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: NGC 6618 in Sagittarius is yet another fine example of a large, bright and impressive emission nebula. Most commonly referred to as the Swan Nebula, it is also referred to as the Omega Nebula, the Horseshoe Nebula as well as the Lobster Nebula. This large expanse of H-II emission lies approximately 5000 to 6000 light-years away and spans another 40 light-years across. Similar to other emission nebulae, star formation is characteristic of the Swan Nebula but with only about 35 such new stars being visible and embedded within the nebulosity. It is believed the greatest proportion of such newborn stars are hidden behind the nebula and which help in its illumination. The Swan nebula was first detected by Philippe Loys de Cheseaux in 1745 and later by Messier in 1764.