40. To tell the difference between a common issue and a rare
variety, a collector needs ____.
A. much knowledge of
history
B. more knowledge of history
C. more knowledge than what appears on stamps
D. techniques to use a magnifying glass
B
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, that is, the center.
In the core of the sun, the pressures are so great against the gases that, in
spite of 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.
Scientists do know that
the sun is divided into five layers. Staring at the outside and going down into
the sun, the layers are the corona, chromosphere, photosphere, convection zone,
and finally the core. The first three layers 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 body of the sun begins.
The sun’s first layer
from the outside 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 the eclipse such as the
one in March, 1997. at any other time, the corona can be seen only when special
parts are used on cameras and telescopes to shut out the burning light of the
sun’s rays.
The corona rays are
made up of gases streaming outward at great 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 rays reach the earth,
they are weak and invisible.