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: The reflection nebula IC 59 at the bottom of the image below and the brighter arc-shaped emission nebula IC 63 at the upper right are two impressive nebulae which are highlighted by the bright and eruptive variable star γ-Cas (mag 2.47 but can vary from mag 1.60 to mag 3.00) lying just outside the field of view (upper left corner). Both of these nebulae are relatively dim in the visible portion of the spectrum and require narrowband signal for their structure to be brought out. IC 59 is illuminated by γ-Cas and the reflected gas leads to the bluish tone of this nebula. In contrast, ionization of gas by γ-Cas highlights the emission nature and reddish tone of IC 63. Also known as Sh2-185, both of these nebulae are part of the larger nebula complex associated with γ-Cas and subject to extensive study owing to their relatively close distance (750 light-years away) and lack of intervening dust. A recent multi-wavelength study by Karr et al found IC 59 to be slightly cooler (590° vs 630° K) and less dense (3.4 x 10¹⁷ cm^-2 vs 5.8 x 10¹⁷ cm^-2) than IC 63.