Messier 51, "Whirlpool" spiral galaxy in Canes Venatici
The Origins of the Catalog
The French astronomer Charles Messier (1730 - 1817) discovered 15 new comets in his lifetime, but he is best known for the Messier Catalog - 110 deep sky objects for the small telescope.
His catalog came about as a direct result of his comet hunting; once in a while during his search Messier encountered a stationary fuzzy spot, so he started a list of these objects to be bypassed during a comet search.
Messier first published a list of 45 objects in 1771. He expanded the list to 68 objects in 1780. The final list that Messier published in 1781 included 103 objects. Six more were later brought to Messier's attention by another French astronomer, Mechain. And Messier 110, one of the companion galaxies of the Andromeda galaxy, M31, was observed by Messier but not added to his catalog until later.
Since Messier observed from France, the catalog does not contain any objects south of declination -35 degrees.
The Object Types
Some of the M objects were called "nebulae," from the Latin word for clouds. Of these, the diffuse nebulae, actually are vast clouds of hydrogen gas excited to shine by the energy of newly formed stars in their midst and/or they shine by reflected light.
Others of the nebulae are clouds surrounding stars at the end of their lives, the planetary nebulae. They form when red supergiants expel shells of gas from their bloated atmospheres. "Planetary Nebula" refers to these objects' resemblance to faint planetary disks.
By 1914 some nebulae were found to actually consist of island universes of billions of stars, like our own Milky Way - the galaxies.
The first object - M1, the Crab Nebula - is arguably the most fascinating. We now know M1 is a cloud of expanding matter surrounding a neutron star. That star is the core of a one-time supergiant star that exploded. The supernova was witnessed on Earth in 1054 AD, and recorded by Chinese astronomers. It is included with the diffuse nebulae.