In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers. Possibly as numerous as conventional galaxies, these galaxies have the same general shape and even the same approximate number of stars as a common type of conventional galaxy, the spiral, but tend to be much larger. Because these galaxies’ mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies. Apparently these low-surface-brightness galaxies, as they are called, take much longer than conventional galaxies to condense their primordial gas and convert it to stars—that is, they evolve much more slowly.
These galaxies may constitute an answer to the long-standing puzzle of the missing baryonic mass in the universe. Baryons—subatomic particles that are generally protons or neutrons—are the source of stellar, and therefore galactic, luminosity, and so their numbers can be estimated based on how luminous galaxies are. However, the amount of helium in the universe, as measured by spectroscopy, suggests that there are far more baryons in the universe than estimates based on
galactic luminosity indicate. Astronomers have long speculated that the missing baryonic mass might eventually be discovered in intergalactic space or as some large population of galaxies that are difficult to detect.
Questions 91–97 refer to the passage above.
91. According to the passage, conventional spiral galaxies differ from low-surface-brightness galaxies in which of the following ways?
(A) They have fewer stars than do low-surface brightness galaxies.
(B) They evolve more quickly than low-surfacebrightness galaxies.
(C) They are more diffuse than low-surfacebrightness galaxies.
(D) They contain less helium than do low-surfacebrightness galaxies.
(E) They are larger than low-surface-brightness galaxies.
91. A The passage states that dim galaxies have approximately the same numbers of stars as a common type of conventional galaxy.
B Correct. Th e passage indicates that dim galaxies evolve much more slowly than conventional galaxies, which entails that conventional galaxies evolve more quickly.
C The passage states that dim galaxies are more spread out, and therefore more diff use, than conventional galaxies.
D The passage does not mention the relative amounts of helium in the two types of galaxies under discussion.
E The passage states that dim galaxies tend to be much larger than conventional galaxies.
The correct answer is B.
92. It can be inferred from the passage that which of the following is an accurate physical description of typical low-surface-brightness galaxies?
(A) They are large spiral galaxies containing fewer stars than do conventional galaxies.
(B) They are compact but very dim spiral galaxies.
(C) They are diffuse spiral galaxies that occupy a large volume of space.
(D) They are small, young spiral galaxies that contain a high proportion of primordial gas.
(E) They are large, dense spirals with low luminosity.
92. A The passage states that the two types of galaxies have approximately the same number of stars.
B The passage indicates that dim galaxies are relatively large and spread out.
C Correct. Th e passage indicates that dim galaxies have the same general shape as spiral galaxies and that their mass is spread out over large areas of space.
D The passage indicates that dim galaxies are relatively large and spread out.
E The passage states that dim galaxies have few stars per unit of volume, suggesting that they are not dense but diff use.
The correct answer is C.
93. It can be inferred from the passage that the “longstanding puzzle” refers to which of the following?
(A) The difference between the rate at which conventional galaxies evolve and the rate at which low-surface-brightness galaxies evolve
(B) The discrepancy between estimates of total baryonic mass derived from measuring helium and estimates based on measuring galactic luminosity
(C) The inconsistency between the observed amount of helium in the universe and the number of stars in typical low-surface-brightness galaxies
(D) Uncertainties regarding what proportion of baryonic mass is contained in intergalactic space and what proportion in conventional galaxies
(E) Difficulties involved in detecting very distant galaxies and in investigating their luminosity
93. A Th e diff erences between the rates of evolution of the two types of galaxies is not treated as being controversial in the passage.
B Correct. Th e passage indicates that measurements using spectroscopy and measurements using luminosity result in puzzling diff erences in estimates of the universe’s baryonic mass.
C The passage does not suggest how helium might relate to the numbers of stars in dim galaxies.
D The passage indicates that astronomers have speculated that the missing baryonic mass might be discovered in intergalactic space or hard-to-detect galaxies, but does not suggest that these speculations are constituents of the long-standing puzzle.
E The passage does not mention how the distance to galaxies aff ects scientists’ ability to detect these galaxies.
The correct answer is B.
94. The author implies that low-surface-brightness galaxies could constitute an answer to the puzzle discussed in the second paragraph primarily because
(A) they contain baryonic mass that was not taken into account by researchers using galactic luminosity to estimate the number of baryons in the universe
(B) they, like conventional galaxies that contain many baryons, have evolved from massive, primordial gas clouds
(C) they may contain relatively more helium, and hence more baryons, than do galaxies whose helium content has been studied using spectroscopy
(D) they have recently been discovered to contain more baryonic mass than scientists had thought when low-surface-brightness galaxies were first observed
(E) they contain stars that are signifi cantly more luminous than would have been predicted on the basis of initial studies of luminosity in lowsurface-brightness galaxies
94. A Correct. Th e passage states that the missing baryonic mass in the universe may be discovered in the dim galaxies that have only recently been noticed by astronomers.
B The passage does not suggest that dim and conventional galaxies both originating from primordial gas clouds help solve the longstanding puzzle of the missing baryonic mass in the universe.
C The passage does not suggest that dim galaxies might contain more helium than do conventional galaxies, or that measures of baryonic mass using spectroscopy do not take some dim galaxies into account.
D The passage does not suggest that dim galaxies contain more baryonic mass than scientists originally believed upon discovering these galaxies.
E The passage suggests that scientists measured the luminosity of galaxies, not of individual stars.
The correct answer is A.
95. The author mentions the fact that baryons are the source of stars’ luminosity primarily in order to explain
(A) how astronomers determine that some galaxies contain fewer stars per unit volume than do others
(B) how astronomers are able to calculate the total luminosity of a galaxy
(C) why astronomers can use galactic luminosity to estimate baryonic mass
(D) why astronomers’ estimates of baryonic mass based on galactic luminosity are more reliable than those based on spectroscopic studies of helium
(E) how astronomers know bright galaxies contain more baryons than do dim galaxies
95. A The passage discussion of baryons does not address the number of stars in individual galaxies.
B The passage discusses how the luminosity of galaxies can be used to estimate baryonic mass, but does not address how total luminosity is measured.
C Correct. Th e passage indicates that because baryons are the source of galactic luminosity, measuring luminosity can be used to estimate baryonic mass of galaxies.
D The passage suggests that estimates based on luminosity may have been less accurate,not more accurate, than those based on spectroscopy.
E The passage does not indicate that bright galaxies contain more baryons than do dim galaxies.
The correct answer is C.
96. The author of the passage would be most likely to disagree with which of the following statements?
(A) Low-surface-brightness galaxies are more difficult to detect than are conventional galaxies.
(B) Low-surface-brightness galaxies are often spiral in shape. (
C) Astronomers have advanced plausible ideas about where missing baryonic mass might be found.
(D) Astronomers have devised a useful way of estimating the total baryonic mass in the universe.
(E) Astronomers have discovered a substantial amount of baryonic mass in intergalactic space.
96. A The passage indicates that low-surfacebrightness galaxies went unnoticed until recently, unlike conventional galaxies.
B The passage indicates that low-surface brightness galaxies have the same general shape as spiral galaxies.
C The passage describes two possible explanations astronomers have given for the missing baryonic mass, one of which was made more plausible by the discovery of
D The passage indicates that astronomers have used spectroscopy to estimate baryonic mass and gives no reason to suspect that this method is not useful.
E Correct. Th e passage does not indicate that astronomers have found any baryonic mass in intergalactic space.
The correct answer is E.
97. The primary purpose of the passage is to
(A) describe a phenomenon and consider itsscientific significance
(B) contrast two phenomena and discuss a puzzling difference between them
(C) identify a newly discovered phenomenon and explain its origins
(D) compare two classes of objects and discuss the physical properties of each
(E) discuss a discovery and point out its inconsistency with existing theory
97. A Correct. The passage describes the phenomenon of dim galaxies and describes their signifi cance in solving the longstanding puzzle of the missing baryonic mass in the universe.
B Although the passage discusses the puzzling difference between the two estimates of baryonic mass, this option does not account for the broader topic of dim galaxies.
C While the passage identifi es the newly discovered phenomenon of dim galaxies, it does not off er a signifi cant explanation for these galaxies’ origins.
D Although the passage compares dim and conventional galaxies in the fi rst paragraph, this option does not account for the important detail that dim galaxies may help solve a long-standing puzzle.
E The discovery of dim galaxies discussed in the passage is not said to be inconsistent with any existing scientific theory.
The correct answer is A.