Stellar parallax is observable for stars that are relatively nearby. Which of the following best captures this idea?

• A. Nearby
• B. Distant
• C. Moving rapidly
• D. In the same direction as the Milky Way center

Answer: (A)

Parallax is the apparent shift in a star’s position caused by Earth's motion around the Sun, so the size of the shift depends on how far away the star is. The closer a star is, the larger the angular shift we observe; for distances in parsecs, the parallax angle is roughly 1 divided by the distance. That means nearby stars show measurable parallax, while distant stars have parallax angles that are too small to detect with ordinary measurements. So the best description is that the observable parallax applies to stars that are nearby. The other ideas don’t fit: distant stars don’t show a detectable parallax with typical methods, parallax is about geometric viewing shift rather than a star’s actual motion, and the direction relative to the Milky Way center is irrelevant to whether parallax is observable.