

Chromosphere lies outside the reversing layer. The chromosphere is known as colour sphere. It acts as a boundary between relatively cool (6000°k) photosphere and much hotter corona. It is 100,000km deep. It is not visible except during total eclipse, when it appears as a zone of rosy colour. Its thickness is 0.02 solar radius. It consists of transparent gases, which are almost homogeneous. Its existence was observed in 1870.

Corona is the outer ring of the sun. It is visible only during total eclipse. In this layer electrons are at very high speeds. Its temperature is 1 million degrees. It has no sharp boundary. This layer is full of extremely hot gases. These gases are mostly in highly ionised and atomic form. These gases can only be seen during the time of eclipse.

Solar Flare are intense, temporary releases of energy from the sun. They can last from minutes to hours sending out radiation from gamma rays to long radio waves.

There are comparatively cooler spots on the bright photosphere. These are called sunspots. They appear as dark spots on the surface of sun. The sunspots show a cycle of nearly 11 years. They are associated with activity that increases the overall energy coming from the sun. There can be dozens of sunspots in an year. However, during a peak in the cycle, thousands of sunspots can be seen.

Several scientists observed that there are cycles of sunspots that have been recognized including the 80 years old and one with a frequency of 205 years. The historical data indicates that ice age has been observed during the period from 1645 to 1723 AD, when the sunspots were almost absent.

On the other hand, warm climate prevailed over the earth in the northern hemisphere during the period from 1100 to 1250, when maximum activities of the sunspots might have taken place.

However, some scientists found close relationship between the sunspots activities and the weather on the earth, whereas others found no relationship. Thus, no conclusion could be drawn about the influence of the cycles of the sunspots on weather and climate of the earth.

These zones suggest that Sun does not act as a perfect black body. The radiation flux is the combined effect of several layers and the energy received from other sources is negligible.

Thermal Energy Received from Other Sources:

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Thermal Energy Sources at Earth Surface

The sun radiates about 56 x 1026 calories of energy per minute. Mean distance of earth from the sun i.e. radius of the spherical shell is 1.5 x 1013cm. The energy falling on unit area is called solar constant (S).

Its value is given below:

Thus, solar constant is defined as the flux of solar radiation falling on the outer boundary of the atmosphere received on a surface held perpendicular to the sun rays at the mean distance between sun and earth. It varies between -3.5 to +3.5 per cent depending upon the distance of the earth from the sun.

The value of the solar constant is 1.95 ly min-1 at a height of 82km. Out of this energy, 35 per cent is contributed by UV and visible radiation whereas infra-red radiation contributes 65 per cent.

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