Mohawk Valley Astronomical Society

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Cosmic Witch

by Perry Pezzolanella, MVAS

The nights grow long and frosty when the trees become bare and cast their spooky shadows across the barren landscape beneath the moonlight. As midnight approaches, mighty Orion, the Hunter, rises high in the east bringing with it an appropriate Halloween icon, the Witch Head Nebula, IC2118, which is located near Orion in the constellation Eridanus, the River.

Max Wolf discovered the Witch Head Nebula nearly a century ago and described it as appearing like “foggy clouds driven by a fresh breeze.” It was Frank E. Ross who named it the “Witch’s Head Nebula,” and Edwin Hubble discovered the source of its illumination in 1922. Most nebulae get their color from nearby stars that illuminate them; the Witch Head Nebula owes its eerie blue color to the brilliant, hot, blue supergiant Rigel. This is a powerhouse star that generates more light in one minute than the Sun generates in one month. It is about 800 light years away and the seventh brightest star in the night sky shining at magnitude +0.2. If it were as close as Alpha Centauri it would be as bright as the full Moon. Oddly, Rigel is too cool to turn the Witch Head red. The more common red nebulae owe their color to extremely hot, blue, O stars, which are far hotter than Rigel, as it is the ultraviolet radiation from these stars that ionizes hydrogen in the nebulae. The radiation tears electrons from hydrogen and other gases, and when the electrons re-join protons or nitrogen ions, red light is emitted.

The dust grains that compose the Witch Head Nebula reflect the blue light from Rigel; this is why the nebula is blue. The Witch Head’s blueness is often thought to be as blue as the nebulosity in the Pleiades, whose blue stars are also too cool to color the nebulae red. The gas and dust that make up the Witch Head has been compressed by the winds from several of Orion’s hot, powerful stars, especially Rigel. This compression is triggering star birth and someday the Witch Head Nebula may be home to planets and new suns similar to our own.

The Witch Head Nebula is about 1000 light-years away, and is diffuse and faint. It is not visible to the unaided eye and even large telescopes may find it hard to locate, but it photographs nicely. In its proper orientation with north facing up, the Witch Head appears upside down about 2½º to the west-northwest of Rigel and is rather large. When upright, the nebula is the perfect profile of a witch’s head with a hat, an indentation for eyes, pointed nose, mouth, and a long, pointed chin.

Several million years from now Rigel will explode as a supernova and the Witch Head Nebula will glow brighter than ever. Then as Rigel fades, the Witch Head will go dark, only to be illuminated from within from the newborn suns. So whether trick-or-treating with family or friends or simply walking under a starlit Halloween night, the Witch Head Nebula rises high among the barren trees, soaring across the vast, hauntingly beautiful cosmos.