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
(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 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
(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 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 permcales 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
(25) eject matter at such tremendous speeds in two narrow jets? And why are such jets not
seen in the Milky Way?
Reading Comprehension 78
The word “celestial” in line 1 could best be replaced by
The word “entities” in line 4 is closest in meaning to
In the first paragraph, the author describes objects in the universe in terms of their
According to the passage, scientists do not fully understand why double radio sources
|cannot be observed with a telescope|
|are beginning to slow down|
|are not as big as some planets and stars|
|have not eventually disappeared|
The word “their” in line 22 refers to
According to the passage, what happens when electrons and gas collide in space?
|The electrons disperse|
|The electrons become negatively charged|
|The gas becomes less radiated|
|The gas becomes more condensed|
The author suggests that astronomers consider the study of cosmic jets to be
|an intriguing challenge|
|a subjective debate|
|an obsolete scientific field|
|an unprofitable venture|
In what lines does the passage compare the size of double radio sources with that of other galaxies?
Where in the passage does the author mention a technology that aided in the understanding of double radio sources?
The paragraph following the passage most likely discusses
|possible reasons for the presence of cosmic jets|
|the discovery of the first double radio sources.|
|an explanation of the synchrotron process|
|specific double radio sources|
There are 10 questions to complete.