NGC 246 -- Planetary Nebula in Cetus
NGC 246 is a fine visual planetary nebula even in modest-sized telescopes. The downside
was my location. From my suburban backyard the Skull Nebula (also known as the Pac-Man
Nebula) is low in the southern sky, fighting for air, in the glow of Washington D.C.
Away from light-pollution the spherical disk has a mouth and two-eyes (darkened areas
within the nebula) that gives it the name of the Skull Nebula. Stunning in the 18” Dobsonian
these features can even be teased out in my 110mm refractor from a dark site. The eye
spots are more noticeable than those in the Owl Planetary Nebula (M97) and the wide-open
mouth is even easier to see than the eyes.
Star hoping from Deneb Kaitos (Beta Ceti) to the location of NGC 246 was easy using my
155mm refractor from the deck of the house, but the Skull was nearly invisible in the telescope
thanks to the light pollution. An Oxygen III nebula filter helped, but unfortunately at all powers,
the Skull remained featureless, except for being subtly mottled, in the eyepiece.
Five stars could be seen across the face of the skull, only one, the middle one is associated
with the nebula, the other four are either behind it or in front of it (you may have to enlarge
the drawing to see the 3 dimmer stars). The middle star, of the three dimmer stars, is the
Central Star of the Planetary Nebula. This White Dwarf (HIP 3678) has a mass of 0.74 times
that of our sun but is very dense -- Earth-size or slightly larger. Seeing an Earth-sized object
that is 1,600 light years away is noteworthy in my book. HIP 3678 is a three-star system
made up of the bright White Dwarf (that I could see) and larger, but far dimmer, mid-sized
K dwarf and mid-sized M dwarf stars (which I could not see).