The discoveries of the white dwarf, the neutron star, and the black

The best answer is D. The central idea of the passage is that the final possible stage in the gravitational collapse of a star, a black hole, produces a state of affairs in which' 'widely accepted theories predict. . . improbable things" (lines 44-45). This situation, the author points out, "may well present physicists with their greatest challenge" since the failure of classical physics (lines 5-6) and' 'poses a fundamental challenge to physics" (lines 43-44). The idea of the challenge is reinforced by the analogy drawn between this situation in physics and the atomic structure paradox of the 1930s. Thus the whole purpose of the passage is to explain the process of gravitational collapse and to suggest how this challenges accepted theories in physics.



The best answer is A because lines 12-16 indicate that all stars with a mass less than 1.4 times the mass of our sun will collapse into white dwarfs. As our sun fits this condition, it must eventually collapse into a white dwarf.



The best answer is C because lines 7-11 indicate that a disturbance in the balance between outward nuclear radiation and stable inward gravitational force occurs "after all of the hydrogen and helium fuel has been burned." Since "slow contraction begins" after the fuel is consumed, and since the gravitational force is stable, the imbalance must be caused by a lessening in the outward radiation pressure.



The best answer is B. Lines 1-6 state that the discoveries of the white dwarf, the neutron star, and the black hole are among the most exciting developments in recent physics and " may well present physicists with their greatest challenge since the failure of classical mechanics." This "challenge" is explained in the rest of the passage, where it is described as "fundamental" (line 43), as being of a magnitude that was last seen in physics "in the 1930's" (lines 48-49), and as conceivably leading to an advance the equivalent of the development of quantum mechanics (lines 57-61). Thus the author considers the discoveries significant because they pose problems of a magnitude that physics has not seen since the 1930s.



The best answer is E. Question I is answered in lines 21-23, where it is stated that in the case of neutron stars, gravitational compression continues until a density of 10 tons per cubic inch is reached. Question II is answered in lines 7-11, where it is stated that stars begin to contract after all hydrogen and helium fuel has been burned. Question III has three answers, depending on what stage a star is in. First, outward nuclear radiation resists gravitational collapse (lines 8-10); next the white dwarf plasma resists gravitational collapse (lines 18-19), and finally, the strong nuclear force resists gravitational collapse (lines 21-24). Thus all three questions are answered in the passage and E is the correct choice.



The best answer is C. In lines 44-48 the author introduces the discussion of the atomic structure paradox by noting that when improbable things happen it means that physicists "are missing some vital insight." The author then states that' 'This last happened in physics in the 1930s, when we faced a fundamental paradox concerning atomic structure" (lines 48- 50). The author closes the discussion by suggesting "that an equivalent advance awaits us" (line 59) in astrophysics. Together these imply that the author sees an analogy between the atomic structure paradox and the gravitational collapse problem in astrophysics, and anticipates that the second will be solved in the same way as the first, by a new vital insight.



The best answer is D. Two paradoxes are described in the passage, the atomic structure problem of the 1930s and the gravitational collapse problem, and both are depicted as calling attention to problems in existing scientific theories. In lines 57-58 the author notes that work in atomic structure physics focused on the inconsistencies of the existing theories, and in lines 44-48 that the discoveries of such paradoxical phenomena as infinite density and infinitely small dimensions tell physicists that "some vital insight" is missing within the existing theory. Thus in both cases, the paradoxes serve to point out problems with existing theories and to draw the attention of physicists toward solving them.