Nebulae represent clouds of gas and dust which appear as hazy or fuzzy objects when viewed through a telescope and are characterized as one of four types (emission, planetary, reflection or dark). Emission nebulae, such as the Lagoon nebula (M8), simply glow, for example, with a stunning shade of red. Planetary nebulae appear as small greenish disks through a telescope, thus emulating the planets Uranus and Neptune, as a result of gas masses being thrown off by dying stars (ex. M27, Dumbbell nebula) or represent supernova remnants (ex. M1, Crab nebula). In contrast, reflection nebulae are characterized with gas surrounding young stars which reflect the stellar light (ex. M45, Pleiades) and, thus, yield beautiful images of nebulosity. Finally, dark nebulae are detectable and studied only using parts other than the visible spectrum and are believed to be associated with the formation of stars (ex. M16 in Serpens).

Note: Perhaps the most unusual entry within the Messier catalog is M40, a double star system in Ursa Major which is easily resolved using even modest telescopes thanks to a separation of 0.8 arc-minutes between the primary and secondary components. More commonly known as Winnecke 4, the primary star is about 510 light-years away with the two members characterized with magnitudes of 9.64 (SAO 28353) and 10.09 (SAO 28355), respectively. It is strange as to what may have prompted Charles Messier to include this entry in his catalog since the two galaxies to the west of the double star system are quite faint (NGC 4290, mag 12.0, 2.2x1.6'; NGC 4284/PGC 39775, mag 14.7, 2.5x1.2') to have been visible through Messier's modest equipment (the third faint galaxy in the image below, PGC 39934, is even more prohibitive at mag 17.4 and which measures only 0.7x0.2'). Since Messier was following up on a reported nebula in the area, his entry may have been more for precautionary measures.