Passage 33
　　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（5） 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（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 physicist⁵ 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 drizzle⁶ 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 constellation⁷ Perseus and found the（30） central galaxy， NGC 1275， to be a strange-looking object　 with irregular， thin strands⁸ of gas radiating from it.
　　According to previous speculation⁹， 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 noted¹⁰ that the wavelengths¹¹　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） illustrate¹³ 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 prospects¹⁴ 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 devoid¹⁵ 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 penetrated¹⁶ 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 peripheral¹⁷ 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 ambiguity¹⁸ in Fabian‘s research findings
　　　（B） illustrate a generalization¹⁹ 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 emanating²⁰ from an explosion will be hotter the more distant it is from the origin.
　　　（B） The wavelength¹² of radiation emitted by a gas as it cools remains²¹ 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 density²² 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 initially²³ because technology was not available to confirm its tenets.
　　　（D） It supports earlier speculation that intergalactic gaswas largely the result of explosions outside thegalaxy.
　　　（E） It was widely challenged until x-ray evidence of gas temperatures in NGC 1275 had been presented.