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 Crescent Nebula in Cygnus is one of the most recognized emission nebulae. Lying at a distance of 4700 light-years away and spanning 16-25 light-years across, NGC 6888 is a prime example of a Wolf-Rayet-type nebula where luminous and massive stars reaching the end of their evolutionary life shed material which is dispersed into an already existing bubble, thus leading to impressive and large-scale emission nebulae. In this particular case, the dominant blue star in the middle of the image, HIP 99546 (mag 7.50), is the star responsible for this stunning nebula. Discovered by Herschel in 1792, the Crescent Nebula lies a few degrees north of Sadr (γ-Cyg, mag 2.20) and is ideally placed during summer where it is directly overhead around midnight.