Little ghost nebula

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Little ghost nebula
NGC 6369HST.jpg
Observational Data
Designation NGC 6369
Right ascension 17h 29m 20.45s[1][2]
Declination -23° 45′ 34.8″[1]
Constellation Ophiuchus
Dimensions 0.467'[1]
Magnitude Apparent Mag: +12.0[1]
Astrometry
Distance from Earth 5,053 ly[3]
Radial velocity -106±5 km/s[1][4]

The Little ghost nebula (NGC 6369) is a planetary nebula in the constellation of Ophiuchus.[5] The nebula was first observed by William Herschel in the 18th century.[6] Images of the nebula taken by the Hubble Space Telescope have revealed the nebula to possess an extraordinarily intricate structure.

Properties and Structure

The nebula is situated around 5,000 light years from Earth in the constellation of Ophiuchus, though some estimates put it at only 2,000 ly.[3][7] Its apparent size of 28 arcseconds therefore corresponds to a physical diameter of 0.3 ly.[7] 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.[8] 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.[8] A white dwarf star can be found at the centre of the nebula.

The nebula is composed of a variety of different elements, the most abundant being helium, nitrogen, oxygen, neon, sulphur and argon.[9] Evidence for H2 molecules as well as polycyclic aromatic hydrocarbons has been observed.[8] The nebula is expanding, as is seen in most other planetary nebulae, at 41 km/s.[7] 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.[7] As a result, it emits vast quantities of ultraviolet radiation. This 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 atoms of all their electrons) 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.[5]

References

  1. 1.0 1.1 1.2 1.3 1.4 NGC 6369 from the SIMBAD Astronomical Database
  2. Cutri, R. (2003). 2MASS All Sky Catalog of point sources. The IRSA 2MASS All-Sky Point Source Catalog. Online
  3. 3.0 3.1 Schwarz, H. and Monteiro, H. (2006). 3D photoionization structure and distances of Planetary Nebulae III. NGC 6781. Exacta, 4(2). arXiv:astro-ph/0604501
  4. Blaauw, A. (1954). General Catalogue of Stellar Radial Velocities. The Astrophysical Journal, 119, p.689. Online
  5. 5.0 5.1 Little ghost nebula NGC 6369 from esa.int
  6. NGC 6369: The Little Ghost Nebula from apod.nasa.gov
  7. 7.0 7.1 7.2 7.3 THE LITTLE GHOST NEBULA from stars.astro.illinois.edu
  8. 8.0 8.1 8.2 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). arXiv:1305.4211
  9. 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. arXiv:astro-ph/0611149