Charles Messier

Many deep sky objects are named by numbers prefaced with an "M." What does the "M" stand for?

The "M" stands for "Messier," and the number refers to the Messier catalog of comet-like objects. As explained in Starstruck Tonight:

In the late 18th century, Charles Messier (MESS-ee-ay) catalogued all the cloudy patches he could find in the sky so that he would not mistake them for comets.

A cloudy patch in Lyra was the 57th nebula listed in Messier's catalog. M57, now known as the Ring Nebula, appears like a little smoke ring peacefully wafting through the starry night. However, this doughnut of glowing hydrogen gas, speaks of the violent explosion of the outer layers of a once massive star. Near the center of the ring, only its hot bluish core remains intact.