Sand and Stars

A Rare and Gorgeous Wolf-Rayet Bubble

NGC 1399 looks like a cosmic bubble ploughing through a sea of stars. Image credit ESO

30 March 2022

Wind-blown Wolf-Rayet bubbles are among the most rare, intriguing and tantalising objects to observe. And NGC 3199’s gorgeous glowing cosmic bubble is even more rare, being one of only a handful of X-ray-emitting Wolf-Rayet bubbles.

Wolf-Rayet stars represent the most advanced evolutionary stage in the lives of luminous massive stars. For very massive stars, mass loss dominates their evolution and Wolf-Rayet stars shed mass rapidly by means of very powerful stellar winds up to 10 million kilometres per hour (6 million miles per hour). These thick, fast-moving winds create nebulae with fabulous shapes as they sweep up surrounding interstellar material. 

These winds can create strong shockwaves as they plough through the comparatively cool interstellar medium, causing them to heat up anything in their vicinity. This process can heat material to such high temperatures that it is capable of emitting X-rays, a type of radiation emitted only by highly energetic phenomena in the Universe…

… and this, astronomers tell us, is what has happened in the case of the nebula NGC 3199 allowing the bubble to join a very select club – that of X-ray-emitting Wolf-Rayet bubbles which, until now, was composed of only three other members – NGC2359 (Canis Major), NGC6888 (Cygnus) and S308 (also Canis Major). 

Lying around 12,000 light years away, WR 18 (HD 89358) is the Wolf-Rayet star that has blown this beautiful bubble. Although there are different types and sub-classes, Wolf-Rayet stars all have one thing in common: they’re teetering on the brink of a massive and rapid change…. ending their lives in a spectacular supernova explosion. When WR 18 eventually blows itself to smithereens it wiil like the earliest massive stars, seed the Universe with the heavy elements necessary for rocky planets to form and for life to eventually arise. It’s a strange experience to gaze at this star through the telescope and wonder if maybe one day, somewhere in the Milky Way, future life will trace its beginnings back to WR 18.

Image courtesy of J. Toalá with help by Don Goldman providing the narrow-band optical images and ESA.


NGC 1399. DSS image

Observing this lovely Wolf-Rayet Bubble

16″ f/5 Dobs, 228x 

This Wolf-Rayet bubble’s location alone is stunning! It lies four degrees NW of the Eta Carina Nebula in a rich Milky Way star field, absolutely ablaze with stars.  With the OIII filter, it appears as a fairly bright, thick crescent oriented N-S, roughly 10′ in size, and open towards the east. The arc of nebulosity is quite irregular with some slightly brighter patches, small knots of brightness, a couple of bright streaks and a few darker lanes that appear as if they are intruding into the nebulosity. It’s noticeably brighter on the south end with well defined edges on both the eastern and the western sides, while its northern end frays into a fine, feathery tangle of very faint nebulosity that simply fades away. The nebulosity appears to continue curving off the brighter southern end in a SE direction, but it is very faint and patchy and simply dissolves away into the background. If one mentally extends the arc into a ring, then WR 18 seems to be very asymmetrically placed with respect to the centre of the entire shell, lying considerably west of where the centre would be, albeit visually it is very attractive seeing its white mag 10.8 spark just east of the glowing arc of nebulosity.

Copyright © Susan Young 2022