Until recently most astronomers1 believed that the space between the galaxies2 in our universe was a near- perfect vacuum. This orthodox view of the universe is now being challenged by astronomers who believe that a heavy rain of gas is falling into many galaxies from the supposedly empty space around them. The gas apparently3 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 galaxy4. The 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 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 physicist5 Fabian reasoned that as intergalactic gas cools, the cooler gas 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- galactic space and ending as a drizzle6 of cool gas called a cooling flow, falling into the central galaxy.
1. The primary purpose of the passage is to
illustrate7 a hypothesis about the origin of galaxies
provide evidence to dispute an accepted theory about the evolution of galaxies
summarize the state of and prospects8 for research in intergalactic astronomy
report new data on the origins of intergalactic gas
reconcile opposing views on the formation of intergalactic gas
2. The author uses the phrase orthodox view of the universe to refer to the belief that
the space between the galaxies is devoid9 of matter
the space between galaxies is occupied by stars that cannot be detected by optical photographs
galaxies have decreased in mass by half since their formation
galaxies contain stars, each the size of Jupiter, which form clusters
galaxies are being penetrated10 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 peripheral11 regions of a galaxy cluster
streams outward into intergalactic space
is hotter than gas in the central regions of the galaxy
is composed primarily of gas left over from the big bang
results in the creation of unusually large stars
expands to increase the size of the galaxy
4. The author of the passage probably mentions Canizares' determination in order to
clarify an ambiguity12 in Fabian's research findings
illustrate a generalization13 about the temperature of gas in a galaxy cluster
introduce a new argument in support of the orthodox view of galaxies
provide support for Fabian's assertions about the Perseus galaxies
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?
It is one-tenth hotter than it was in the outer regions of the galaxy cluster.
It emits radiation with wavelengths15 that change as the gas moves toward the center of the galaxy.
The total amount of radiation emitted diminishes as the gas cools.
It loses 90 percent of its energy as it moves to the center of the galaxy.
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?
Gas emanating16 from an explosion will be hotter the more distant it is from the origin.
The wavelength14 of radiation emitted by a gas as it cools remains17 constant.
If pressure remains constant, the volume of a gas will decrease as it is cooled.
The volume of a gas will increase as the pressure increases.
As gas cools, its density18 decreases.
7. It can be inferred from the passage that which of the following is true of Fabian's theory?
It did not receive approval until Canizares' work was published.
It was not widely accepted in the 1970's.
It did not receive support initially19 because technology was not available to confirm its tenets.
It supports earlier speculation20 that intergalactic gas was largely the result of explosions outside the galaxy.
It was widely challenged until x-ray evidence of gas temperatures in NGC 1275 had been presented
