Also known as NGC 2359, Thor’s Helmet Nebula is an emission nebula in the constellation Canis Major, approximately 11.96 thousand light years away and 30 light-years in size. The power source for this complex nebula appears to be the Wolf Rayet star HD 56925 (the brightest star in the middle of the nebula).
The nebula has an overall bubble shape, but with complex filamentary structures containing several hundred solar masses of ionised material, plus several thousand more of unionised gas. It is largely interstellar material swept up by winds from the central star, although some material does appear to be enriched with the products of fusion and is likely to come directly from the star. The expansion rate of different portions of the nebula varies from 10km/s to at least 30km/s.
Regions dominated by hydrogen gas, which are typical of the interstellar medium, will tend to glow red. Regions with significant amounts of other gases, particularly oxygen, often glow green or blue. Based on spectra, 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 bluish-green.
Wolf-Rayet stars begin life very massive, perhaps 25-40 times as massive as the sun, and about 280,000 times brighter than the Sun, having a surface temperature of 112,000 K. These stars are extremely rare (only about 300 are known presently) and are short-lived super-hot blue giant stars. They are also highly luminous, from tens of thousands to several million times the luminosity of the Sun, although not exceptionally bright visually since most of their output is in far ultraviolet and even soft X-rays. They are doomed, within a few million years, to explode as supernovae. One of these stars can blow material away with velocities that approach 2000 km/sec., that is 4.5 million miles per hour. At that rate they quickly lose mass, eventually becoming a more normal, stable star.
Credits: Anne’s Astronomy News, NASA, Observing At Skyhound, Wikipedia