单选题
The temperature of the sun is over 5, 000 degrees Fahrenheit at the surface, but it rises to perhaps more than 16 million degrees at the center. The sun is so much hotter than the earth that matter can exist only as a gas, except at the core. In the core of the sun, the pressures are so great against the gases that, despite the high temperature, there may be a small solid core. However, no one really knows, since the center of the sun can never be directly observed. Solar astronomers do know that the sun is divided into five layers or zones. Starting at the outside and going down into the sun, the zones are the corona, chromosphere, photosphere, convection zone and finally the core. The first three zones are regarded as the sun's atmosphere. But since the sun has no solid surface, it is hard to tell where the atmosphere ends and the main body of the sun begins. The sun's outermost layer begins about 10, 000 miles above the visible surface and goes outward for millions of miles. This is the only part of the sun that can be seen during an eclipse such as the one in February 1979. At any other time, the corona can be seen only when special instruments are used on cameras and telescopes to shut out the glare of the sun's rays. The corona is a brilliant, pearly white, filmy light, about as bright as the full moon. Its beautiful rays are a sensational sight during an eclipse. The corona's rays flash out in a brilliant fan that has wispy spikelike rays near the sun's north and south poles. The corona is thickest at the sun's equator. The corona rays are made up of gases streaming outward at tremendous speeds and reaching a temperature of more than 2 million degrees Fahrenheit. The rays of gas thin out as they reach the space around the planets. By the time the sun's corona rays reach the Earth, they are weak and invisible.
单选题
Matter on the sun can exist only in the form of gas because of the sun's______.