Skull Nebula (NGC 246)

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 27 November 1785 by William 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]
Designations PN G118.8-74.7: NGC 246, PK 118-74.1, ARO 43, VV 4, VV' 7
Right Ascension (J2000.0) 00h 47m 04s
Declination (J2000.0) -11° 52' 22"
Dimensions 245." (optical), 280." (radio)
Distance 0.57 kpc
Radial Velocity -46.0 ± 6.0 km/s
Expansion Velocity 39.0 (O-III) km/s
C-Star Designations AG82 8, BD -12 134, CSI -12 -00445, PHL 829, PLX 152, TD1 31081, UBV 644
C-Star Magnitude U: 10.41, B: 11.60, V: 11.96
C-Star Spectral Type WC OVI, O(C)
Discoverer HERSCHEL 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.

Finder Chart Skull Nebula (NGC 246)
Skull Nebula (NGC 246) in constellation Cetus. Charts created using SkySafari 6 Pro and STScI Digitized Sky Survey. Limiting magnitudes: Constellation chart ~6.5 mag, DSS2 close-ups ~20 mag. [149, 160]

Visual Observation

400 mm Aperture: In the 21 mm Tele Vue Ethos eyepiece (85x) and with O-III filter, NGC 246 appears as a large, round, slightly elongated nebula. Two very faint dark spots can be seen in the nebula and one edge is slightly brighter. — 400 mm f/4.5 Taurus Dobsonian, Glaubenberg, 6. 11. 2020 Bernd Nies

Objects Within a Radius of 20°

References