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NGC 246
NGC 246
NGC 246
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NGC 246
Emission nebula
Planetary nebula
An infrared Spitzer Space Telescope image of NGC 246.
Credit: NASA/JPL.
Observation data: J2000 epoch
Right ascension00h 47m 03.338s[1]
Declination−11° 52′ 18.94″[1]
Distance3,500+720
−850[2] ly
Apparent magnitude (V)8[3] / 11.8 (central star)[1]
Apparent dimensions (V)3.8′[3]
ConstellationCetus
Physical characteristics
Radius2-3[4] ly
DesignationsSkull Nebula,[5] Pac-Man Nebula,[6] Caldwell 56, HIP 3678, PMN J0047-1152, 2E 178, PN VV 4, IRAS 00445-1207, MCT 0044-1208[1]
See also: Lists of nebulae

NGC 246 (also known as the Skull Nebula[5] or Caldwell 56) is a planetary nebula in the constellation Cetus. It was discovered in 1785 by William Herschel. The nebula and the stars associated with it are listed in several catalogs, as summarized by the SIMBAD database.[1]

The nebula is roughly 3,500+720
−850 light-years away.[2] NGC 246's central star is the 12th magnitude[7] white dwarf HIP 3678 A.[8] In 2014, astronomers discovered a second companion to NGC 246's central star, which has a comoving companion star called HIP 3678 B. The second companion star, a red dwarf known as HIP 3678 C, was discovered using the European Southern Observatory's Very Large Telescope. This makes NGC 246 the first planetary nebula to have a hierarchical triple star system at its center.[9]

NGC 246 is not to be confused with the Rosette Nebula (NGC 2237), which is also referred to as the "Skull."[10] Among some amateur astronomers, NGC 246 is known as the "Pac-Man Nebula" because of the arrangement of its central stars and the surrounding star field.[6]

[edit]
  • HaRGB image of The Skull Nebula (NGC 246). Data from the Liverpool Telescope, processed by Göran Nilsson. Total exposure time 1.1 hours.
    HaRGB image of The Skull Nebula (NGC 246). Data from the Liverpool Telescope, processed by Göran Nilsson. Total exposure time 1.1 hours.
  • NGC 246 acquired using the Schulman 0.8m Telescope atop Mount Lemmon, AZ. This is a full color (visual) image.
    NGC 246 acquired using the Schulman 0.8m Telescope atop Mount Lemmon, AZ. This is a full color (visual) image.

References

[edit]
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NGC 246

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NGC 246, commonly known as the Skull Nebula or Caldwell 56, is a situated in the constellation Cetus approximately 1,800 light-years from Earth. Discovered by astronomer on November 27, 1785, it consists of an expanding shell of ionized gas and dust ejected from a dying central star, spanning an angular diameter of about 3.7 arcminutes. The nebula's central system is a hierarchical triple, comprising a primary closely orbited by a low-mass companion, with a third star in a wider orbit—a configuration that marks NGC 246 as the first known with such a stellar arrangement. This nebula's structure reveals a roughly with prominent wispy filaments of gas, illuminated by radiation from the hot central , which has a visual magnitude of 11.76 and exhibits a of -15.7 km/s. observations highlight a distinctive offset red ring of molecular surrounding the core, alongside green emissions from expelled gases, providing insights into the late stages of for Sun-like stars. The system's is -16.9 mas/yr in and -9.2 mas/yr in , indicating its path through space. Positioned at celestial coordinates RA 00h 47m 03.3s and Dec -11° 52' 19", NGC 246 is visible to amateur astronomers under with telescopes of 4-inch aperture or larger. Notable observations include images capturing the nebula's intricate filaments to study temporal changes, Gemini South's 2006 multi-wavelength view emphasizing its "skull-like" appearance in sulfur, , and oxygen emissions, and ESO's imaging in 2020 that detailed the triple-star dynamics in pink and red hues. These studies underscore NGC 246's role in understanding binary interactions during formation, particularly how the close companions may influence mass loss and asymmetry in the ejecta.

Overview

Discovery and designation

NGC 246 was discovered by the German-born British astronomer on November 27, 1785, during one of his systematic sweeps of the night sky in the constellation Cetus, using his 18.7-inch Newtonian reflector with a 20-foot . Herschel initially cataloged the object as H V-25 in his class of very large nebulae and described it as "four or five rather large stars form a about 5' in diameter, with the enclosed space filled with weakly demarcated, milky nebula." In 1888, Danish-Irish astronomer John Louis Emil Dreyer incorporated Herschel's observation into the of Nebulae and Clusters of Stars, assigning it the designation NGC 246 with equatorial coordinates of 00h 47m 04s and −11° 52′ 22″ (epoch 1860.0). Dreyer's entry abbreviated the description as "vF, L, irr fig, 5 or 6 st inv" (very faint, large, irregular figure, 5 or 6 stars involved). Subsequent spectroscopic observations in the early confirmed its classification as a , distinct from other diffuse nebulae due to its ionized gas shell surrounding a hot central star. The object has acquired alternative designations reflecting its visual characteristics and catalog inclusions. It is commonly known as the Skull Nebula owing to the skull-like shape revealed by its arrangement of brighter patches and embedded stars in deep imaging. Additionally, British astronomer designated it as Caldwell 56 in his 1995 Caldwell Catalogue, a supplementary list of 109 deep-sky objects for amateur observers, published in Sky & Telescope magazine to extend beyond the Messier Catalogue.

Physical properties

NGC 246 is classified as a planetary nebula, consisting of an ionized gas envelope expelled by a low- to intermediate-mass asymptotic giant branch star during its post-main-sequence evolution. The distance to NGC 246 is estimated at 538^{+20}_{-17} parsecs (about 1,750 light-years) based on Gaia Early Data Release 3 parallax measurements of the central star (as of 2021). Earlier estimates from the hierarchical triple central star system, using photometry of the G8–K0 V companion fitted to the zero-age main sequence combined with spectroscopic data, yielded around 495 parsecs, but Gaia data provide the current benchmark. This measurement aligns with the parallax method, where distance in parsecs is given by d=1πd = \frac{1}{\pi} with π\pi in arcseconds. Given its of roughly 3.8 arcminutes, the physical diameter of NGC 246 is approximately 0.55 parsecs (1.8 light-years), reflecting the expanded shell's scale at this distance. The exhibits a systemic of -16 km/s relative to the Sun, indicating motion toward our solar system, while the central star system moves at about 80 km/s through space, primarily out of the . NGC 246 is regarded as a relatively old , with kinematic age estimates derived from expansion velocities and size ranging from 6,600 to 16,100 years (early 2000s studies), placing it among more evolved examples of its class.

Location and observation

Coordinates and visibility

NGC 246 has equatorial coordinates (J2000) of 00h 47m 03.3s and −11° 52′ 19″. Its corresponding galactic coordinates are longitude 118.9° and latitude −74.7°. The is situated in the constellation Cetus, positioned near the border with Pisces amid a notably sparse star field that facilitates its identification once located. This location places it in a relatively uncrowded region of the sky, away from prominent bright stars or dense clusters. NGC 246 lies close to the , rendering it observable from virtually all latitudes on for part of the year, specifically from 78°S to 78°N, though it never rises above the horizon at higher northern latitudes or remains visible at extreme southern ones. It reaches its highest point in the evening sky during the northern hemisphere's autumn and winter months, typically from to , providing optimal viewing conditions under clear, . With an of about 8, the nebula is not visible to the and requires at least a small (4-inch or larger) for detection, where it appears as a faint, ghostly ring.

Apparent size and magnitude

NGC 246 has an integrated visual magnitude of 8.0, but its low surface brightness, resulting from its extended nature, makes it appear fainter to the eye than this value suggests. The nebula's angular diameter measures approximately 3.8 arcminutes for its outer envelope, presenting as a faint, irregular patch in telescopes. The central star system shines at a visual magnitude of 11.8 to 12.0, allowing it to be resolved and separated from the surrounding nebulosity even in small telescopes. Observing the nebula itself requires a telescope aperture of at least 4 to 6 inches for detection, as its low surface brightness poses a significant challenge; OIII filters can enhance contrast by isolating key emission lines. Additionally, its southern declination of about -11° limits visibility for northern observers, particularly those above latitude 50° N, where it remains low on the horizon. Visually, NGC 246 appears as a pale green-gray glow, though broadband reveals reddish hues in the hydrogen-rich regions.

Structure and morphology

Overall appearance

NGC 246 exhibits a distinctive morphology that resembles a human skull in wide-field images, characterized by a bright central cavity and an irregular outer shell, with prominent dark lanes and voids evoking the illusion of eye sockets. This visual form arises from the nebula's asymmetric expansion, where the outer shell displays a complex, braided structure in its blue-hued ring, shaped by shock waves from the central star's winds. The nebula's overall structure is elliptical and ring-like, with a faint outer halo extending beyond the brighter interior arcs, contributing to its bipolar or asymmetric appearance. It spans approximately 4 arcminutes along its major axis, inclined to the , which enhances the observed irregularity. The brighter western limb results from interactions between the expanding shell and the , creating a sharper leading edge compared to the dimmer trailing side.

Internal features

NGC 246 exhibits several resolved substructures within its , including prominent ansae manifesting as double-loop features in [Ne V] emission, forming an hourglass-like pattern near the central star. These wing-like extensions appear on the northern and southern sides, likely originating from earlier episodes of mass ejection during the phase of the progenitor star. High-resolution imaging reveals these ansae with significant emission fluxes, such as 9.90 × 10^{-13} erg cm^{-2} s^{-1} for the northern feature and 6.67 × 10^{-13} erg cm^{-2} s^{-1} for the southern one. The outer envelope contains faint, bright knots observed primarily in [O III] emission, resembling cometary globules but far less prominent than those in the . These knots, located in the peripheral regions, have fluxes ranging from 7.31 × 10^{-14} to 2.33 × 10^{-13} erg cm^{-2} s^{-1}, indicating localized denser material sculpted by the . The nebula displays notable asymmetry, with the western limb appearing brighter and sharper, attributed to the interaction of the central star's —approximately 50 km s^{-1} westward—with the surrounding , compressing the leading edge. At its core, the inner shell is clumpy and features radial streaks, spanning a diameter of about 1.5 arcminutes (corresponding to roughly 0.24 pc at a distance of 560 pc). This clumpy morphology, evident in multiple emission lines including Hα and [O III] 500.7 nm, suggests turbulent interactions between the ionized gas and the expanding shell. Evidence for multiple ejection phases emerges from the varied structures and numerical modeling, which incorporate evolving parameters across the to transition, as supported by imaging data from facilities like . The braided and lobed patterns in the inner regions further imply episodic outflows shaping these features over time, with recent 2020 ESO observations highlighting how the central triple-star system contributes to the observed asymmetries.

Composition and dynamics

Chemical composition

The of NGC 246 is characterized by strong emission lines from key ions, with the forbidden [O III] line at 500.7 nm dominating the optical emission and imparting a color to the inner regions. Ionized contributes prominently through the Hα line at 656.3 nm, producing red hues in the ionized shell, while ultraviolet spectroscopy reveals [Ne V] emission at 342.6 nm, a tracer of highly ionized excited by the central star's hard UV . Chemical abundance determinations from optical recombination and collisionally excited lines indicate an oxygen-rich composition, with log(O/H) + 12 = 8.62, consistent with a Type II or III rather than the helium- and nitrogen-enriched Type I subclass. abundance is He/H ≈ 0.11 by number, while is slightly enhanced relative to solar values, yielding N/O ≈ 0.3. Spatial mapping from VLT/FORS2 integral-field highlights oxygen enrichment in the compact inner blue structures, whereas mid-infrared imaging identifies the outer red ring as molecular hydrogen (H₂) emission from regions. The ionization structure is stratified, with high-ionization zones (e.g., Ne⁵⁺) confined to the inner regions directly illuminated by the central star's UV flux, giving way to lower-ionization plasma (e.g., H⁺, O²⁺) in the extended outer .

Expansion and age

The radial expansion velocity of NGC 246 is measured at 39.5 km/s, determined from Doppler shifts observed in emission lines such as [O III]. This kinematic measurement reveals the nebula's outward motion from its central star system. The dynamical age of NGC 246 is approximately 6,600 years, derived from the ratio of its physical to the expansion velocity using the formula age = radius / expansion velocity. The expansion follows a largely homologous pattern, where velocities scale proportionally with distance from the center, though clumpy structures indicate some likely induced by the motion of the central stars. Due to the high space velocity of about 85 km/s relative to the ambient , NGC 246 exhibits evidence of interaction, including a possible on its leading edge that compresses and fragments material. As a post-asymptotic giant branch remnant in its late evolutionary stage, the nebula is fading as its ionized shell disperses over time.

Central star system

Primary star

The primary star of the planetary nebula NGC 246 is the hot, hydrogen-deficient designated HIP 3678 A, classified as a PG 1159-type star of the GW Vir subclass. These stars represent a transitional phase between post-asymptotic giant branch objects and cooling s, exhibiting spectra dominated by broad absorption lines of , carbon, and oxygen due to their extreme surface conditions. HIP 3678 A has a visual magnitude of 11.8 and an effective temperature of approximately 150,000 K, with a surface gravity of log g ≈ 5.7. Its mass is estimated at 0.57–0.87 M_⊙ based on asteroseismic modeling (best-fit ~0.57 M_⊙; alternatives ≥0.57 or ≥0.87 M_⊙ depending on mode identification), consistent with typical values for PG 1159 stars around 0.6 M_⊙, while the radius is roughly 0.02 R_⊙. The resulting luminosity is about 50 L_⊙, driven by the high temperature despite the compact size. This intense ultraviolet emission makes HIP 3678 A the primary ionizing source for the nebula, exciting its gases through photoionization, while a weak stellar wind with a mass-loss rate of log Ṁ ≈ –6.9 M_⊙ yr⁻¹ contributes to the dynamical interaction. As a member of the rare triple system HIP 3678, the primary exhibits multiperiodic g-mode pulsations with periods of 1460–1823 s, characteristic of GW Vir variables and indicative of its pre-white dwarf evolutionary stage. These low-amplitude variations, driven by the κ-mechanism in the partial helium ionization zone, provide key probes into the star's internal structure via asteroseismology.

Companion stars

The central star system of NGC 246 forms a hierarchical triple, with the primary (HIP 3678 A) orbited by two low-mass companions that share its , confirming their gravitational association. The inner companion, HIP 3678 C, is a mid-M dwarf (spectral type M5–M6) discovered in 2014 through high-resolution imaging with the . It resides at an angular separation of approximately 1 arcsecond from the primary, corresponding to a projected orbital distance of about 500 AU assuming a system distance of roughly 500 pc. The outer companion, HIP 3678 B, is an early to mid-K dwarf (spectral type –K5 V) with a visual magnitude of about 14, located at a wider angular separation of 3.9 arcseconds (projected distance ~1900 AU). This outer star was identified early in observations of the , with its companionship confirmed through shared . The large orbital separations in this triple system—far exceeding typical close-binary scales—indicate that neither companion participated in a common-envelope phase with the progenitor of the . No photometric eclipses have been detected, consistent with the wide orbits precluding transit events. The entire system moves with a tangential velocity of approximately 50 km/s relative to the local standard of rest (based on DR3 data as of 2021), which interacts with the and influences the nebula's elongated, asymmetric structure by compressing material on the leading edge.

Scientific significance

Historical studies

NGC 246 was discovered on November 27, 1785, by using his 18.7-inch reflector telescope, who described it as "4 or 5 pretty large stars, forming a trapezium, with a faint nebula between them." In the , observations with larger telescopes confirmed its nature as a due to its disk-like appearance and central star, as documented in John Louis Emil Dreyer's of 1888, where it is listed with the description "very faint, large, 4 stars involved." Early spectroscopic investigations began in the mid-20th century. This was followed by Jesse L. Greenstein and Rudolph Minkowski's 1964 study of low-surface-brightness planetary nebulae central stars, which included photoelectric measurements of NGC 246's nucleus, estimating its and confirming its status as a hot, evolved star with an unidentified emission line at 3868 . A significant advancement came in 1975 with image-tube of the nebula and central star conducted by Myron W. Heap and colleagues using the 36-inch telescope at the Goddard Space Flight Center's Laboratory for Optical Astronomy. This work identified prominent [O III] emission lines at 4959 and 5007 , along with He II at 4686 , and provided early abundance estimates, such as O++/H+ ratios ranging from 9.95 × 10^{-5} to 2.98 × 10^{-5} and He/H between 0.08 and 0.14, depending on the nebular region and ionization model. These observations also estimated the central star's temperature at approximately 200,000 K, linking it to late stages of . In the and , photoelectric photometry further characterized the central star. Alan D. Grauer and collaborators in 1987 performed multi-band photometry as part of a search for pulsating hot stars, establishing the nucleus as a non-variable hot with exceeding 100,000 K and confirming its hydrogen-deficient composition consistent with PG 1159-type stars. Multiplicity was noted during this period, with the central star resolved as a visual featuring a hot primary and a cooler G8-K0 V companion separated by 3.8 arcseconds, as detailed in Howard E. Bond's 1999 analysis using CCD photometry to derive the companion's properties and estimate the nebula's distance. Early ground-based photographic plates from the mid-20th century, such as those from the Palomar Observatory Sky Survey, revealed the nebula's ring-like structure, with a bright elliptical shell and fainter inner features, highlighting its irregular, clumpy morphology. These historical studies contributed to understanding NGC 246 as a prototype for post-asymptotic giant branch (post-AGB) evolution, with the 1975 analysis explicitly comparing the central star's parameters to Paczyński's evolutionary tracks for stars nearing helium exhaustion in the core.

Recent observations

In 2004, NASA's captured an image of NGC 246, revealing an unexpected ring of emission at 5.8 and 8.0 microns, interpreted as molecular hydrogen (H₂) excited by UV radiation from the central star, along with a distribution of cooler dust grains that trace the nebula's outer envelope. This observation highlighted the presence of warm dust and molecular material not prominent in optical wavelengths, providing insights into the nebula's mass-loss history. A 2006 image from the Gemini South telescope, using the GMOS instrument, showcased the nebula's fine structure, including braided filaments in the outer shell formed by shock interactions between the fast-moving stellar wind and the interstellar medium. These "shocking" gas dynamics create instabilities that compress and heat the leading edge, resulting in an asymmetric oval ring with brighter knots and internal voids from colliding winds. High-resolution imaging in 2014, combining data from the Hubble Space Telescope's WFPC2 and the VLT's NACO system, discovered a low-mass companion (HIP 3678 C) to the central and its known G8-K0 V companion (HIP 3678 B), forming a hierarchical triple system at separations of approximately 500 AU and 1800 AU. Astrometric analysis confirmed their common , ruling out a background alignment at over 8σ , and enabled initial dynamical modeling of the system's stability and influence on the nebula's bipolar morphology. In 2020, the ESO Very Large Telescope's FORS2 instrument produced a detailed image composite that mapped the spatial distributions of (in , via Hα emission) and oxygen (in blue, via [O III] lines), revealing chemical gradients with oxygen enrichment in the inner regions and hydrogen dominance in the outer shell, consistent with nucleosynthetic processing in the progenitor star. This observation also clearly resolved the triple central system for the first time in ground-based data, supporting refined dynamical models that attribute the nebula's asymmetries to orbital interactions within the triple. Archival imaging has captured the nebula's intricate structure, while similar high-contrast techniques from space facilities have informed these studies; as of November 2025, the has yet to release targeted observations of NGC 246, though its capabilities hold potential for future dust and molecular mapping.

References

  1. https://www.spitzer.caltech.edu/image/ssc2004-13a1-spitzer-view-of-planetary-nebula-ngc-246
  2. https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-caldwell-catalog/caldwell-56/
  3. https://simbad.u-strasbg.fr/simbad/sim-id?Ident=NGC246
  4. https://www.eso.org/public/news/eso2019/
  5. https://academic.oup.com/mnras/article/444/4/3459/1022889
  6. https://noirlab.edu/public/images/gemini0603a/
  7. https://www.eso.org/public/images/eso2019a/
  8. https://www.constellation-guide.com/skull-nebula-ngc-246/
  9. https://www.deepskycorner.ch/obj/ngc246.en.php
  10. https://ui.adsabs.harvard.edu/abs/1888MmRAS..49....1D/abstract
  11. https://adsabs.harvard.edu/full/1918PLicO..13...55C
  12. http://www.messier.seds.org/xtra/similar/caldwell.html
  13. https://www.aanda.org/articles/aa/full_html/2021/12/aa41916-21/aa41916-21.html
  14. https://www.gemini.edu/news/press-releases/gemini0603
  15. https://simbad.cds.unistra.fr/simbad/sim-basic?Ident=NGC+246
  16. https://astronomynow.com/2014/10/09/deep-sky-challenge-skull-nebula/
  17. https://www.skyhound.com/observing/archives/oct/NGC_246.html
  18. https://theskylive.com/sky/deepsky/ngc246-object
  19. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=NGC+246
  20. https://www.researchgate.net/publication/26476887_The_Structure_and_Dynamics_of_NGC_246
  21. https://iopscience.iop.org/article/10.1086/377123/pdf
  22. https://ui.adsabs.harvard.edu/abs/2003ApJ...594..874S/abstract
  23. https://ui.adsabs.harvard.edu/abs/2007ApJ...670..442H/abstract
  24. https://academic.oup.com/mnras/article/362/2/424/1015947
  25. https://doi.org/10.1051/0004-6361/202349103
  26. https://iopscience.iop.org/article/10.1086/311383
  27. https://rogerivester.com/wp-content/uploads/2015/11/november-2019-observers-challenge-ngc-246.pdf
  28. http://spider.seds.org/ngc/ngc.cgi?ngc246
  29. https://ui.adsabs.harvard.edu/abs/1964ApJ...140.1601G/abstract
  30. https://ui.adsabs.harvard.edu/abs/1975ApJ...196..195H/abstract
  31. https://ui.adsabs.harvard.edu/abs/1999PASP..111..217B/abstract
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