It spans nearly 30 light years of space… and resides approximately 15,000 light-years from Earth. Its heartbeat is an extremely hot giant star thought to be in a brief, pre-supernova stage of evolution. Interactions with a nearby dense, warn and large molecular cloud are what may have contributed to its complex shape and curved bow-shock structure. Step back into mythology and see if you have what it takes to capture “Thor’s Helmut”…
Unlike many nebula, this unusual character is the product of the central Wolf-Rayet star, its stellar winds, and the surrounding interstellar matter. The powerful star emits a high velocity wind, pushing matter ahead of it. This process both compresses and expands its ring-like shell. As it grows, it collects even more gas and dust from the interstellar medium. But how many times and how many events?
“We have detected three different velocity components, and determined their spatial distribution and physical properties. The kinematics, morphology, mass and density are clearly stratified with respect to the W-R star.” says JR Rizzo (et al). “These features allow us to learn about the recent evolutionary history of HD 56925, because the multiple layers could be associated to several energetic events which have acted upon the surrounding circumstellar medium. Hence, a careful study of the different shockfronts contain clues in determining the present and past interaction of this evolved massive star with its surroundings.”
While most planetary nebulae contain old stars nearing the end of their lives, the central Wolf-Rayet star in NGC 2359 is very young. Its ultraviolet photons are the fueling source of the emission nebula. Wolf-Rayets are evolved, massive and extremely hot – up to ~50,000 K. Not only that, but their luminosity is incredible, too… up to 10L to the fifth or sixth power. Their surface composition is extremely exotic, being dominated by helium rather than hydrogen and the stars themselves are rare, simply because they are so short-lived. It was only three short decades ago that astronomers also realized that WRs suffered from heavy mass loss as well. Their ejecta bursts outward at speeds comparable to a nova. The whole process of formation simply isn’t clearly understood yet. The layers may be from differential rotation – but they could be the results of the exposed stellar core.
“The overall emission in the nebula is dominated by the overwhelming contribution of the H II region and is characteristic of photoionization processes. The embedded, photoevaporating cloud contributes enough mass over a dynamical lifetime to account for the shell mass of 5.0 solar mass.” says TE Jernigan. “In NGC 2359, imagery reveals variations in density, temperature, and ionization structure on scales ranging from the size of the nebula down to the seeing limit of approximately 2.1 seconds. The structure of the H II region can be understood in terms of a photoionized conical cavity protruding into the surrounding molecular cloud. The emission in the bubble region is characteristic of that produced in the incomplete cooling region behind a stellar-wind shock wave.”
No matter what explanation lay behind it, observing “Thor’s Helmut” is a pure pleasure. You’ll find it located about a fistwidth east-northeast of Sirius (07h 18m 30s, ?13° 13′ 48″). This Herschel object is a delightful 8th magnitude and well worth the effort!
And many thanks to John Chumack of Galactic Images for making the effort and sharing it with us!