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The '''Medusa nebula''' (Sharpless 2-274, Abell 21, PK 205+14 1) is a [[planetary nebula]] in the [[constellation]] of [[Gemini]].<ref name=apod>[https://apod.nasa.gov/apod/ap150612.html The Medusa Nebula] from apod.nasa.gov</ref> The nebula was discovered by Abell in 1955 and designated it number 14 in his list of planetary nebulae.<ref name=hist>Johnson, H. and Rubin, R. (1971). Observation and Classification of the Nebula YM 29. ''The Astrophysical Journal'', 163, p.151. [http://adsabs.harvard.edu/abs/1971ApJ...163..151J Bibcode:&nbsp;1971ApJ...163..151J]</ref> Later in 1966, Abell redesignated it to number 21 in a catalogue of what he consider to be "odd planetary nebulae".<ref name=hist/> It was debated for a while whether the object was in fact a [[supernova]] remnant, but measurements of its expansion velocity made in the 1970s determined it to be a planetary nebula instead. It's expansion velocity of 50&nbsp;km/s is much lower than that of supernova remnants such as the [[Crab nebula]] which is expanding at approximately 1,500&nbsp;km/s.<ref name=eso/><ref>Bietenholz, M., Kronberg, P., Hogg, D. and Wilson, A. (1991). The expansion of the Crab Nebula. ''The Astrophysical Journal'', 373, p.L59. [https://ui.adsabs.harvard.edu/#abs/1991ApJ...373L..59B/abstract Bibcode:&nbsp;1991ApJ...373L..59B]</ref>

The '''Medusa nebula''' (Sharpless 2-274, Abell 21, PK 205+14 1) is a [[planetary nebula]] in the [[constellation]] of [[Gemini]].<ref name=apod>[https://apod.nasa.gov/apod/ap150612.html The Medusa Nebula] from apod.nasa.gov</ref> The nebula was discovered by Abell in 1955 and designated as number 14 in his list of planetary nebulae.<ref name=hist>Johnson, H. and Rubin, R. (1971). Observation and Classification of the Nebula YM 29. ''The Astrophysical Journal'', 163, p.151. [http://adsabs.harvard.edu/abs/1971ApJ...163..151J Bibcode:&nbsp;1971ApJ...163..151J]</ref> Later in 1966, Abell redesignated it as number 21 in a catalogue of what he considered to be "odd planetary nebulae".<ref name=hist/> It was debated for a while whether the object was in fact a [[supernova]] remnant, but measurements of its expansion velocity made in the 1970s determined it to be a planetary nebula instead. Its expansion velocity of 50&nbsp;km/s is much lower than that of supernova remnants such as the [[Crab nebula]], which is expanding at approximately 1,500&nbsp;km/s.<ref name=eso/><ref>Bietenholz, M., Kronberg, P., Hogg, D. and Wilson, A. (1991). The expansion of the Crab Nebula. ''The Astrophysical Journal'', 373, p.L59. [https://ui.adsabs.harvard.edu/#abs/1991ApJ...373L..59B/abstract Bibcode:&nbsp;1991ApJ...373L..59B]</ref>

The nebula's name derives from [[Medusa]], one of the [[gorgon]]s in [[Greek mythology]], a creature capable to turning flesh into stone when looked upon. Medusa possessed [[snake]]s instead of hair and filamentary structures of [[gas]] can be seen to snake their way across the nebula. Though large, the nebula has a low surface brightness making it very faint and tricky to observe.

The nebula's name derives from [[Medusa]], one of the [[gorgon]]s in [[Greek mythology]], whose "hair" was a mass of writhing [[snake]]s. Filamentary structures of [[gas]] can be seen to snake their way across the nebula. Though large, the nebula has a low surface brightness making it very faint and tricky to observe.

==Properties and Structure==

==Properties and structure==

The nebulae is situated some 1,500 [[light year]]s in the [[constellation]] of [[Gemini]], close to its southern border with [[Canis Major]].<ref name=eso/> It has an apparent size of 10.25 arcseconds, meaning it is roughly 4.5 light years across.<ref name=simbad/> The nebula houses a small but hot star, a [[white dwarf]] as most planetary nebulae do. However this is not the bright star that can be seen in images of the nebula; the bright star that can be seen is in fact a foreground star that happens to be located in front of the nebula, and is designated TYC 776-1339-1.<ref name=eso/> The white dwarf can be seen lying just off the crescent shaped region.

The nebula is situated some 1,500 [[light year]]s in the [[constellation]] of [[Gemini]], close to its southern border with [[Canis Major]].<ref name=eso/> It has an apparent size of 10.25 arcseconds, meaning it is roughly 4.5 light years across.<ref name=simbad/> The nebula houses a small but hot star, a [[white dwarf]] as most planetary nebulae do. The bright star that can be seen in images of the nebula is a foreground star that happens to be located in front of the nebula, and is designated TYC 776-1339-1.<ref name=eso/> The white dwarf can be seen lying just off the crescent shaped region.

The white dwarf emits vast amounts of [[ultraviolet]] radiation, [[ionization|ionizing]] the enveloping gas and causing it to glow.<ref name=eso/> The different colors correspond to different ionization levels of different [[elements]]. The green for example is due to doubly ionized [[oxygen]], <nowiki>[O III]</nowiki>, that is [[atom]]s of oxygen that have lost two of their [[electron]]s. This particular emission is extremely rare, so rare that it was originally believed to indicate the presence of an unknown element designated "nebulium".<ref name=eso/> Later it was realised it was only another ionization state of oxygen and not a new element. The transition responsible for the green color is rare due to the effects of [[quantum mechanics]].

The white dwarf emits vast amounts of [[ultraviolet]] radiation, [[ionization|ionizing]] the enveloping gas and causing it to glow.<ref name=eso/> The different colors correspond to different ionization levels of different [[elements]]. The green for example is due to doubly ionized [[oxygen]], <nowiki>[O III]</nowiki>, that is [[atom]]s of oxygen that have lost two of their [[electron]]s. This particular emission is extremely rare, so rare that it was originally believed to indicate the presence of an unknown element designated "nebulium".<ref name=eso/> Later it was realised it was only another ionization state of oxygen and not a new element. The transition responsible for the green color is rare due to the effects of [[quantum mechanics]].