For 6-inch or larger telescopesYou won't find it in Burnham's or in Houston's Deep Sky Wonders. Yet Thor's Helmet (NGC 2359) is an excellent nebula for large and small telescopes alike. This patch of irregular nebulosity lies in an otherwise unremarkable patch of sky about 8o northeast of Sirius. See the finder charts link at the bottom of this page for star hopping directions.
Observing Thor's Helmet
In 6-inch or smaller instruments look for a rectangular-shaped hazy patch of sky. The challenge here is not only to see the nebula at all, but to perhaps see the associated "bubble" that lies at one end of the brighter rectangle. A UHC or OIII filter will help bring out the bubble.
A 10-inch scope will reveal much more, even without a filter. A UHC filter will reveal some of the faint arcs that extend away from the bubble. An OIII filter may show detail in the round "bubble" portion of the nebula, including arcs near the edges.
I observed Thor's Helmet in January 2000 with my 18-inch, without knowing anything about it. It made for a very pleasing find. The bright bar which runs east/west was immediately obvious. Soon I could make out the "bubble" that lies off the northeast corner of the bar. The bubble was fascinating! The edges appeared brighter than the inside and they were quite sharp. The inside of the bubble seemed at once smooth and mottled. The most intriguing thing was that the character of the light from the bubble was distinctly different from that of the bar.
In time I thought I could see a faint arc running out to the northwest from the north end of the bubble, but I couldn't convince myself that this wasn't the effect from a chain of stars. The photographs plainly show that this is real, and now I wonder if I might have been able to make out the similar bar to the southeast if I had looked for it. This observation was made without a filter under average skies. I'm certain that filters and/or an excellent night would bring out much more detail.
At Thor's Heart Beats a Wolf Rayet Star
So what is this nebula? Is it a planetary? An HII region? A reflection nebula? Or perhaps something completely different? There is no simple answer.
The power source for this complex nebula appears to be the Wolf Rayet star HD 56925. These stars begin life very massive, perhaps 25-40 times as massive as the sun. Such stars live their lives in the fast lane -- burning their candles at both ends, so to speak. Their lives are short but exciting.
Wolf Rayet stars are typically very bright. They represent a temporary phase in the life of a massive star where it becomes very unstable, losing mass from its surface at an enormous rate. One of these stars can blow material away with velocities that approach 2000 km/sec. That's 4.5 million miles per hour! At that rate they quickly lose mass, eventually becoming a more normal, stable star.
The color photograph above may be telling us a lot about what is going on here. Regions dominated by hydrogen gas, which are typical of the interstellar medium, will tend to glow red in photographs. Regions with significant amounts of other gases, particularly oxygen, often glow green or blue. That's why planetary nebulae often appear green or blue in the telescope and why an oxygen (OIII) filter works so well to show them. Based on spectra obtained with large telescopes, the "bubble" region appears to be made up of "shells" of material ejected from the star. These shells are made of nearly pure "star stuff" and glow blue in the photograph.
The Wolf Rayet star also appears to be embedded in a region of hydrogen gas clouds, which are being caused to glow by this bright star. The large, bright "bar" to the southwest appears to be such a cloud--perhaps a typical HII region.
Thor's Helmet -- NGC 2359