The Sun is exactly the same kind of heavenly object as a star that you see at night. This may seem hard to believe, because the Sun is so bright and hot and lights up the whole sky, while a star is faint-so faint that it can be seen only when the Sun has set and the sky is dark. Yet, despite its apparent brightness the Sun really is a star and a perfectly ordinary one too. The only real difference between it and the other stars is that it is so much nearer to the Earth.
The Sun's light and heat are essential for life on Earth. The light and heat come from nuclear reactions deep within the Sun's interior. The energy produced in these reactions makes its way to the surface and emerges as radiation, including heat and light.
Observation of the Sun
It is possible to observe the Sun safely by projecting it. This is done by means of a telescope and a piece of card. First, put a cap over the eyepiece so that if you accidentally look through the telescope you won't damage your sight; point the telescope in the direction of the Sun. When the telescope is set up, place the card behind it and remove the cap. Move the telescope until the Sun's image falls on the card. By adjusting the card's position, you can bring the image into sharp focus. Now you can study the Sun in safety.
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 had thought that the Sun went round the Earth. Investigation of the Sun's appearance began with discovery of the markings called sunspots in 1610 by Galileo and others. Records of sunspots have been kept since the beginning of the 18th century. Observations all over the world now takes photographs of the Sun on every clear day, and takes moving pictures of the behavior of its surface. Since the mid-1970s many observations of the Sun have been made from space.
Dimensions and Nature
The Sun lies at the center of the Solar System. The Earth on which we live is one of the nine known planets that travel around the Sun, along with asteroids, comets, and other bodies. The Earth orbits the Sun at an average distance of 149,600,000 kilometers (92,957,000 miles). The planets and other bodies shine by 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 kilometers (864,900 miles) in diameter, which is nearly 10 times the diameter of Jupiter and about 109 times the diameter of the Earth. The volume of space inside the Sun is about 1,300,000 times greater than the volume of space inside the Earth; but the amount of material contained in the Sun (its mass)-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. The center of the Sun, however, is more than 30 times dense as the Earth, 150 times as dense as water.
© 1988 Encyclopaedia Britannica, Inc.