The sun is a star. It is the star round which the earth and other planets revolve. In a way, we can say that 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. The sun appears to be larger and brighter because it is much more nearer to the earth than any other star. Thus, the star nearest to the earth is the sun. Though sun is the nearest star to the earth, even then it is at a distance of 150 x 106 kilometres from the earth and light, travelling at a great speed of 300,000 kilometres per second, takes about 8 minutes to reach us from the sun.
Image Source: noaanews.noaa.gov/stories2004/images/sun111004-1319z.jpg
But light takes about 4.3 years to reach us from the next nearest star called alpha centuari. The sun is a mass of hot gases, about 109 times bigger in size than the earth; 330,000 times as heavy as the earth and about 150 x 106 kilometres away from the earth. The diameter of the sun is about 14 x 105 kilometres. The sun is a big ball of fire. The temperature at the surface of the sun is about 6000°C while the temperature at its centre is about 20 million degrees Celsius. Please note that sun is the only star having a known system of planets with life on one of its planets, called earth.
The sun is not a solid body. It consists mostly of hydrogen gas. In the centre of the sun, hydrogen atoms are continuously combining to produce atoms of helium. Each time an atom of helium is formed, tremendous amount of energy is released in the form of heat and light. It is this energy which makes the sun shine. The planets and other objects in the sky reflect a part of the sunlight falling on them due to which they become visible to us.
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Even our earth reflects a part of the sunlight falling on it. So, if the earth is seen from a spaceship or moon, the earth will also appear as a bright object in space, just like other planets. It should be noted that sun is the main source of heat and light energy for all the members of the solar system including the earth. Like the earth, the sun spins on its axis and also moves through space. The planets travel with the sun through millions of stars in our galaxy at a speed of about 70,000 kilometres per hour. The sun has nine major planets revolving round it.