NGC 2359 = Sharpless 2-298 -- Thor's Helmet in Canis Major
The star HD 56925 is going through a stage that occurs only during the deaths of massive
stars. HD 56925 and others like it are called Wolf-Rayet stars. Over the past 2.3 million
years this 25-solar mass, Type-O star has blown off material that was caught in its own violent
solar-wind (12km/second) to form a bubble that now covers 230 by 120 light years. This
enlarging bubble is butting up against a diffuse HII region in space and causing a spectacular
collision. The resulting emission/reflection nebula is known officially as NGC 2350 or
Sharpless 2-298, but among amateur astronomers it is best known from it shape, as seen
through a large telescope – Thor’s Helmet.
From my suburban backyard using the 6-inch refractor a nebula filter is needed to tease out
Thor’s Helmet. The southern bar of the nebula is the easiest to see while the outer edges
of the bubble to the north of the bar is more difficult. The center of the bubble, where the star
HD 56925 shines at magnitude 11.4, appears nearly free of the ghost-like nebula. I like the
view best using an eyepiece that provides plenty of space around Thor’s Helmet. Increasing
the magnification seemed to make it harder to distinguish the edges of the nebula.
This nebula begs for dark skies and large aperture telescopes, but I was reasonably
surprised, that if I applied a little patience, of the detail I could get from NGC 2359 from the
deck of my house with a modest-sized telescope.