The Sun

The Sun

The Sun, a great ball of immensely hot gases about 865,000 miles in diameter, is at once the central body of the solar system, the sustainer of life on the Earth, and the nearest star. It contains over 99 per cent of all the material in the solar system, and if it were divided into a million equal parts, each part would be larger than the Earth. Even Jupiter, the largest and most massive of the planets, is a tiny dwarf compared to the Sun.

As a star the sun is relatively small, faint and cool. Although much larger than many stars, it is only a speck compared with supergiant stars like Antares and Betelgeuse. One of the nearest stars is 500,000 less luminous than the sun. On the other hand, some stars are thousands of times more brilliant. True, the sun appears to be much larger than other stars, but these appear like pin-points because they are so far away. The sun is distant 93 million miles, so its light takes only about eight minutes to reach us. Proxima Centauri, the nearest star, is so remote that its light takes just over four years.

Prominences of the Sun. Prominences of the edge of the sun.

Sunspots

The sun’s bright disk, called the lightsphere or photosphere, is often marked by dark spots. These invariably change in shape, size and number as they move across the disk. They are seldom found near the sun’s poles. Studies of their movements show that the sun rotates once in about 25 days at its equator, and then more slowly with increasing latitude to reach about 34 days near the poles. The photosphere cannot therefore be a solid surface. It merely represents the level through which, owing to the opacity of the solar gases, we cannot see.

The spots are really hot, bright regions. They appear dark because we see them in contrast to the hotter, brighter surrounding regions of the photosphere. The spots have temperatures of approximately 3,500 degrees centigrade, but the temperature of the photosphere is nearly 6,000 degrees centigrade. The number of spots varies from a minimum though to a maximum and then to a minimum again in an average period of 11 years. This period, known as the sunspot cycle, has been as short as six years and as long as 17. Large sport usually appear at sunspot maxima. The largest seen so far occurred in 1947. It grew from a few small spots to a group visible to the unaided eye, At its maximum development covered some 6,200 million square miles, or about one per cent of the sun’s disk.

Prominences

The hot gases above the level of the photosphere form a tenuous and fairly transparent “atmosphere”. This region consists largely of hydrogen and helium, but also contains a small percentage of other gases representing probably all 92 permanent elements from hydrogen to uranium. Here are found flame-like clouds of shining gases named prominences. Some form shapes like pyramids, trees and arches and keep fairly still for hours and even days. Others surge outwards with explosive violence to form spectacular steamers, loops and plumes or cascade sunwards like fountains of fire. Their observation used to be restricted to the times of total solar eclipses. But they now can be studied in full daylight with the coronograph, a telescope that artificially eclipses the sun, and also with telescopes equipped with special filters.

More violent than even the most active prominences are sudden explosive disturbances known as solar flares. Large ones can be seen directly with the telescope, The great majority of them are more readily detected when the sun is photographed in the red light of hydrogen. A large flare not only emits liberal doses of x-rays and ultraviolet rays. It also ejects streams of fast moving electrified particles (protons and electrons). If the flare occurs on the sun’s near side, both the rays and the particles have a good chance of reaching the earth. The rays make the journey in just over eight minutes, disturb the earth’s ionosphere, or electrified upper atmosphere, and upset radio communications. The particles take much longer. They eventually stream into the earth’s atmosphere to upset the directions of compass needles. They bring about vivid displays of northern and southern lights.

The Corona

During the total eclipse of the Sun, when the photosphere is completely hidden by the dark body of the moon, the sun’s atmosphere is seen to extend far into space. It then appears as a pearly-white aureole, often with a delicate structure of tufts and curved wisps of light. It consists of extremely thin gases, whose temperature, measured by the velocity of their atoms, is about one million degrees centigrade near the sun. Most of its light is sunlight scattered by atomic particles. It also continuously emits radio waves. These sometimes come in intense bursts usually associated with active sunspots or large solar flares.

The corona is no static medium, nor is it limited to the sun in the way that the earth’s atmosphere’s limited to the earth. Its form and appearance are definitely influenced by the sun’s activity. At sunspot’s minimum, long curved streamers of light reach to great distances from the sun’s equatorial regions, while relatively short brush-like streaks appear at the poles. At sunspot maximum, the long streamers disappear and the corona takes on a more regular and uniform appearance.

We now know that the corona, and therefore the sun, extends in a very real sense far beyond the earth. Hot hydrogen gas flows from the sun in all directions and rushes past the earth at about 900,000 miles an hour. It blows through the solar system like a swift wind, sweeping any fine particles, meteoric dust, and gases into interstellar space. The streams of particles blasted off by solar flares are like temporary gales in the outward flow.

How the Sun Shines

Sunspot, prominences, flares, and the corona are all outward visible signs of activity deep in the sun’s interior. The mainspring of the sun’s energy output is a process known as a thermonuclear reaction. Under the enormous pressures and temperatures of several million degrees which exist in the sun’s central part of core, hydrogen is transmitted into helium. Vast quantities of energy are released in the process. They pass through the photosphere and, in the form of light and heat, pour into space. Calculations show that the sun converts about four million tons of its mass into energy every second. But since the total mass of the sun is so enormous, the loss is only a few percent over several thousand million years.

Considerations based on theory indicate that the sun has been shining for some 6,000 to 8,000 million years. Also, that it has enough hydrogen in its core to keep it going without much change for at least another 2,000 to 3,000 million years. But when the available hydrogen has been used up, the sun will gradually swell into a reddish star several times larger than its present size. The change will probably take many millions of years, As the sun approaches giant status its immense heat could boil away our seas and oceans and bring about the end of life on earth.

Just as man is destined to destruction, so also is the sun. Its nuclear fire cannot burn forever. Modern theories of the evolution of the stars suggests that the sun will dwindle into a white dwarf. It will become a hot, small, and extremely dense star, the last stage towards final extinction. So if intelligent life on the earth escaped a hot death, it eventually would have to face death through darkness and extreme cold.