A guide star (GS) assists in guiding something: the term is used for identifiable stars that indicate a position in the celestial sphere, and for other assistance in observation. Such guide stars have been used throughout history for astronomical observations made for the practical benefits of knowing your position and direction, e.g., for navigation at sea. In astronomy, determination of an observed astronomical object's position involves comparisons of angular distances to objects for which positions have been established, termed guide stars. For telescopes with very small fields of view, guide stars may be necessary simply to determine whether the telescope is anywhere near pointing in the right direction. Catalogs of guide stars have been compiled, such as the Guide Star Catalog for the Hubble Space Telescope
The term guide star is also used regarding modern adaptive optics systems that adjust the optics to compensate for varying seeing issues, basing it on the apparent position of some star that is within the field and is of known position. For directions without (enough) guide stars suitable for this, lasers are used to produce a spot in the sky. Given this practice, the terms laser guide star (LGS) and natural guide star (NGS) have been coined for such laser-created spots versus actual stars.