NGC 246, Skull Nebula
The planetary nebula NGC 246 was already discovered in 1785 by Friedrich Wilhelm Herschel. With a brightness of 10.9mag and an apparent size of 4.0x3.5', it can
even be seen in a small telescope. Due to its southern declination of -12°, however, it rises only about 28° above the horizon in Germany. The PN probably got its
spooky-sounding name because of its shape.
NGC 246 is located at a distance of about 2,050 light years. If we convert the apparent diameter, we get the true size of the nebula of 2.4 light years. The dynamic
age of the PN is 6,600 years. During this time, the nebular envelope has expanded into interstellar space at a speed of about 40km/s. Over time, the nebular
envelope will expand, it will continue to thin out and the PN will become invisible in a few thousand years.
The central star (ZS) in NGC 246 is one of the brightest of its kind with an apparent brightness of 11.8mag - making it visible even in small telescopes with only a
few inches of aperture. It bears the designation "HIP 3678" and belongs to the group of white dwarfs. Its spectral class was determined to be B0, and its effective
temperature is 150,000 Kelvin. This extremely high temperature is more than sufficient to excite the nebular envelope to glow. The mass of the white dwarf is 0.74
solar masses, its luminosity is 20,000 times that of the Sun. It rotates at a speed of 77km/s. The ZS is not a single star, but belongs to a triple star system. The other
two stars are dwarf stars: Component B is a K dwarf with 0.8 solar masses, while component C is an M dwarf with only 0.1 solar masses.
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In my 20" telescope NGC 246 offers a stunning view at 200x with an OIII filter. The nebula envelope shows great structures. While it gently blends into the sky
background towards the east, the edge appears strongly brightened in the other areas. In the northern part another filament can be made out within the PN, which
appears slightly curved. Further inside, a darker area follows. The shape of the "skull" can be seen nicely.
The central star appears quite bright and together with four other bright stars a great star pattern is formed, which borders the PN to the south and west.
