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.

　　1. The primary purpose of the passage is to

　　(A) illustrate⁷ 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 prospects⁸ 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 devoid⁹ 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 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

　　(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 ambiguity¹² in Fabian's research findings

　　(B) illustrate a generalization¹³ 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 wavelengths¹⁵ 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 emanating¹⁶ from an explosion will be hotter the more distant it is from the origin.

　　(B) The wavelength¹⁴ of radiation emitted by a gas as it cools remains¹⁷ 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 density¹⁸ 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 initially¹⁹ because technology was not available to confirm its tenets.

　　(D) It supports earlier speculation²⁰ 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