NGC 246 - Skull Nebula

Discovered in 1785 by William Herschel, NGC 246 is a relatively large planetary nebula in the constellation Cetus. It lies approximately 6° north-northeast of the 2nd-magnitude star β Cet.

Although visible to a trained observer under dark skies in telescopes as small as 4 inches, most observers will require at least a 6 inch telescope to see its faint, diffuse glow. In a small telescope, the nebula's feeble light is almost overpowered by the foreground stars superimposed on it. Larger telescopes show the nebula more clearly, especially with an oxygen III filter.

This beautiful 10.9-magnitude planetary has a bright disk that gives the impression of transparency because of the sprinkling of stars across it. The disk is mottled, and the notch at its SE edge makes the planetary look like a donut with a bite taken from it. Within the disk, about 45" SW and 65" ESE of the 12th magnitude central star respectively, are 12th and 13th magnitude field stars. Another 12th magnitude field star lies near the NNW edge, and 1' SW of the disk is a second 13th magnitude star.

Appropriately nicknamed "the Skull Nebula", this lovely, intricate planetary nebula is the outer atmosphere of a once sun-like star some 1,600 light-years away. Expelled over a period of thousands of years, its large angular dimensions of 4.6' x 4.1' imply an actual size of 6 light years.

The central star of this nebula (HIP 3678) is actually a binary system, with a close angular separation (3.8"). NGC 246's dying central star, now a white dwarf, is worth monitoring. In 1930, its photographic magnitude was as bright as 9th magnitude. In 1969 it dipped to 11.2, and today it shines at about 12th magnitude.

The central binary star is slightly displaced toward the shell's leading edge, which appears brighter and sharper than its trailing edge. The complex braided structure of NGC 246's blue outer ring is thought to result from the action of shock waves. As the compressed outer shell piles up against the interstellar medium, it heats up and becomes unstable, causing it to fragment.