For near-by stars their distance is measured by parallax: the star is observed in, say, December and then in June, and the direction of the star with respect to the sun is measured in both cases. Knowing these angles and the diameter of the orbit of the Earth around the sun, one can determine the distance to the star (see Fig. 8.5).
As we look at farther and farther stars the angles measured come closer and closer to 90o. For stars more than 100 l.y. from Earth one cannot distinguish the angles from right angles and the method fails.