The emission nebula Thor's Helmet in the winter constellation Canis Major looks like its name! Catalogued as NGC 2359, Sharpless 2-298, Gum 4 and the Duck Nebula, this glowing gas bubble is powered by a super-hot Wolf-Rayet WR7 star on a brief evolutionary path to certain supernova obliteration. This object lies almost 12,000 light years distant and covers 30 light years in size. In your telescope though, it subtends 8x6 arc-minutes, a good size target that leaves no doubt you've found it once located. A narrowband filter such as Orion Telescope and Binocular's Ultrablock or OIII will darken the field while allowing the wavelength's of light emitted by the nebula to pass through to your eye. The effect is stunning!
What instrument will show this to you? There are views recorded in a 13x80 finderscope with an OIII filter, although the famous wings of the "helmet" were not discerned. It was discovered in January 1785 by Sir William Herschel who published a sketch along with this description: "A broad elongated nebulosity, in the form of a parallelogram with a short ray southwards from the south preceding corner. The nebulosity between the milky and resolvable, almost of an equal brightness; but very faint. The parallelogram about 8' long and 5 or 6' broad, but ill-defined." It will be visible to those with even six inch telescopes, although to bring out detail in the helmet's horns may require more aperture. The well known amateur Steve Gottlieb has given this description through a 17.5" telescope: "The central region is a 5' bubble (illuminated by a Wolf-Rayet star) with a brighter rim along the west side giving a "C" appearance with irregular knots, filamentary wisps of nebulosity and areas of thinner nebulosity in the interior."
Here is a sky view of the location:
A is Sirius, the Alpha star in Canis Major. The inset shows in a grey oval a few bright star hop locations to orient your self. Start at Sirius with a wide field eyepiece. Start increasing magnification when you see a glow at the area the arrow indicates. Add a filter once you?re there and magnify to bring out the intricate details.
To whet your appetite here is an excellent sketch using large aperture by French observer Bertrand Laville, note the wings on the helmet? Can you see them in your telescope?
Image courtesy of Bertrand Laville. Used by permission
Here is an image from wikisky.org showing much of the tenuous nebulosity throughout the helmet's area:
Bundle up, get out, and view this somewhat obscure showpiece object!
