GMAT综合阅读精解之三十一

2022-05-18 20:05:28

  

  Until recently most astronomers believed that the

  space between the galaxies in our universe was a near-

  perfect vacuum. This orthodox view of the universe is

  now being challenged by astronomers who believe that a

  (5) heavy “rain” of gas is falling into many galaxies from

  the supposedly empty space around them. The gas

  apparently condenses into a collection of small stars,

  each a little larger than the planet Jupiter. These stars

  vastly outnumber the other stars in a given galaxy. The

  (10) amount of “intergalactic rainfall” into some of these

  galaxies has been enough to double their mass in the

  time since they formed. Scientists have begun to suspect

  that this intergalactic gas is probably a mixture of gases

  left over from the “big bang” when the galaxies were

  (15) formed and gas was forced out of galaxies by supernova

  explosions.

  It is well known that when gas is cooled at a constant

  pressure its volume decreases. Thus, the physicist Fabian

  reasoned that as intergalactic gas cools, the cooler gas

  (20) shrinks inward toward the center of the galaxy. Mean-

  while its place is taken by hotter intergalactic gas from

  farther out on the edge of the galaxy, which cools as it is

  compressed and flows into the galaxy. The net result is a

  continuous flow of gas, starting as hot gases in inter-

  (25) galactic space and ending as a drizzle of cool gas called a

  “cooling flow,” falling into the central galaxy.

  A fairly heretical idea in the 1970’s, the cooling-flow

  theory gained support when Fabian observed a cluster

  of galaxies in the constellation Perseus and found the

  (30) central galaxy, NGC 1275, to be a strange-looking object

  with irregular, thin strands of gas radiating from it.

  According to previous speculation, these strands were

  gases that had been blown out by an explosion in the

  galaxy. Fabian, however, disagreed. Because the strands

  (35) of gas radiating from NGC 1275 are visible in optical

  photographs, Fabian suggested that such strands consisted

  not of gas blown out of the galaxy but of cooling flows

  of gas streaming inward. He noted that the wavelengths

  of the radiation emitted by a gas would changes as the

  (40) gas cooled, so that as the gas flowed into the galaxy and

  became cooler, it would emit not x-rays, but visible light,

  like that which was captured in the photographs. Fabian’s

  hypothesis was supported by Canizares’ determination in

  1982 that most of the gas in the Perseus cluster was at a

  (45) temperature of 80 million degrees Kelvin, whereas the

  gas immediately surrounding NGC 1275 (the subject of

  the photographs) was at one-tenth this temperature.

  1. The primary purpose of the passage is to

  (A) illustrate a hypothesis about the origin of galaxies

  (B) provide evidence to dispute an accepted theory

  about the evolution of galaxies

  (C) summarize the state of and prospects for research in

  intergalactic astronomy

  (D) report new data on the origins of intergalactic gas

  (E) reconcile opposing views on the formation of

  intergalactic gas

  2. The author uses the phrase “orthodox view of the

  universe” (line 3) to refer to the belief that

  (A) the space between the galaxies is devoid of matter

  (B) the space between galaxies is occupied by stars that

  cannot be detected by optical photographs

  (C) galaxies have decreased in mass by half since their

  formation

  (D) galaxies contain stars, each the size of Jupiter, which

  form clusters

  (E) galaxies are being penetrated by gas forced out of

  other galaxies by supernova explosions.

  3. It can be inferred from the passage that, if Fabian is

  correct, gas in the peripheral regions of a galaxy cluster

  (A) streams outward into intergalactic space

  (B) is hotter than gas in the central regions of the galaxy

  (C) is composed primarily of gas left over from the big

  bang

  (D) results in the creation of unusually large stars

  (E) expands to increase the size of the galaxy

  4. The author of the passage probably mentions Canizares’

  determination in order to

  (A) clarify an ambiguity in Fabian’s research findings

  (B) illustrate a generalization about the temperature of

  gas in a galaxy cluster

  (C) introduce a new argument in support of the orthodox

  view of galaxies

  (D) provide support for Fabian’s assertions about the

  Perseus galaxies

  (E) provide an alternate point of view concerning the

  movement of gas within a galaxy cluster

  5. According to the passage, Fabian believes that gas

  flowing into a central galaxy has which of the following

  characteristics?

  (A) It is one-tenth hotter than it was in the outer regions

  of the galaxy cluster.

  (B) It emits radiation with wavelengths that change as

  the gas moves toward the center of the galaxy.

  (C) The total amount of radiation emitted diminishes as

  the gas cools.

  (D) It loses 90 percent of its energy as it moves to the

  center of the galaxy.

  (E) It condenses at a rate much slower than the rate of

  decrease in temperature as the gas flows inward.

  6. According to the passage, Fabian’s theory makes use of

  which of the following principles?

  (A) Gas emanating from an explosion will be hotter the

  more distant it is from the origin.

  (B) The wavelength of radiation emitted by a gas as it

  cools remains constant.

  (C) If pressure remains constant, the volume of a gas

  will decrease as it is cooled.

  (D) The volume of a gas will increase as the pressure

  increases.

  (E) As gas cools, its density decreases.

  7. It can be inferred from the passage that which of the

  following is true of Fabian’s theory?

  (A) It did not receive approval until Canizares’ work

  was published.

  (B) It was not widely accepted in the 1970’s.

  (C) It did not receive support initially because

  technology was not available to confirm its tenets.

  (D) It supports earlier speculation that intergalactic gas

  was largely the result of explosions outside the

  galaxy.

  (E) It was widely challenged until x-ray evidence of gas

  temperatures in NGC 1275 had been presented.

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