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Difficulty: HardCommand of Evidence: Textual

Astrophysicist Dr. Niloufar Esfandiari and colleagues investigated the rotational dynamics of middle-aged main-sequence stars (stars of similar mass and evolutionary stage to the Sun). According to standard models of stellar evolution, magnetic braking—the process by which a star's magnetic field interacts with its stellar wind to shed angular momentum—continues steadily throughout a star's main-sequence lifetime, causing a continuous decline in rotation speed and magnetic activity as the star ages. However, Esfandiari and her team hypothesized that once stars reach middle age, their magnetic braking efficiency drops sharply, resulting in a state where rotation rates and magnetic activity levels remain relatively stable thereafter.

Which finding from the team's observations, if true, would most directly support the team's hypothesis?

  1. A
    High-resolution observations show that middle-aged stars with faster rotation rates tend to possess stronger magnetic fields than those with slower rotation rates.
  2. Main-sequence stars that are billions of years older than the Sun exhibit rotation rates and magnetic activity levels that are nearly identical to those of the Sun.Answer
  3. C
    A study of stellar winds reveals that the density of particles ejected from stars decreases gradually and predictably as the stars progress from youth to middle age.
  4. D
    Stellar models that incorporate the new hypothesis suggest that stars with stabilized rotation rates are more likely to support close-in planets with stable magnetospheres.

Answer

The finding that main-sequence stars billions of years older than the Sun exhibit rotation rates and magnetic activity levels that are nearly identical to those of the Sun.
The finding that older stars have rotation and magnetic activity levels identical to the Sun's (a middle-aged star) shows that the continuous decline has stopped, confirming the hypothesis that magnetic braking efficiency drops at middle age to produce stable rotation rates.

Step-by-Step Solution

1
Identify the team's hypothesis from the text.
The hypothesis is that once stars reach middle age, their magnetic braking efficiency drops sharply, causing their rotation rates and magnetic activity levels to remain relatively stable thereafter.
Understanding the specific claim is necessary to evaluate which finding supports it.
2
Determine what kind of evidence would support this hypothesis.
Evidence showing that older stars do not continue to slow down or lose magnetic activity compared to middle-aged stars (such as the Sun).
This establishes the criteria for the correct answer.
3
Evaluate the choices to find the one that fits the criteria.
The finding that stars billions of years older than the Sun have rotation and magnetic activity levels nearly identical to the Sun's shows that these properties stabilized after middle age, supporting the hypothesis.
This identifies the correct choice by matching the evidence to the hypothesis.

Key Concept

Identifying textual evidence that supports a scientific claim.
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Topics

Command of Evidence: Textual

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