Structure of the Sun
Compared to other members of the solar system, the sun is almost unbelievably vast. Its diameter of some 865,000 miles (1,392,000 km) is more than 100 times that of the earth and nearly 10 times the diameter of Jupiter, the largest of the nine planets. More than one million earths could fit inside the space occupied by the sun. Although it may appear to be a solid body, the sun is entirely gaseous, since liquids and solids cannot exist at the temperature of even the coolest part of the sun.
The sun is made up of four general regions: the interior, the photosphere, the chromosphere, and the corona. Relatively little is known of the sun's interior, although calculations of its temperature, pressure, and density have been made, based on the sun's mass and diameter. These calculations indicate that the gas deep within the sun's core has a density more than eight times that of lead and a temperature greater than 30,000,000 °F. (16,667,000 °C.).
The photosphere is the yellowish, luminous surface and outermost layer of the sun's sphere. It is about 200 miles (320 km) thick and has an average temperature of about 10,000 °F. (5,540 °C.). Seen through a telescope, the photosphere has a distinctive granular or mottled appearance.
The lower portion of the sun's atmosphere, the chromosphere, is a constantly changing, turbulent region that reaches some 6,000 miles (9,660 km) above the photosphere. Temperatures in the lower part of the chromosphere are similar to those of the photosphere. With increasing distance from the surface, however, the temperature rises rapidly, reaching more than 1,000,000 °F. (556,000 °C.).
Even higher temperatures exist in the corona, the outer atmosphere. Like the chromosphere, the corona is of such low density that it is transparent and can be seen from earth only during a total eclipse or with the aid of a special instrument called a coronagraph. The corona's shape and extent vary from time to time, but normally the corona reaches out more than 1,000,000 miles (1,600,000 km) from the sun's surface.
By careful study of sunlight using spectroscopes and other instruments, scientists have found in the sun more than 70 of the natural chemical elements known on earth. Of these, hydrogen is by far the most abundant. Helium makes up most of the remainder, followed by smaller amounts of oxygen, nitrogen, and carbon. Trace amounts of heavier elements such as copper, gold, iron, and lead are also present, particularly in the cooler parts of the chromosphere.
Like most other bodies in space, the sun rotates about its axis. However, because the sun is entirely gaseous, some parts of it rotate faster than others. Regions near the sun's equator make a complete rotation in about 25 days, but toward the poles the period of rotation is as long as 34 days.
All of the sun's major characteristics—size, brightness, density, and temperature distribution—depend on two opposing forces. These are gravitation, which tends to compress the sun's mass; and pressure within the interior, which tends to expand the sun. Each force balances the other, keeping the sun in a state of equilibrium.