Question

1. how can astronomers tell whether there is an unseen second star system?

a. they observe the effects of its gravity on the brighter star.
b. they measure the parallax of the second star.
c. they send a probe to the second star.
d. they observe regular changes in the brightness of the star

Explanation:

When there is an unseen second star in a system, astronomers can observe the effects of its gravity on the brighter star. Parallax is used for distance - measuring of visible objects, sending a probe is not feasible for most stars, and observing brightness changes (like in eclipsing binaries) is a different method (not relevant here as the question is about an unseen star's presence through gravity effects).

Answer:

a. They observe the effects of its gravity on the brighter star.