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.