托福阅读练习4.doc

Passage 1:
Stars may be spheres, but not every celestial object is spherical. Objects in the universe show a variety of shapes: round s (some with rings), ets,
wispy cosmic gas and dust clouds, ringed nebulae, pinwheel-shaped spiral galaxies,
Line and so on. But none of the shapes on this list describes the largest single entities in the
(5) 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.
(10) 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 ic 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 case producing radiation. Somehow new electrons must be continually
(15) 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 a galaxy. Remarkably narrow and highly directional,
(20) 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 e from. Why should a galaxy
(25) eject mat