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论文作者:佚名论文属性:短文 essay登出时间:2009-09-24编辑:lisa点击率:14170
论文字数:1000论文编号:org200909241539166996语种:英语 English地区:中国价格:免费论文
关键词:托福试题
Question 9-19
Stars may be spheres, but not every celestial object is
spherical. Objects in the universe show a variety of shapes:
round planets (some with rings), tailed comets, wispy cosmic
gas and dust clouds, ringed nebulae, pinwheel-shaped spiral
galaxies, and so on. But none of the shapes on this list describes
the largest single entities in the universe. These are the
double radio sources, galaxies with huge clouds of radio emission
that dwarf the visible galaxies, sometimes by a factor of a
hundred or more. Stretching over distances greater than a million
light-years, these radio-emitting regions resemble twin turbulent
gas clouds, typically forming dumbbell-like shapes with
the visible galaxy (when it is visible) in the center.
These double radio sources present astronomers with a
puzzle. Their radio emission arises from the synchrotron
process, in which electrons accelerated to nearly the speed of light
move through magnetic fields. However, in view of the rate at
which the radio sources emit energy, they should disappear in
a few million years as their electrons slow down and cease
producing radiation. Somehow new electrons must be continually
accelerated to nearly the speed of light, otherwise, by now
almost none of the double radio sources would be observed.
With the advent of high-resolution radio interferometers
during the late 1970's, part of the answer became clear: the
electrons are produced in jets that are shot out in opposite
directions from the center of galaxy. Remarkably narrow and
highly directional, the jets move outward at speeds close to the
speed of light. When the jets strike the highly rarefied gas
that permeates intergalactic space, the fast-moving electrons
lose their highly directional motion and form vast clouds of
radio-emitting gas.
Cosmic jets have ranked among the hottest topics of
astronomical research in recent years as astronomers strive to
understand where they come from. Why should a galaxy eject
matter at such tremendous speeds in two narrow jets? And
why are such jets not seen in the Milky Way?
9. The word "celestial" in line 1 could best be replaced by
(A) visible
(B) astronomical
(C) glowing
(D) scientific
10. The word "entities" in line 4 is closest in meaning to
(A) factors
(B) processes
(C) objects
(D) puzzles
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