These tend to be the brightest stars, or stars that form part of constellation patterns with the brightest stars.
As the resolving power of telescopes increased, numerous objects that were thought to be a single object were found to be multiple star systems that were too closely spaced in the sky to be discriminated by the human eye.
In the 19th century, the exact nature of galaxies was not yet understood, and the early catalogs such as the Messier catalog simply grouped together open clusters, globular clusters, nebulas, and galaxies.
The GuideStarCatalog I (GSC I) is an all-sky optical catalog of positions and magnitudes of approximately 19 million stars and other objects in the 6th to 15th magnitude range.
The GuideStar Photometric Catalog (GSPC-I) is an all-sky set of photoelectrically determined BV sequences in the magnitude range from 9 to 15, generally near the centers of the fields of the GSC-I plates.
GSPC II is an all-sky catalog of secondary "standard-stars" that were used to perform the photometric calibration of the GSC-II.
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