Alternate: Sharpless 2-298, Gum 4, Thor’s Helmet
RA 7h 18.0 m 30 s
Dec -13.0º 13.8′
Thor’s Helmet, NGC 2359 a winter month object that deserves more attention from amateur astronomers. The nebula is approximately 11.96 thousand light years away and 30 light-years across.
The central star illuminating NGC 2359 is an extremely hot star thought to be in a pre-supernovastage of evolution, called a Wolf-Rayet star. Wolf-Rayet stars are the final stage of giant stars before going supernova. Their spectra shows broad emission lines of ionized helium and highly ionized nitrogen and carbon. The hydrogen to helium fusion that typifies our Sun has long since expired in a Wolf-Rayet. Helium fusion, followed by nitrogen, etc marks the end stage of these stars, ending with a massive supernova event. The properties of these stars were first described 1867 by Charles J. Wolf and Georges A. Rayet. There are 164 Wolf-Rayet stars are known in the Galaxy. These luminous Wolf-Rayet stars of spectral type O or B, and are hot stars, with effective temperatures between 30,000 and 50,000 K. Their very peculiar spectra show emission lines of a a stellar wind is ejecting matter into space at velocities between 1,000 and 3,000 kilometers per second, and indicate that an envelope of ejected matter exists around the star. The rate of mass loss by the stellar wind is significant, of the order of 10-4 solar masses per year. Wolf-Rayet stars are often seen surrounded by nebulosities as seen in NGC 2359.
Thor’s Helmet is similar to the Bubble Nebula. Interactions with a nearby large molecular cloud are thought to have contributed to the more complex shape and curved bow-shock structure of Thor’s Helmet.
NGC 2361 is a bright knot of nebulosity on one edge of the central ring of NGC 2359, thereby qualifying Thor’s Helmet as a cosmic duet.
My first view of NGC 2359 was at the 2014 Winter Star Party in the Florida Keys. I climbed to the top of a ten-foot ladder to reach the eyepiece of a 24” Dobsonian to get my first view of Thor’s Helmet. Definitely a memorable experience. Returning home from the trip, I was able to view NGC 2359 through my 11” SCT, as this deep space treasure can be seen through the more common 8” telescopes.
A nebula filter, such as an UHC or O-III filter, will be of benefit.