Celeste Lynn, aage 13, of Pittsburgh, Pa., for her question:
HOW IS THE SUN'S TEMPERATURE MEASURED?
Largest object in the solar system is our sun. The sun is a brilliant giant star that is 109 times the diameter of the earth. The sun is nearer the earth than is any other star, and for this reason scientists are able to study it and learn about stars that are
much farther away. Only about one two billionth of the sun's light and heat reaches the earth as the rest is lost in space.
As energy produced at the sun's center reaches the surface, it is sent out into space as radiant energy in the form of light and heat. Once scientists believed that this heat and light came from something that was burning but today they know it comes from thermonuclear reactions in the center of the sun.
Thermonuclear reactions happen when lightweight atoms join and form heavier atoms.
Astronomers and other scientists can't measure the sun's temperature directly, of course. But they are sure they have accurately estimated the temperature at the sun's center to be about 27 million degrees Fahrenheit.
The sun's temperature is determined from indirect measurements on sunlight and from mathematical equations based on known and proven physical laws.
Scientists say the sun's temperature midway between the interior and thhe surface is about 4.5 million degrees Fahrenheit and that it drops at the surface to about 10,000 degrees Fahrenheit.
The amount of heat and light that reaches the earth from the sun stays fairly constant at all times. The changes in brightness that seem to take place result from weather conditions in the earth's atmosphere. These conditions affect the amount of sunlight that reaches any particular place on the earth.
The sun controls the orbits of the planets through the force of gravity. Because the sun is so large, the force of gravity on it is much greater than on any other of the planets.
The force of the sun's gravity also pulls the gases toward the center of the sun. If there were nothing to balance the force of gravity on the sun, it would collapse.
But the sun does not collapse because its gases ae extremely hot. Hot gases have high pressure and try to expand. The pressure of the gases balances the force of gravity and, as a result, the sun keeps its size and shape.
All life on earth depends on the light and heat from the sun. The earth's atmosphere helps trap the heat but lets sunlight through to the surface. The light warms the earth but the heat it creates cannot pass easily through the atmosphere into space. As a result, the earth is warmed by the sun.