This wispy witch hides near the prominent winter constellation Orion the Hunter, her spooky face lit up by the star she appears to be gazing at. That star is the super giant Rigel, which marks one of Orion’s legs. Perhaps the Witch Head is casting a spell on Orion as he confronts the raging bull Taurus to his right.
The Witch Head’s eerie blue glow is caused entirely by the reflection of Rigel’s super hot brilliance only 40 Astronomical Units (AU) away from her face (40 times the distance between the Earth and Sun). That’s right next door as cosmic distances go. There are two reasons the Witch Head is blue: One is because Rigel is so hot that it shines blue like the blue flame on a gas stove; the other reason is because there are copious amounts of dust in the nebula, which scatters blue light much better than the other visible wavelengths. That’s similar to why our sky is blue, although in Earth’s case it is primarily nitrogen and oxygen gas that’s scattering the blue light.
Even though the Witch Head nebula is 50 light years long, Rigel has no problem illuminating it because Rigel is a class B star that is 120,000 times brighter than our Sun. Distances to nebulas are difficult to estimate but, thanks to the Hipparcos satellite, the distance to Rigel is known to be 863 light years plus or minus nine percent. So we can make an educated guess that the Witch Head is about the same distance.
By its shape and composition, a reasonable assumption is that the Witch Head is an ancient supernova remnant. It’s probably associated with the Orion hydrogen gas super shell that includes the brightest stars in Orion, such as the three famous belt stars, Alnitak, Alnilam and Mintaka. The Spitzer Space Telescope has imaged the Witch Head in infrared light, revealing protostars forming deep within the nebula. Spitzer also detected Polycyclic Aromatic Hydrocarbons (PAHs,) which is a scientific name for soot. PAHs are precursors for life. In the form of air pollution, they also harm existing life on Earth. So, we can ask metaphorically: Do the potions in this nebula make her a good or a bad witch?
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