In addition to conventional galaxies, the universe contains

Official Answers and Explanations

1. According to the passage, conventional spiral galaxies differ from low-surfacebrightness
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-surface-brightness galaxies.
C. They are more diffuse than low-surface-brightness galaxies.
D. They contain less helium than do low-surface-brightness galaxies.
E. They are larger than low-surface-brightness galaxies.
Supporting ideas
This question requires recognizing information that is provided in the passage.
The first paragraph describes and compares two types of galaxies: conventional galaxies and dim, or low-surface-brightness, galaxies. It states that dim galaxies have the same approximate number of stars as a common type of conventional galaxy but tend to be larger and more diffuse because their mass is spread over wider areas . The passage also indicates that dim galaxies take longer than conventional galaxies to convert their primordial gases into stars, meaning that dim galaxies evolve much more slowly than conventional galaxies , which entails that conventional galaxies evolve more quickly than dim galaxies.
A. The passage states that dim galaxies have approximately the same numbers of stars as a common type of conventional galaxy.
B. Correct. The 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 diffuse, 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.

2. 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 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.
Inference
This question requires drawing an inference from information given in the passage. The first paragraph compares dim galaxies and conventional galaxies. Dim galaxies are described as having the same general shape (lines 4–5) as a common type of conventional galaxy, the spiral galaxy, suggesting that dim galaxies are, themselves, spiral shaped. The passage also indicates that, although
both types of galaxies tend to have approximately the same number of stars, dim galaxies tend to be much larger and spread out over larger areas of space than conventional galaxies.
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. The 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 diffuse.
The correct answer is C.

3. It can be inferred from the passage that the “long-standing 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 galaxiesD. 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
Inference
This question requires drawing an inference from information given in the passage. The second paragraph describes the long-standing puzzle of the missing baryonic mass in the universe. The passage states that baryons are the source of galactic luminosity, and so scientists can estimate the amount of baryonic mass in the universe by measuring the luminosity of galaxies (lines 17–21). The puzzle is that spectroscopic measures of helium in the universe suggest that the baryonic mass in the universe is much higher than measures of luminosity would indicate.
A. The differences between the rates of evolution of the two types of galaxies is not treated as being controversial in the passage.
B. Correct. The passage indicates that measurements using spectroscopy and measurements using luminosity result in puzzling differences 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 affects scientists’ ability to detect these galaxies.
The correct answer is B.

4. 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 firstobserved
E. they contain stars that are significantly more luminous than would have been predicted on the basis of initial studies of luminosity in low-surfacebrightness galaxies
Inference
This question requires drawing an inference from information given in the passage. The puzzle is that estimates of the baryonic mass of the universe based on luminosity are lower than those based on spectroscopy . The passage states that astronomers did not notice dim galaxies until recently and that these galaxies may help account for the missing baryonic mass in the universe . The passage also suggests that astronomers measure the luminosity of specific galaxies . Thus it can be inferred that, prior to their being noticed by astronomers, the luminosity of these dim galaxies was not measured, and their baryonic mass was not taken into account in the estimates of luminosity that led to the long-standing puzzle.
A. Correct. The 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 long-standing 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.

5. 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 heliumE. how astronomers know bright galaxies contain more baryons than do dim galaxies
Evaluation
This question requires understanding how one aspect of the passage relates to the reasoning in a larger portion of the passage. The second paragraph explains that scientists have been puzzled over missing baryonic mass in the universe as measured by luminosity (lines 21–25). Given that baryons are the source of luminosity in the galaxy (lines 17–19), astronomers can estimate the baryonic mass of a galaxy by measuring its luminosity.
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. The 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.

6. 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.
Inference
This question involves identifying which answer option potentially conflicts with the information the author has provided in the passage. The second paragraphindicates that astronomers’ estimates of the baryonic mass of the universe is lower when measured using luminosity than it is when measured using spectroscopy . The final sentence states that astronomers have speculated that the missing baryonic mass might be discovered in intergalactic space or in hard-to-detect galaxies (lines 25–29). Although the passage does indicate that the discovery of dim, low-surface-brightness galaxies might help account for the missing baryonic mass (lines 15–17), the passage provides no support for the possibility that baryonic mass has been discovered in intergalactic space.
A. The passage indicates that low-surface-brightness 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 low-surface-brightness galaxies.
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. The passage does not indicate that astronomers have found any baryonic mass in intergalactic space.
The correct answer is E.

7. The primary purpose of the passage is to
A. describe a phenomenon and consider its scientific 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
Main idea
This question requires understanding, in broad terms, the purpose of the passage as a whole. The first paragraph describes a phenomenon: the discovery of dim galaxies and some of their general attributes. The second paragraph describes how this discovery may help astronomers to solve a long-standing puzzle about the baryonic mass of the universe.
A. Correct. The passage describes the phenomenon of dim galaxies and describes their significance in solving the long-standing 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 identifies the newly discovered phenomenon of dim galaxies, it does not offer a significant explanation for these galaxies’ origins.
D. Although the passage compares dim and conventional galaxies in the first 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.