NGC 2392, Eskimo Nebula, Clown Face Nebula

History

On January 17 1787, the relatively bright planetary nebula was discovered by William Herschel. He cataloged it as IV 45 (class IV = planetary nebulae) and wrote about it: «A star 9 mag with a pretty bright milky nebulosity equally dispersed all around. A very remarkable phaenomenon.» [464] Dreyer added it as NGC 2392 in 1888. [313]

NGC 2392
NGC 2392: Eskimo Nebula in Gemini; 500 mm Cassegrain 5800 mm f/11.4; SBIG STL11K; 4+4+4 min LRGB; Bernese Highlands; © 2005 Radek Chromik
NGC 2392
NGC 2392: Eskimo Nebula in Gemini; RC 500 on Herkules V48 mount; SBIG STL-11000M/C2; -30 °C chip temperature , R/G/B 2 x binned: R 7×5 min, G 6×5 min, B 5×5 min; Observatory Son Bi, Mallorca; © 25. – 26. 1. 2017 Beat Kohler, Stefan Meister, Hansjörg Wälchli

Physical Properties

NGC 2392
NGC 2392: Eskimo Nebula; 500/2500mm-Newton + SBIG ST-6; Observatory Bülach; © 1996 Stefan Meister

In medium-sized amateur telescopes one can already see that the area around the 10.5 mag central star appears brighter than the outer part of the nebula. Detailed investigations revealed that the central star is surrounded by two concentric gas shells that are expanding at different speeds.

The shells are excited to glow by the intense radiation from the very hot central star. The outer one is expanding at a speed of about 100 km/s. Of course, the shell that is closer to the central star and expands less quickly is irradiated more intensively and therefore appears brighter to us. The central star is an O8 dwarf with about 40 times the luminosity of the Sun. The absolute brightness is given as about +0.7 mag and the surface temperature is about 40'000 Kelvin.

NGC 2392
NGC 2392: Image taken with the Hubble Space Telescope [225]

Distances are given from 1370 to 3600 light years, but a distance of 2900 light years seems the most appropriate value. The age of the PN is probably about 1700 years. So he's still relatively young.

— 1996, Philipp Reza Heck

«Strasbourg-ESO Catalogue of Galactic Planetary Nebulae» Acker et al., 1992 [141]
DesignationsPN G197.8+17.3: NGC 2392, PK 197+17.1, ARO 24, VV 38, VV' 63
Right Ascension (J2000.0)7h 29m 11s
Declination (J2000.0)20° 54' 37"
Dimensions 19.5" (optical)
Distance0.5 kpc - 1.6 kpc ?
Radial Velocity+75.0 km/s ± 2.1 km/s
Expansion Velocity> 52.5 km/s (O-III), > 53. km/s (N-II)
C-Star DesignationsAG +21 821, AG82 81, BD +21 1609, CSI +21 01609, CSV 100874, DC 13050, EM* CDS 689, GCRV 4981, HD 59088, NSV 3604, PLX 1765, SV* ZI 631
C-Star Magnitude9.30 mag (U filter), 10.38 mag (B filter), 10.53 mag (V filter)
C-Star Spectral TypeO6 f, Of(H)
DiscovererLASELL 1853

How to find the Eskimo Nebula?

The Eskimo Nebula in the constellation of Gemini is relatively easy to find: You start with the 3.5m star Wasat (Delta Gem), which forms an almost equilateral triangle with the two 5mag stars 63 and 58 Gem. The telescope can be easily aligned with this using the Telrad. Immediately about two arc minutes south of an 8.3mag bright star, a diffuse patch can be seen even at low magnification.

Chart NGC 2392
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

Visual Observation

NGC 2392
NGC 2392: Drawing; 150mm Refractor; © 16. 12. 1996 Beat Kohler
NGC 2392
NGC 2392: Drawing; 200mm SCT; © 1996 Philipp Reza Heck

200 mm aperture: Once found and centered - start magnifying! Gradually increase the magnification and try out which one gives the greatest contrast between the outer and inner shell. I prefer to observe the Eskimo Nebula with a short focal length, orthoscopic eyepiece, with which I can achieve a magnification of 226x with my telescope (20 cm aperture; f/10). The «Face of the Eskimo» can best be recognized visually with an H-alpha filter - the O-III filter, which is otherwise helpful for planetary nebulae, is not recommended here. — 1996, Philipp Reza Heck


NGC 2392
NGC 2392: Pencil drawing; 300mm f/4 Newton, 500x, ohne Filter; © 8. 2. 1997 Bernd Nies

300 mm aperture: Even at 27x magnification, the planetary nebula NGC 2392 can be seen with averted vision as a small diffuse speck, which, however, disappears immediately, leaving only the central star when viewed directly. From about 46x the PN remains visible even with direct viewing and from 200x the slightly brighter central region of the PN slowly emerges. The best magnification is more than 500x, but for this the air must be very calm and the telescope optics must be good. A high magnification is necessary in order to be able to clearly see the inner structures of the Eskimo Nebula (the face in the fur collar). The PN is bright enough to allow magnifications of more than 500x. An O-III or an H-Beta do not achieve a significant improvement in contrast. The halo of the PN is circular with a darker rim to the east and west. The central region is slightly brighter than the halo (which is clearly visible in transparent and calm air) and has an asymmetrical shape, resembling an egg with the apex pointing north. The central star is not exactly in the middle. In this central region, three darker spots around the central star can be seen. — 300 mm f/4 Popp-Newton, 1996, Bernd Nies

762 mm aperture: On the Eskimo Nebula NGC 2392, the face consisting of three black circular areas and the central star as a nose can be seen with a UHC filter and a magnification of 625x, the bright part of the nebula as a head boundary and the fainter part of the nebula as a fur hat. — 30" f/3.3 Slipstream Dobsonian, Hasliberg Reuti, 5. 3. 2022, Eduard von Bergen

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
225NGC 2392; hubblesite.org/contents/media/images/2000/07/940-Image.html (2021-01-10)
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