Our Star Sun

Our Star Sun

Source- google wiki

In the cosmos the Sun is just one of the billions of stars.  There are larger and brighter star in the universe.

The Sun is important to us due to many reasons.-

1. It is the nearest star in our planetry system

2. It provides us almost all of our energy.

Photosphere

All the visible radiation from the Sun comes from its surface layer called the photosphere, and above that there is the atmosphere of the Sun having two distinct layers called the chromosphere and the corona.

Sun radiates like a black body at approximately 6000K. We do not recieve the radiation in the same form as generated in the interior of the Sun. At the center, the energy is generated in form of high energy photons called γ- rays. when these photons speed up outwards, they collide with particles of matter and lose energy continuously. By the time they reach the surface the photosphere they are reduced to photons of visible region of electromagnetic spectrum. Therefore, visible radiation gets emitted from the photosphere.

Density of Photosphere- 3400 times less than the density of the air around us.

Thickness- 500km

Temperature- 6500K

The chemical composition of the photosphere is 79% H and the remaining 21% with about and 60 other chemical elements..

Chromosphere

It is just above the photosphere and stretches upto 2000km, normally this layer is not visible from earth, its faint, but its visible during a solar eclipse.

After a total solar eclipse, a bright pink flash appears above the photosphere. That pink color appears due to the emission of the Balmer line (Hₐ) which occurs in the red region. The clue to this occurrence is provided by observation of chromosphere just prior the total solar eclipse. Hot gas, in shape of jets called spicules.

Corona

It is the outermost layer of the sun's atmosphere. We cannot see corona at normal times because of the fact that the density of matter in chromosphere as well as corona is very low, they are extremely faint. In the bright light of the photosphere, they are not visible to us.

The spectrum of corona is made up of bright lines superimposed on continuous  spectrum.

Due to high temperature, electrons in the corona region have high energies. These 

electrons interact with ionised atoms and give rise to emission of X-rays. The coronal 

X-ray emission is much larger than that of the photosphere. Remember that the 

temperature of the photosphere is only 6000 K. So, it emits very little energy in the 

X-ray region.

Solar wind

Unlike its visual appearance, the solar corona extends much beyond into the space. 

The outer layer of the solar atmosphere, in fact, continuously emits charged particles 

which fill the entire solar system. This emission is called the solar wind. It comprises 

streams of charged particles (mainly protons and electrons) and causes continuous loss 

of mass from the Sun. The phenomenon of solar wind was predicted much before its 

detection. Its characteristics can be investigated using rockets and satellites.

 For instance, the solar wind velocities range from 200-700 km s−1

 at the distance of the 

Earth from the Sun. The number density of the solar wind at this distance 

is ~ 7 particles per cm3.

The electrically charged particles carried by the solar wind cannot cross the lines of 

force of the Earth’s magnetic field. These particles are deflected by the Earth’s 

magnetic field, spiral around the field lines and move back and forth between the 

magnetic poles of the Earth. As a result, two doughnut-shaped zones of highly 

energetic charged particles are created around the Earth and they are collectively 

called the van Allen radiation belts. 

Sunspots

The dark spots visible on sun, called sunspots.

The sunspot temperature is ~ 4000 K. With such high temperature, you may 

wonder, why they appear dark! Sunspots appear darker because they are cooler 

than their surrounding areas in the photosphere that have an effective temperature of 

6000 K. A typical white light picture of a large sunspot . Note -

that it consists of a dark central region, called umbra, surrounded by a less dark 

region, called penumbra. 

In the presence of a magnetic field, a spectral line emitted by an atom at a single wavelength is split into three lines. This is called the Zeeman Effect. Such 

Zeeman splitting is observed in the spectrum of sunspots. Since the line separation, ∆λ

is proportional to the applied magnetic field, a magnetic field up to ~ 3000 Gauss has been estimated in sunspots.

The sunspots are the dark spots on the solar disk. 

©Kirti Tomar