GRE考试

【单选题】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-type star or a cluster of such stars in a giant molecular cloud, around which 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, 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 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 coron
A、Far from its source of energy, the coronal gas slowly starts to cool and condense into clouds. Over eons, these clouds fall back to the 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 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.　　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 power for the interstellar medium. The loop above theCassiopeia superbubble1looks 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
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

网考网参考答案：E
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