Planetary Nebula NGC 6818

NGC 6818
NGC 6818: Image taken with Hubble Space Telescope. © ESA/Hubble & NASA, postprocessing Judy Schmidt [165, 536]

History

This planetary nebula was discovered on 8 August 1787 by the German-British astronomer William Herschel using his 18.7 inch reflecting telescope. He cataloged it as IV 51 (class IV = planetary nebula). In his publication of 1789 he wrote: «A considerably brigth, small, beautiful planetary nebula; but considerably hazy on the edges, of a uniform light; 10 or 15" diameter, perfectly round. I shewed it to M. De la Lande.» [464] The nebula was added in 1888 by J. L. E. Dreyer as NGC 6818 to his famous catalogue. [313]

NGC 6818 is sometimes also referred to «Little Gem Nebula», a name which is also used for the planetary nebula NGC 6445.

Physical Properties

NGC 6818 is located at a distance of about 6000 light-years and has a diameter of about 0.5 light-years. A fast wind from the hot central star is creating the elongated shape and in fact has caused a «blowout» at the two ends of the major axis. The planetary nebula has an outer spherical envelope, an inner brighter elongated bubble, and it shows a blowout orifice at one end of the major axis. [535, 536]

«Strasbourg-ESO Catalogue of Galactic Planetary Nebulae» Acker et al., 1992 [141]
DesignationsPN G025.8-17.9: NGC 6818, PK 25-17.1, ARO 12, Sa 2-392, VV 241, VV' 511
Right Ascension (J2000.0)19h 43m 58s
Declination (J2000.0)-14° 09' 07"
Dimensions 20." (optical)
Distance 2.2 kpc
Radial Velocity-13.0 km/s ± 3.0 km/s
Expansion Velocity 27.6 km/s (O-III), 31. km/s (N-II)
C-Star DesignationsAG82 381
C-Star Magnitude16.90 mag (B filter)
C-Star Spectral TypeWNb ?
DiscovererHERSCHEL 1787

Finder Chart

The planetary nebula NGC 6818 is located about 40 arc minutes from dwarf galaxy NGC 6822 in the constellation Sagittarius, actually closer to Capricornus. It is best observed in the months from June to August.

Chart Planetary Nebula NGC 6818
Planetary Nebula NGC 6818 in constellation Sagittarius. Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

More Objects Nearby (±15°)

References

141Strasbourg-ESO Catalogue of Galactic Planetary Nebulae; A. Acker, F. Ochsenbein, B. Stenholm, R. Tylenda, J. Marcout, C. Schohn; European Southern Observatory; ISBN 3-923524-41-2 (1992); cdsarc.unistra.fr/viz-bin/cat/V/84 (2021-02-18)
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
165Flickr: Judy Schmidt; flickr.com/people/geckzilla (2021-01-02)
313«A New General Catalogue of Nebulae and Clusters of Stars, being the Catalogue of the late Sir John F.W. Herschel, Bart., revised, corrected, and enlarged» Dreyer, J. L. E. (1888); Memoirs of the Royal Astronomical Society. 49: 1–237; Bibcode:1888MmRAS..49....1D
464«Catalogue of a second thousand of new nebulae and clusters of stars; with a few introductory remarks on the construction of the heavens» William Herschel, Philosophical Transactions of the Royal Society of London, 1 January 1789; DOI:10.1098/rstl.1789.0021
535Planetary Nebula NGC 6818; hubblesite.org/contents/media/images/1998/11/622-Image.html (2022-09-21)
536Little gem; esahubble.org/images/potw1531a (2022-09-22)