Click on the image for a full resolution version
See this image also on Instagram
NGC 246, also called the Skull Nebula, is a large and bright enough planetary nebula to have been discovered by William Herschel as early as 1785. As usual with planetary nebulae, this is the ejected outer envelope of a once sun-like star. Remarkable in this nebula is that its central star, a white dwarf (as usual), is a double star (as it can be seen in the picture). This has been known for many years, but quite recently, a German team (C. Adam & M. Mugrauer, 2014) determined, using HST observations during the last 20 years, that it is a triple star. Better said, the central white dwarf (called HIP 3678 A, mag +11.8, here showing a bluish color) is indeed a double star, having a smaller close companion, HIP 3678 C, separated less than 1 arcsec (approximately 500 AU at its distance), and hence not visible in this image. HIP 3678 B is the reddish dimmer star (mag +14.3) at its upper left and is 3.9 arcsec away. They also determined the possible masses at: HIP 3678A: 0.84 M⊙ (progenitor calculated to have been about five times that mass), HIP 3678B: 0.85 M⊙ and HIP 3678C: 0.1 M⊙.
Given its proximity to the celestial equator, it is visible from most locations on both hemispheres during the months of September through March.
Additional Information
Object
Name(s): NGC 246. Caldwell 56. Skull Nebula
Type: Planetary Nebula
RA: 00h 47m 02.9s
Dec: -11º 52’ 28.3”
Constellation: Cetus
Size (arcmin): 4.1 x 4.1 arcmin
Magnitude: +10.9
Distance: 1,500 ly
Image
Date: 2025-09-26 to 2025-09-28
Location: Obstech, Río Hurtado, Chile
Size (arcmin): 7 x 7 arcmin
Telescope: 24” f/6.5 Reflector
Camera: Moravian C5A-100M (11760x8896pix)
Guiding: off-axis guider
Total exposure: 17 hours (Ha: 7h 50m; OIII: 6h 10m; RGB: 3h)
Processing: CCDStack, PixInsight (one process) and Photoshop CC 2025