The Sun

The single most important factor responsible for sustaining Life in the Earth, the Sun is the nearest star, seen in greatest detail. Our earth gets most of its energy from the sun. We call this energy solar energy. Solar energy travels from the sun to the earth in rays. Some are light rays that we can see. Some are rays we can’t see, like X-rays. Energy in rays is called radiant energy. The sun is a giant ball of gas. It sends out huge rays of energy every day. Most of the rays go of into space. Only a small part reaches the earth. When the rays reach the earth, some bounce of clouds back into space - the rays are reflected. The earth absorbs most of the Sun’s energy and turns it into heat. This heat in turn, warms the earth and the air around it - the atmosphere. Without the sun, Life would be impossible on the earth - it would be too cold.

The Sun has a diameter of about 870,000 miles (1.4 million km). It contains 99.8% of the mass of the solar system. Our Sun is about 4.5 to 5 billion years old. Astronomers established this age primarily through radioactive dating of the oldest Meteorites. Since its birth the sun has used up approximately half of the hydrogen in its core. It will continue to radiate for another 5 billion years or so. But eventually it will run out of hydrogen and will be forced into radical changes. These changes will inevitably result in the total destruction of the Earth and probably the creation of a planetary nebula.

The Sun, indeed most of the Universe, is in the fourth state of matter - the so-called plasma state - in which a gas is so hot that it has become ionized. Its behavior differs from that of a normal gas mainly because it interacts in a complex and subtle way with any magnetic field that is present. The sun's energy comes from nuclear fusion. Each second about 7 x 10⁸ tons of hydrogen is converted to about 695,000,000 tons of helium and 5,000,000 tons of energy in the form of gamma rays. As it travels toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light.

There are three main regions of the solar atmosphere, namely, the surface layer of the Sun (The Photosphere), the overlying hotter and rarer Chromosphere with a temperature of about 10⁴ K and the much hotter (about 10⁶ K) and rarer Corona, which stretches out to the Earth and beyond. The plasma that possesses temperatures between 10⁴K and 10⁶ K comprise the Solar transition region.

The Sun has always been a fascinating object of speculation. Ever since his evolution, Man has revered and worshipped the Sun. Annual events called Solar Eclipses (in which the Moon comes between the Sun and the Earth) have always attracted people across the World to witness the mesmerizing Solar Corona. Besides all these, there is a lot that we don’t understand about the Sun. Astronomers are continuously discovering fascinating properties of the Sun and yet, the more we know, the more we are amazed by its sheer mystique.

Questions to Ponder

• Is the distance from the Earth to the Sun changing?
• How do you calculate the lifetime of the Sun?
• Does the Sun rotate?
• How are sunspots formed?
• How often does the Sun pass through a spiral arm in the Milky Way?