Messier 4 (M4) – the NGC 6121 globular cluster

During the late 18th century, Charles Messier began to notice that a series of "nebulous" objects in the night sky that he originally mistook for comets. In time, he would notice that they were in fact something significantly different. With the hope of preventing other astronomers from making the same mistake, he began compiling a list of these in what would come to be known as the Messier Catalog.