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  3. That large regions of the galaxy can be influenced by the formation of massive stars in a few localized regions requires

That large regions of the galaxy can be influenced by the formation of massive stars in a few localized regions requires

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问题             That large regions of the galaxy can be influenced by the formation of
       massive stars in a few localized regions requires that star formation somehow be
       coordinated over long periods of time. The process commences with a single O-
Line    type star or a cluster of such stars in a giant molecular cloud, around which
(5)     stellar radiation, winds and explosions carve a modest cavity from of the
       surrounding interstellar medium,  concomitantly destroying the progenitor
       cloud. Perchance this disturbance triggers star formation in a nearby cloud, and
       so on, until the interstellar medium in this corner of the galaxy resembles Swiss
       cheese, composed of what theorists have recently begun to refer to as bubbles,
(10)    or superbubbles, one of the most exciting discoveries in astrophysics in years.
           Within a hot bubble, the sun has revealed itself in x-rays emitted by highly
       ionized trace ions such as oxygen and at some point in the process of its
       formation, nearby bubbles commence to overlap, coalescing into a superbubble.
       The energy from more and more O-type stars feeds this expanding superbubble
(15)    until its natural buoyancy stretches it from the midplane up toward the halo,
       forming a chimney-the superbubble thus becomes a pathway for hot interior
       gas to spread into the upper reaches of the galactic atmosphere, producing a
       widespread corona. Far from its source of energy, the coronal gas slowly starts
       to cool and condense into clouds. Over cons, these clouds fall back to the
(20)    galaxy’s mid-plane, completing the fountain-like cycle and replenishing the
       galactic disk with cool clouds from which star formation begins anew.
           Star formation often occurs in sporadic but intense bursts, but in the Milky
       Way the competing feedback effects almost balance out, so that stars form at an
       unhurried pace-just 10 per year on average, except in some galaxies where
(25)    positive feedback has gained the upper hand. 20 million to 50 million years ago,
       star formation in the central parts of M82 began running out of control. Our
       galaxy, too, may have had sporadic bursts, and the way these starbursts occur,
       and what turns them off must be tied to the complex relation between stars and
       the tenuous atmosphere from which they precipitate.
(30)         Progress will be made in this complicated subject as astronomers continue
       to study how the medium is cycled through stars, through the different phases
       of the medium, and between the disk and the halo. Observations of other
       galaxies give astronomers a bird’ s-eye view of the interstellar goings-on, and
       answer such a critical question as whether stars are really the main source of
(35)    power for the interstellar medium. The loop above the Cassiopeia superbubble
       looks uncomfortably similar to the prominences that arch above the surface of
       the sun, prominences which owe much to the magnetic field in the solar
       atmosphere, and scientists have begun to wonder if magnetic activity dominates
       our galaxy’s atmosphere.
According to the explanation given in the passage, which of the following causes superbubbles to gradually assume the shape of a chimney?
选项 A、The gravitational attraction of supernovas adjacent to the region of the chimney’s formation
B、The effect of entropy on the relative abundance of hot interior gas within the superbubble
C、A chemical reaction involving ionized trace ions at the heart of the superbubble
D、A tendency toward coalescence in the gases that contribute to the formation of the superbubble
E、A natural buoyancy of the superbubble, which causes the chimney to stretch and become a gaseous conduit
答案E
解析
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