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 281 in Cassiopeia is an impressive emission nebula which also happens to host a stellar nursery. The central portion of the nebula in the image below is home to a young and developing open cluster and which is dominated by a bright double star system that helps illuminate the surrounding hydrogen gas. Also present are the dark lanes and globules of interstellar dust which help obscure light from the visible parts of the spectrum. This stunning nebula is quite large with an apparent diameter which exceeds the full moon and lies at a distance of 9,600 light-years away. First discovered by Barnard in 1881, this celestial gem commonly referred to as the Pacman Nebula is one of the most prominent deep-sky objects in Cassiopeia and is well-placed during late summer and fall when it is reaches its greatest altitude above the north celestial pole around midnight.