The sun is exactly the same kind of heavenly object as the stars that you see at night. This is hard to believe, because the Sun is so bright and hot and lights up the whole sky, while the stars are faint, so faint that they can be seen only when the sky is dark. The Sun is an ordinary star and the only real difference between it and the other stars is that it is so much nearer to the Earth. It is this that makes it seem so much the brightest and hottest, although there are many stars that are bigger, brighter and hotter than the Sun.
The Sun's light and heat make life possible on Earth. The light and heat come from nuclear reactions which take place deep inside the Sun. Its light is so bright that it can hurt your eyes.
Men have been interested in the Sun since ancient times, when it was worshipped as a god. Modern astronomical study of the Sun began in the 16th century, when Copernicus stated that the Earth went round the Sun. Until that time people thought that the Sun went round the Earth. Investigations of the Sun's appearance began with Galileo's discovery of the markings called sunspots in 1610. Records of sunspots have been kept since the beginning of the 18th century. Observatories all over the world now makes photographs of the Sun on every clear day, and take moving pictures of the behaviour of its surface. The outermost parts of the Sun, called the corona, are photographed during eclipses, when its bright surface is hidden by the Moon or, at other times, with the special cameras called coronagraphs.
The sun lies at the centre of the solar system, which includes the planets and many other bodies moving around the sun under its gravitational pull. The Earth on which we live is one of the planets. The planets and other bodies shine be reflecting the light of the Sun, not by their own light, and even the largest of them, the planet Jupiter, is small compared to the Sun.
The Sun is about 1,390,000 km in diameter, which is nearly 10 times the diameter of Jupiter, and about 109 times the diameter of the Earth. If the Earth were placed at the Sun's centre, then even the whole path of the Moon would lie inside the Sun. The volume of space inside the Sun is about 1,300,000 times greater than the volume of space inside the Earth, but the mass of the Sun is only about 333,000 times the amount in the Earth, so the material of the sun is on average only about a quarter as dense as the material of the Earth. However, the centre of the Sun is more than 30 times as dense as the Earth.
On the surface of the Sun, the pull of gravity is 28 times as much as on the surface of the Earth. If a man who weighed 70 kg on the Earth could survive on the surface of the Sun, he would weigh 2,000 kg there. But of course a man could not live on the Sun, because the temperature of the surface is about 6,000 degrees.
The temperature at the centre exceeds 10 million degrees. The Sun is a ball of hot gas. It is gaseous all the way through, containing no solid or liquid material. Most of what is known about the Sun comes from the study of the light it sends out, which is photographed with special cameras attached to telescopes. Its surface also emits radio waves, X-rays, gamma rays and cosmic rays, and from deep within the Sun come a few very light, fast-moving particles called neutrinos.
Like most other stars, the Sun is composed largely of hydrogen, with a good deal of helium and smaller amounts of other elements. More than 60 other elements are known to be present in the Sun, and it is probable that there are very small amounts of all of the rest. The temperature is so high that even metals like iron are present in gaseous form.
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