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Difficulty: MediumMain Idea and Primary Purpose

For much of the nineteenth century, astronomers overwhelmingly viewed spiral nebulae as localized, embryonic star systems coalescing within the boundaries of the Milky Way. This prevailing paradigm relied heavily on visual observations through low-resolution telescopes, which rendered these celestial features as faint, diffuse clouds of interstellar gas. However, the introduction of high-precision astronomical spectroscopy in the early twentieth century destabilized this consensus. When Vesto Slipher conducted systematic radial velocity measurements of spiral nebulae between 1912 and 1917, his spectroscopic data revealed extraordinary Doppler shifts indicating velocities far exceeding those of any known stars within our galaxy. Rather than interpreting these anomalous speeds as mere observational errors—as several prominent contemporaries initially urged—Slipher suggested that these objects were independent galactic systems moving rapidly away from our own. While Slipher’s findings did not immediately settle the debate over the scale of the universe, they fundamentally reconfigured the theoretical framework of observational astronomy by forcing astrophysicists to decouple nebular motion from internal galactic dynamics, thereby paving the way for Edwin Hubble’s definitive distance determinations a decade later.

Based on the passage, which of the following statements accurately describe the primary purposes of the author? Select all that apply.

  1. To trace how novel spectroscopic measurements challenged an established astronomical consensus regarding spiral nebulae.Answer
  2. B
    To argue that Slipher's spectroscopic techniques were methodology superior to Hubble's later distance determinations.
  3. To explain how empirical findings led astrophysicists to reevaluate the relationship between spiral nebulae and internal galactic dynamics.Answer
  4. D
    To demonstrate that low-resolution nineteenth-century telescopes were inherently incapable of yielding valid scientific insight.
  5. E
    To condemn early twentieth-century astronomers for actively suppressing revolutionary discoveries in astrophysics.

Answer

The primary purposes of the passage are to trace how novel spectroscopic measurements challenged the consensus surrounding spiral nebulae and to explain how empirical findings prompted astrophysicists to reevaluate the relationship between nebular motion and galactic dynamics.
The correct options accurately capture the central thesis of the passage. The text details how Slipher's spectroscopic radial velocity data challenged the older view that spiral nebulae were local features within the Milky Way, leading astrophysicists to separate nebular motion from internal galactic dynamics.

Step-by-Step Solution

1
Analyze the structural arc of the passage
The text begins by identifying a historical consensus (nebulae as internal Milky Way structures), introduces new empirical evidence (Slipher's spectroscopic velocity measurements), and explains the theoretical shift that resulted (decoupling nebular movement from internal galactic dynamics).
Determining the overall passage structure reveals the author's primary intent.
2
Evaluate candidate main idea statements against passage evidence
The statement highlighting the challenge posed by spectroscopic measurements aligns directly with the central narrative transition. Similarly, the statement regarding the reevaluation of nebular relationship to galactic dynamics reflects the passage's concluding synthesis.
Main idea claims must encompass the central thesis without narrowing in on minor details or distorting the author's academic tone.
3
Eliminate incorrect distractors based on scope, extrapolation, and tone errors
Statements claiming methodological superiority, total invalidity of earlier instruments, or active suppression introduce exaggerated claims and unwarranted extrapolations not supported by the text.
Distractors often take qualified historical claims and turn them into absolute or exaggerated assertions.

Key Concept

Main Idea and Primary Purpose
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Main Idea and Primary Purpose

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