6.1. Sun

The sun is the head of solar family or solar system. Compared with the millions of other stars, the sun is a medium sized star and of average brightness... Though sun is the nearest star to the earth, even then it is at a distance of 150×106kilometres from the earth and light, travelling at a great speed of 300,000 kilometres per second, takes about 8 minutes and 20 seconds to reach us from the sun. However, light takes about 4.3 years to reach us from the next nearest star called proxima centauri.

Sun is not a solid body. The sun is a sphere of hot gases. It consists mostly of hydrogen gas.. The nuclear fusion reactions taking place in the centre of the sun( in which hydrogen is converted into helium) produce a tremendous amount of energy in the form of heat and light. It is this energy, which makes the sun shine From the Earth, we see only the surface of the sun. The shining surface of the sun is called photosphere. The surface of the sun (photosphere) appears

1 Details of dark matter and dark energy will be discussed in Science and Technology notes.

like a bright disc to us, it is also known as disc of the sun. It is this bright, shining disc of the sun (or photosphere) which radiates energy and acts as a source of energy for us. The temperature at the sun (or the temperature of the bright disc of the sun) is about 6000°C. The temperature at the centre of the sun is about 15 million °C. The outer layer of the sun’s atmosphere made up of thin, hot gases is called corona. The corona is visible only during a total eclipse of the sun.