Changes

==Properties and Structure==

==Properties and Structure==

The nebula is situated around 5,000 [[light year]]s from [[Earth]] in the constellation of [[Ophiuchus]], though some estimates put it at only 2,000 ly.<ref name=arxivDist/><ref name=illinois>[http://stars.astro.illinois.edu/sow/n6369.html THE LITTLE GHOST NEBULA] from stars.astro.illinois.edu</ref> Its apparent size of 28 arcseconds therefore corresponds to a physical diameter of 0.3 ly.<ref name=illinois/> The nebula is a double-shelled planetary nebula meaning the [[gas]] and dust it is composed from is arranges in two shells as can be seen in images.<ref name=arxiv>Ramos-Larios, G., Guerrero, M., Vázquez, R. and Phillips, J. (2011). Weak and Extended H2 Emission in NGC 6369. ''Proceedings of the International Astronomical Union'', 7(S283). [https://arxiv.org/abs/1305.4211 arXiv:1305.4211]</ref> Its outer envelope has a filamentary structure as well as rather faint bipolar extensions. These knots are separated from the main nebula and have been observed in other planetary nebulae such as the [[Hourglass nebula]]. These are located upto 80 arcseconds from the main body of the nebula.<ref name=arxiv/> A [[white dwarf]] [[star]] can be found at the centre of the nebula.

The nebula is situated around 5,000 [[light year]]s from [[Earth]] in the constellation of [[Ophiuchus]], though some estimates put it at only 2,000 ly.<ref name=arxivDist/><ref name=illinois>[http://stars.astro.illinois.edu/sow/n6369.html THE LITTLE GHOST NEBULA] from stars.astro.illinois.edu</ref> Its apparent size of 28 arcseconds therefore corresponds to a physical diameter of 0.3 ly.<ref name=illinois/> The nebula is a double-shelled planetary nebula meaning the [[gas]] and dust it is composed from is arranges in two shells as can be seen in images.<ref name=arxiv>Ramos-Larios, G., Guerrero, M., Vázquez, R. and Phillips, J. (2011). Weak and Extended H2 Emission in NGC 6369. ''Proceedings of the International Astronomical Union'', 7(S283). [https://arxiv.org/abs/1305.4211 arXiv:1305.4211]</ref> Its outer envelope has a filamentary structure as well as rather faint bipolar extensions. These knots are separated from the main nebula and have been observed in other planetary nebulae such as the [[Hourglass nebula]]. These are located up to 80 arcseconds from the main body of the nebula.<ref name=arxiv/> A [[white dwarf]] [[star]] can be found at the centre of the nebula.

The nebula is composed of a variety of different [[element]]s, the most abundant being [[helium]], [[nitrogen]], [[oxygen]], [[neon]], [[sulphur]] and [[argon]].<ref>Girard, P., Köppen, J. and Acker, A. (2006). Chemical compositions and plasma parameters of planetary nebulae with Wolf-Rayet and wels type central stars. ''Astronomy & Astrophysics'', 463(1), pp.265-274. [https://arxiv.org/abs/astro-ph/0611149 arXiv:astro-ph/0611149]</ref> Evidence for H₂ molecules as well as polycyclic [[Aliphatic and aromatic compounds|aromatic]] [[hydrocarbons]] has been observed.<ref name=arxiv/> The nebula is expanding, as is seen in most other planetary nebulae, at 41 km/s.<ref name=illinois/> The central white dwarf is cool compared to most planetary nebulae and has a [[luminosity]] 1,000 greater than that of the [[Sun]]. However it is still very hot by everyday standards at 58,000 k.<ref name=illinois/> As a result it emits vast quantities of [[ultraviolet]] radiation. This [[ionization|ionizes]] the enveloping gas causing it to glow. However the in the outer region, this radiation is not as intense so does not fully ionize (strip the [[atom]]s of all their [[electron]]s) as it does in the inner regions. Consequently the nebula can be seen to be green/blue (due to hydrogen and oxygen) in the centre but red (due to nitrogen) in its outer region.<ref name=esa/>

The nebula is composed of a variety of different [[element]]s, the most abundant being [[helium]], [[nitrogen]], [[oxygen]], [[neon]], [[sulphur]] and [[argon]].<ref>Girard, P., Köppen, J. and Acker, A. (2006). Chemical compositions and plasma parameters of planetary nebulae with Wolf-Rayet and wels type central stars. ''Astronomy & Astrophysics'', 463(1), pp.265-274. [https://arxiv.org/abs/astro-ph/0611149 arXiv:astro-ph/0611149]</ref> Evidence for H₂ molecules as well as polycyclic [[Aliphatic and aromatic compounds|aromatic]] [[hydrocarbons]] has been observed.<ref name=arxiv/> The nebula is expanding, as is seen in most other planetary nebulae, at 41 km/s.<ref name=illinois/> The central white dwarf is cool compared to most planetary nebulae and has a [[luminosity]] 1,000 greater than that of the [[Sun]]. However it is still very hot by everyday standards at 58,000 k.<ref name=illinois/> As a result, it emits vast quantities of [[ultraviolet]] radiation. This [[ionization|ionizes]] the enveloping gas causing it to glow. However the in the outer region, this radiation is not as intense so does not fully ionize (strip the [[atom]]s of all their [[electron]]s) as it does in the inner regions. Consequently, the nebula can be seen to be green/blue (due to hydrogen and oxygen) in the centre but red (due to nitrogen) in its outer region.<ref name=esa/>

==References==

==References==