Here's why:
* Parallax is the apparent shift in a star's position against the background of more distant stars as Earth orbits the Sun.
* The larger the parallax angle, the closer the star is.
* The smaller the parallax angle, the further the star is.
When a parallax angle becomes too small to measure with current technology, it means the star is so distant that the apparent shift in its position is too slight to detect. This is why astronomers use increasingly powerful telescopes and sophisticated measurement techniques to determine the distances to more and more distant stars.
