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 6523 in Sagittarius is one of the largest, brightest and most impressive examples of an emission nebula. More commonly referred to as the Lagoon Nebula thanks to the dark curved lane which runs through the center of the nebula and just west of the small open cluster NGC 6530, this large expanse of H-II emission lies approximately 5200 light-years away spanning 140 light-years horizontally and another 60 light-years vertically. The Lagoon Nebula is effectively an active stellar nursery and where the large volumes of gas are condensing to form young stars. In fact, careful examination of the image(s) below will reveal various dark areas, commonly referred to as Bok globules, and which represent collapsing clouds of protostellar material. EE Barnard has catalogued a number of dark nebulae (B88, B89 and B296) within NGC 6523 and recent studies have discovered four Herbig-Haro objects (ex. HH 213, HH 870) in the immediate vicinity of the Hourglass Nebula (see the brightest section of the core in the image below). The nebula was first detected by Giovanni Battista Hodierna in 1654 and later by Messier and Herschel.