NASA image release June 22, 2012
Like many of the most famous objects in the sky, globular cluster Messier 10 was of little interest to its discoverer. Charles Messier, the 18th century French astronomer, cataloged over 100 galaxies and clusters, but was primarily interested in comets. Through the telescopes available at the time, comets, nebulae, globular clusters and galaxies appeared just as faint, diffuse blobs and could easily be confused for one another.
Only by carefully observing their motion — or lack of it — were astronomers able to distinguish them: comets move slowly relative to the stars in the background, while other more distant astronomical objects do not move at all…..(etc)