ἀπλανής (ἀστήρ)

"Fixed star" is another term for "star" but does not leave the freedom to be used metaphorically.

Astrophysics defines stars as hot gas balls that emit energy that they produce by nuclear fusion. These stars are similar to the Sun but in much larger distances from us. Therefore, they appear in the sky rather "fixed". Their positions relative to each other don't change but form the same patterns (constellations) for millennia. The stability of these patterns was systematically proven by Hipparchus and again verified by Ptolemy (Alm, VII, 1-3).

Modern astrophysics is able to measure the tiny shifts of the stars but their proper motions typically range in the order milli-arcseconds per year (or arcseconds per millennium) which explains that the shift have not been observed in antiquity.