NGC 246, Skull Nebula

NGC 246
NGC 246: Image taken with ESO Very Large Telescope (VLT) [208]

History

The planetary nebula NGC 246 is also known under the name «Skull Nebula». It was discovered on November 27, 1785 by Wilhelm Herschel with his 18.7 inch reflector telescope with a focal length of 20 feet. He cataloged it as V 25, whereby its class «V» stood for very large nebulae. In his second part of his discoveries, published 1789 in the «Philosophical Transactions of the Royal Society of London», he described the nebula as follows: «Four or five rather large stars form a trapezoid about 5' in diameter. The enclosed space is filled with weakly demarcated, milky nebula. The stars do not appear To have a connection with the nebula.» [464]

Physical Properties

The distance of the nebula is 1'600 light years. Diameters range from 2.2 to 4-6 light years.

In 2014 it was discovered that the central white dwarf star in 500 AU is orbited by a red dwarf star (not visible in the photo). Their interaction is probably related to the formation of the planetary nebula. This pair is orbited by a brighter star at a distance of 1900 AU. This is the first known triple star system within a planetary nebula. [208]

«Strasbourg-ESO Catalogue of Galactic Planetary Nebulae» Acker et al., 1992 [141]
DesignationsPN G118.8-74.7: NGC 246, PK 118-74.1, ARO 43, VV 4, VV' 7
Right Ascension (J2000.0)0h 47m 04s
Declination (J2000.0)-11° 52' 22"
Dimensions 245." (optical), 280." (radio)
Distance 0.57 kpc
Radial Velocity-46.0 km/s ± 6.0 km/s
Expansion Velocity 39.0 km/s (O-III)
C-Star DesignationsAG82 8, BD -12 134, CSI -12 -00445, PHL 829, PLX 152, TD1 31081, UBV 644
C-Star Magnitude10.41 mag (U filter), 11.60 mag (B filter), 11.96 mag (V filter)
C-Star Spectral TypeWC OVI, O(C)
DiscovererHERSCHEL 1785

Finder Chart

The Skull Nebula is located in the belly of the whale. Connect the stars η Ceti - ι Ceti, halve the distance and place the Telrad underneath so that the outermost ring comes to rest on the line. The best observation time is September to December.

Chart NGC 246
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

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
208New ESO’s VLT image of the Skull Nebula; eso.org/public/images/eso2019a (2020-12-30)
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